INFO-DIR-SECTION Programming
START-INFO-DIR-ENTRY
* Gambit: (gambit).		A portable implementation of Scheme.
* gsi: (gambit) interpreter.	Gambit interpreter.
* gsc: (gambit) compiler.	Gambit compiler.
END-INFO-DIR-ENTRY

   This file documents Gambit, a portable implementation of Scheme.

   Copyright (C) 1994-2022 Marc Feeley.

   Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

   Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

   Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that this permission notice may be stated in a
translation approved by the copyright holder.

Gambit
******

This manual documents Gambit.  It covers release v4.9.4.

1 The Gambit system
*******************

The Gambit programming system is a full implementation of the Scheme
language which conforms to the R4RS, R5RS, R7RS and IEEE Scheme
standards.  It consists of two main programs: ‘gsi’, the Gambit Scheme
interpreter, and ‘gsc’, the Gambit Scheme compiler.

   The Gambit Scheme compiler translates Scheme code to another target
language, currently C or JavaScript.  The C target is the most mature
and it offers portability and fast execution.  The JavaScript target
allows writing web apps in Scheme.

   Most of the Gambit system, including the interpreter and compiler, is
written in Scheme and compiled to portable C code using the compiler.
The high portability of the generated C code allows the interpreter,
compiler and user programs to be easily compiled and executed on any
platform for which a C compiler is available.  With appropriate
declarations in the source code the compiled Scheme programs run roughly
as fast as equivalent C programs.

   For the most up to date information on Gambit and related resources
please visit the Gambit web page at <https://gambitscheme.org>.  Issues
should be reported on the github source code repository
<https://github.com/gambit/gambit>.

1.1 Accessing the system files
==============================

Files related to Gambit, such as executables, libraries and header
files, are stored in multiple “Gambit installation directories”.  Gambit
may be installed on a system according to two different installation
models.

   In the first model there is a single directory where all the Gambit
installation directories are stored.  This “central installation
directory” is typically ‘/usr/local/Gambit’ under UNIX,
‘/Library/Gambit’ under macOS and ‘C:/Program Files/Gambit’ under
Microsoft Windows.  This may have been overridden when the system was
built with the command ‘configure --prefix=/my/Gambit’.  If the system
was built with the command ‘configure --enable-multiple-versions’ then
the central installation directory is ‘prefix/version’, where ‘version’
is the system version string (e.g.  ‘v4.9.4’ for Gambit v4.9.4).
Moreover, ‘prefix/current’ will be a symbolic link which points to the
central installation directory.  In this model, the Gambit installation
directory named X is simply the subdirectory X of the central
installation directory.

   In the second model some or all of the Gambit installation
directories are stored in installation specific directories.  The
location of these directories is assigned when the system is built using
the command ‘configure --bindir=/my/bin --includedir=/my/include
--libdir=/my/lib’.

   The advantage of the first model is that it is easy to have multiple
versions of Gambit coexist and to remove all the files of a given
version.  However, the second model may be necessary to conform to the
package installation conventions of some operating systems.

   Executable programs such as the interpreter ‘gsi’ and compiler ‘gsc’
can be found in the ‘bin’ installation directory.  Adding this directory
to the ‘PATH’ environment variable allows these programs to be started
by simply entering their name.  This is done automatically by the macOS
and Microsoft Windows installers.

   The runtime library is located in the ‘lib’ installation directory.
When the system’s runtime library is built as a shared-library (with the
command ‘configure --enable-shared’) all programs built with Gambit,
including the interpreter and compiler, need to find this library when
they are executed and consequently this directory must be in the path
searched by the system for shared-libraries.  This path is normally
specified through an environment variable which is ‘LD_LIBRARY_PATH’ on
most versions of UNIX, ‘LIBPATH’ on AIX, ‘SHLIB_PATH’ on HPUX,
‘DYLD_LIBRARY_PATH’ on macOS, and ‘PATH’ on Microsoft Windows.  If the
shell is ‘sh’, the setting of the path can be made for a single
execution by prefixing the program name with the environment variable
assignment, as in:

     $ LD_LIBRARY_PATH=/usr/local/Gambit/lib gsi

   A similar problem exists with the Gambit header file ‘gambit.h’,
located in the ‘include’ installation directory.  This header file is
needed for compiling Scheme programs with the Gambit compiler.  When the
C compiler is being called explicitly it may be necessary to use a
‘-I<DIR>’ command line option to indicate where to find header files and
a ‘-L<DIR>’ command line option to indicate where to find libraries.

   Access to both of these files can be simplified by creating a link to
them in the appropriate system directories (special privileges may
however be required):

     $ ln -s /usr/local/Gambit/lib/libgambit.a /usr/lib # name may vary
     $ ln -s /usr/local/Gambit/include/gambit.h /usr/include

   Alternatively these files can be copied or linked in the directory
where the C compiler is invoked (this requires no special privileges).

   Another approach is to set some environment variables which are used
to tell the C compiler where to find header files and libraries.  For
example, the following settings can be used for the ‘gcc’ C compiler:

     $ export LIBRARY_PATH=/usr/local/Gambit/lib
     $ export CPATH=/usr/local/Gambit/include

   Note that this may have been done by the installation process.  In
particular, the macOS and Microsoft Windows prebuilt installers set up
the environment so that the ‘gcc’ compiler finds these files
automatically.

2 The Gambit Scheme interpreter
*******************************

Synopsis:

     gsi [-:RUNTIMEOPTION,...] [-i] [-f] [-h] [-help] [-v]
         [[-] [-e EXPRESSIONS] [-install] [-uninstall] [-update]
          [SEARCH-DIRECTORY-OR-MODULE-OR-FILE]]...

   The interpreter is executed in “batch mode” when the command line
contains a module or file or a ‘-’, or ‘-e’ option.  The interpreter is
executed in “module management mode” when the command line contains the
‘-install’, ‘-uninstall’, or ‘-update’ option.  Otherwise the
interpreter is executed in “interactive mode”.  The ‘-i’ option is
ignored by the interpreter.  The initialization file will be examined
unless the ‘-f’ option is present (*note GSI customization::).  The ‘-h’
and ‘-help’ options print brief usage information on standard output and
exit.  The ‘-v’ option prints the system version string, system time
stamp, operating system type, and configure script options on standard
output and exits.  Runtime options are explained in *note Runtime
options::.

2.1 Interactive mode
====================

In interactive mode a read-eval-print loop (REPL) is started for the
user to interact with the interpreter.  At each iteration of this loop
the interpreter displays a prompt, reads a command and executes it.  The
commands can be expressions to evaluate (the typical case) or special
commands related to debugging, for example ‘,q’ to terminate the process
(for a complete list of commands see *note Debugging:: or use the
‘,help’ command).  Most commands produce some output, such as the value
or error message resulting from an evaluation.

   The input and output of the interaction is done on the “interaction
channel”.  The interaction channel can be specified through the runtime
options but if none is specified the system uses a reasonable default
that depends on the system’s configuration.  Typically the program’s
standard input and output are used as the interaction channel.  When
using the runtime option ‘-:debug=c’, the interaction channel is the
user’s “console”, also known as the “controlling terminal” in the UNIX
world.

   When the REPL starts, the ports associated with
‘(current-input-port)’, ‘(current-output-port)’ and
‘(current-error-port)’ all refer to the interaction channel.

   Expressions are evaluated in the global “interaction environment”.
The interpreter adds to this environment any definition entered using
the ‘define’ and ‘define-macro’ special forms.  Once the evaluation of
an expression is completed, the value or values resulting from the
evaluation are output to the interaction channel by the pretty printer.
The special “void object” is not output.  This object is returned by
most procedures and special forms which are defined as returning an
unspecified value (e.g.  ‘write’, ‘set!’, ‘define’).

   Here is a sample interaction with ‘gsi’:

     $ gsi
     Gambit v4.9.4

     > (define (fact n) (if (< n 2) 1 (* n (fact (- n 1)))))
     > (map fact '(1 2 3 4 5 6))
     (1 2 6 24 120 720)
     > (values (fact 10) (fact 40))
     3628800
     815915283247897734345611269596115894272000000000
     > ,q

   What happens when errors occur is explained in *note Debugging::.

2.2 Batch mode
==============

In batch mode the command line arguments denote modules and files to
execute, REPL interactions to start (‘-’ option), and expressions to be
evaluated (‘-e’ option).  Those options can be interspersed with the
search directories, modules, and files on the command line and can occur
multiple times.

   In addition to these options the command line may contain 3 types of
non-options: “search directories”, “modules”, and “files”.

“Search directories”
     Search directories are locations in the file system that are
     searched to resolve references to modules.  Any command line
     argument that ends with a path separator or a ‘.’ is treated as a
     search directory.  By default the module search order is initially
     ‘~~lib’ (which contains builtin modules) followed by ‘~~userlib’
     (which contains user installed modules and is typically the
     directory ‘.gambit_userlib’ in the user’s home directory).  Search
     directories on the command line are added to the front of the
     search order, and thus take precedence over the default module
     search order.

“Modules”
     Modules are either “unversioned” or “versioned” (managed by the
     ‘git’ version-control system).  There are two flavors of versioned
     modules: “hosted” modules have a ‘git’ repository on a network
     accessible repository manager site such as ‘github.com’ and
     ‘gitlab.com’, and “local” modules have a ‘git’ repository on the
     local file system.  Module names have a syntax similar to the paths
     used to identify files.  They consist of one or more non-empty
     parts separated by ‘/’.  The last part may end with a suffix of the
     form ‘@VERSION’.  Only the first part and version may contain ‘.’,
     otherwise only the characters a-z, A-Z, 0-9, ‘-’, and ‘_’ are
     permitted.  If there are at least 3 parts and the first part
     contains at least one ‘.’ and no ‘_’, then it refers to a hosted
     module (1st part = host, 2nd part = account, 3rd part = repository
     name).  For example ‘github.com/gambit/hello@1.0’ is a hosted
     module reference.  Otherwise it refers to a local versioned module
     or an unversioned module, for example ‘foobar’ or ‘A/B/C/D’.

“Files”
     Files are simple code containers located on the local file system.
     They are also identified by a path.  If a path is a valid module or
     file, it is interpreted as a module.  Note that a path with a last
     component containing an extension, such as ‘.scm’, and no ‘@’, is
     always interpreted as a file.

   The interpreter processes the command line arguments from left to
right.  Search directories are added to the head of the module search
order.  Files are executed using the ‘load’ procedure.  Modules are
requested using the ‘##demand-module’ special form (this form is
explained in *note Modules::, but essentially it causes that module to
be searched in the module search order and executed once).  The ‘-e’
option uses the ‘eval’ procedure to evaluate expressions in the global
interaction environment.  After this processing the interpreter exits.

   The ports associated with ‘(current-input-port)’,
‘(current-output-port)’ and ‘(current-error-port)’ initially refer
respectively to the standard input (‘stdin’), standard output (‘stdout’)
and the standard error (‘stderr’) of the interpreter.  This is true even
in REPLs started with the ‘-’ option.  The usual interaction channel is
still used to read expressions and commands and to display results.
This makes it possible to use REPLs to debug programs which read the
standard input and write to the standard output, even when these have
been redirected.

   Here is a sample use of the interpreter in batch mode, under UNIX:

     $ cat h.scm
     (display "hello") (newline)
     $ cat w.six
     display("world"); newline();
     $ gsi h.scm - w.six -e "(pretty-print 1)(pretty-print 2)"
     hello
     > (define (display x) (write (reverse (string->list x))))
     > ,c
     (#\d #\l #\r #\o #\w)
     1
     2
     $ gsi . h w   # add . to search order to load modules h and w
     hello
     world

2.3 Module management mode
==========================

Package management operations are executed using the command line
options ‘-install’, ‘-uninstall’, and ‘-update’ which respectively
install, uninstall and update packages.  Package installation is
explained in detail in *note Modules::, but here are a few examples:

     $ gsi -install github.com/gambit/hello
     installing github.com/gambit/hello to /Users/feeley/.gambit_userlib/
     $ gsi github.com/gambit/hello@1.0
     hello world!
     $ gsi -uninstall github.com/gambit/hello
     uninstalling github.com/gambit/hello from /Users/feeley/.gambit_userlib/

2.4 Customization
=================

There are two ways to customize the interpreter.  When the interpreter
starts off it tries to execute a ‘(load "~~lib/gambext")’ (for an
explanation of how file names are interpreted see *note Host
environment::).  An error is not signaled when the file does not exist.
Interpreter extensions and patches that are meant to apply to all users
and all modes should go in that file.

   Extensions which are meant to apply to a single user or to a specific
working directory are best placed in the “initialization file”, which is
a file containing Scheme code.  In all modes, the interpreter first
tries to locate the initialization file by searching the following
locations: ‘.gambini’ and ‘~/.gambini’ (with no extension, a ‘.sld’
extension, a ‘.scm’ extension, and a ‘.six’ extension in that order).
The first file that is found is examined as though the expression
‘(include INITIALIZATION-FILE)’ had been entered at the read-eval-print
loop where INITIALIZATION-FILE is the file that was found.  Note that by
using an ‘include’ the macros defined in the initialization file will be
visible from the read-eval-print loop (this would not have been the case
if ‘load’ had been used).  The initialization file is not searched for
or examined when the ‘-f’ option is specified.

2.5 Process exit status
=======================

The status is zero when the interpreter exits normally and is nonzero
when the interpreter exits due to an error.  Here is the meaning of the
exit statuses:

‘0’
     The execution of the primordial thread (i.e.  the main thread) did
     not encounter any error.  It is however possible that other threads
     terminated abnormally (by default threads other than the primordial
     thread terminate silently when they raise an exception that is not
     handled).

‘64’
     The runtime options or the environment variable ‘GAMBOPT’ contained
     a syntax error or were invalid.

‘70’
     This normally indicates that an exception was raised in the
     primordial thread and the exception was not handled.

‘71’
     There was a problem initializing the runtime system, for example
     insufficient memory to allocate critical tables.

   For example, if the shell is ‘sh’:

     $ gsi -e "(pretty-print (expt 2 100))"
     1267650600228229401496703205376
     $ echo $?
     0
     $ gsi -e "(pretty-print (expo 2 100))"
     *** ERROR IN (string)@1.16 -- Unbound variable: expo
     $ echo $?
     70
     $ gsi -:debug=0 -e "(pretty-print (expo 2 100))"
     $ echo $?
     70
     $ gsi -:debug=0,unknown # try to use an unknown runtime option
     $ echo $?
     64
     $ gsi -:debug=0 nonexistent.scm # try to load a file that does not exist
     $ echo $?
     70
     $ gsi nonexistent.scm
     *** ERROR IN ##load-module-or-file -- No such file or directory
     (load "nonexistent.scm")
     $ echo $?
     70

   Note the use of the runtime option ‘-:debug=0’ that prevents error
messages from being output.

2.6 Scheme scripts
==================

The ‘load’ procedure treats specially files that begin with the two
characters ‘#!’ and ‘@;’.  Such files are called “script files” and the
first line is called the “script line”.  In addition to indicating that
the file is a script, the script line provides information about the
source code language to be used by the ‘load’ procedure.  After the two
characters ‘#!’ and ‘@;’ the system will search for the first substring
matching one of the following language specifying tokens:

‘scheme-r4rs’
     R4RS language with prefix syntax, case-insensitivity, keyword
     syntax not supported

‘scheme-r5rs’
     R5RS language with prefix syntax, case-insensitivity, keyword
     syntax not supported

‘scheme-ieee-1178-1990’
     IEEE 1178-1990 language with prefix syntax, case-insensitivity,
     keyword syntax not supported

‘scheme-srfi-0’
     R5RS language with prefix syntax and SRFI 0 support (i.e.
     ‘cond-expand’ special form), case-insensitivity, keyword syntax not
     supported

‘gsi-script’
     Full Gambit Scheme language with prefix syntax, case-sensitivity,
     keyword syntax supported

‘gsc-script’
     Full Gambit Scheme language with prefix syntax, case-sensitivity,
     keyword syntax supported

‘six-script’
     Full Gambit Scheme language with infix syntax, case-sensitivity,
     keyword syntax supported

   If a language specifying token is not found, ‘load’ will use the same
language as a nonscript file (i.e.  it uses the file extension and
runtime system options to determine the language).

   After processing the script line, ‘load’ will parse the rest of the
file (using the syntax of the language indicated) and then execute it.
When the file is being loaded because it is an argument on the
interpreter’s command line, the interpreter will:

   • Setup the ‘command-line’ procedure so that it returns a list
     containing the expanded file name of the script file and the
     arguments following the script file on the command line.  This is
     done before the script is executed.  The expanded file name of the
     script file can be used to determine the directory that contains
     the script (i.e.  ‘(path-directory (car (command-line)))’).

   • After the script is loaded the procedure ‘main’ is called with the
     command line arguments.  The way this is done depends on the
     language specifying token.  For ‘scheme-r4rs’, ‘scheme-r5rs’,
     ‘scheme-ieee-1178-1990’, and ‘scheme-srfi-0’, the ‘main’ procedure
     is called with the equivalent of ‘(main (cdr (command-line)))’ and
     ‘main’ is expected to return a process exit status code in the
     range 0 to 255.  This conforms to the “Running Scheme Scripts on
     Unix SRFI” (SRFI 22).  For ‘gsi-script’ and ‘six-script’ the ‘main’
     procedure is called with the equivalent of ‘(apply main (cdr
     (command-line)))’ and the process exit status code is 0 (‘main’’s
     result is ignored).  The Gambit system has a predefined ‘main’
     procedure which accepts any number of arguments and returns 0, so
     it is perfectly valid for a script to not define ‘main’ and to do
     all its processing with top-level expressions (examples are given
     in the next section).

   • When ‘main’ returns, the interpreter exits.  The command line
     arguments after a script file are consequently not processed
     (however they do appear in the list returned by the ‘command-line’
     procedure, after the script file’s expanded file name, so it is up
     to the script to process them).

2.6.1 Scripts under UNIX and macOS
----------------------------------

Under UNIX and macOS, the Gambit installation process creates the
executable ‘gsi’ and also the executables ‘six’, ‘gsi-script’,
‘six-script’, ‘scheme-r5rs’, ‘scheme-srfi-0’, etc as links to ‘gsi’.  A
Scheme script need only start with the name of the desired Scheme
language variant prefixed with ‘#!’ and the directory where the Gambit
executables are stored.  This script should be made executable by
setting the execute permission bits (with a ‘chmod +x SCRIPT’).  Here is
an example of a script which lists on standard output the files in the
current directory:

     #!/usr/local/Gambit/bin/gsi-script
     (for-each pretty-print (directory-files))

   Here is another UNIX script, using the Scheme infix syntax extension,
which takes a single integer argument and prints on standard output the
numbers from 1 to that integer:

     #!/usr/local/Gambit/bin/six-script

     function main(n_str)
     {
       scmobj n = \string->number(n_str);
       for (scmobj i=1; i<=n; i++)
         \pretty-print(i);
     }

   For maximal portability it is a good idea to start scripts indirectly
through the ‘/usr/bin/env’ program, so that the executable of the
interpreter will be searched in the user’s ‘PATH’.  This is what SRFI 22
recommends.  For example here is a script that mimics the UNIX ‘cat’
utility for text files:

     #!/usr/bin/env gsi-script

     (define (display-file filename)
       (display (call-with-input-file filename
                  (lambda (port)
                    (read-line port #f)))))

     (for-each display-file (cdr (command-line)))

2.6.2 Scripts under Microsoft Windows
-------------------------------------

Under Microsoft Windows, the Gambit installation process creates the
executable ‘gsi.exe’ and ‘six.exe’ and also the batch files
‘gsi-script.bat’, ‘six-script.bat’, ‘scheme-r5rs.bat’,
‘scheme-srfi-0.bat’, etc which simply invoke ‘gsi.exe’ with the same
command line arguments.  A Scheme script need only start with the name
of the desired Scheme language variant prefixed with ‘@;’.  A UNIX
script can be converted to a Microsoft Windows script simply by changing
the script line and storing the script in a file whose name has a ‘.bat’
or ‘.cmd’ extension:

     @;gsi-script %~f0 %*
     (display "files:\n")
     (pretty-print (directory-files))

   Note that Microsoft Windows always searches executables in the user’s
‘PATH’, so there is no need for an indirection such as the UNIX
‘/usr/bin/env’.  However the script line must end with ‘%~f0 %*’ to pass
the expanded filename of the script and command line arguments to the
interpreter.

2.6.3 Compiling scripts
-----------------------

A script file can be compiled using the Gambit Scheme compiler (*note
GSC::) into a standalone executable.  The script line will provide
information to the compiler on which language to use.  The script line
also provides information on which runtime options to use when executing
the compiled script.  This is useful to set the default runtime options
of an executable program.

   The compiled script will be executed similarly to an interpreted
script (i.e.  the list of command line arguments returned by the
‘command-line’ procedure and the invocation of the ‘main’ procedure).

   For example:

     $ cat square.scm
     #!/usr/local/Gambit/bin/gsi-script -:debug=0
     (define (main arg)
       (pretty-print (expt (string->number arg) 2)))
     $ gsi square 30        # gsi will load square.scm
     900
     $ gsc -exe square      # compile the script to a standalone program
     $ ./square 30
     900
     $ ./square 1 2 3       # too many arguments to main
     $ echo $?
     70
     $ ./square -:debug=1 1 2 3  # ask for error message
     *** ERROR IN ##start-main -- Wrong number of arguments passed to procedure
     (main "1" "2" "3")

3 The Gambit Scheme compiler
****************************

Synopsis:

     gsc [-:RUNTIMEOPTION,...] [-i] [-f] [-h] [-help] [-v]
         [-target TARGET]
         [-prelude EXPRESSIONS] [-postlude EXPRESSIONS]
         [-dynamic] [-exe] [-obj]
         [-nb-gvm-regs N] [-nb-arg-regs N] [-compactness LEVEL]
         [-cc COMPILER] [-cc-options OPTIONS]
         [-ld-options-prelude OPTIONS] [-ld-options OPTIONS]
         [-pkg-config PKG-CONFIG-ARGS] [-pkg-config-path PKG-CONFIG-PATH]
         [-warnings] [-verbose] [-report] [-expansion] [-gvm] [-cfg] [-dg]
         [-debug] [-debug-location] [-debug-source]
         [-debug-environments] [-track-scheme]
         [-o OUTPUT] [-c] [-keep-temp] [-link] [-flat] [-l BASE]
         [-module-ref MODULE-REF] [-linker-name LINKER-NAME]
         [[-] [-e EXPRESSIONS] [-preload] [-nopreload]
          [SEARCH-DIRECTORY-OR-MODULE-OR-FILE]]...

   The ‘-h’ and ‘-help’ options print brief usage information on
standard output and exit.  The ‘-v’ option prints the system version
string, system time stamp, operating system type, and configure script
options on standard output and exits.

   The ‘-i’ option can be used to force ‘gsc’ to process the command
line like the interpreter.  The only difference with the interpreter is
that the compilation related procedures listed in this chapter are also
available (i.e.  ‘compile-file’, ‘compile-file-to-target’, etc).

3.1 Interactive mode
====================

When no command line argument is present other than options ‘gsc’
behaves like ‘gsi’ in interactive mode.

3.2 Customization
=================

Like the interpreter, the compiler will examine the initialization file
unless the ‘-f’ option is specified.  Runtime options are explained in
*note Runtime options::.

3.3 Batch mode
==============

In batch mode ‘gsc’ accepts on the command line 3 types of non-options
which are processed from left to right: “search directories”, “modules”,
and “files”.  Search directories are added to the list of module search
order directories.  Every command line argument that is the name of a
module that is found in the list of module search order directories will
cause that module to be compiled.  Similarly, file names (with either no
extension, or a C file extension, or some other extension) on the
command line will cause that file to be compiled.  The compilation is
done for the target language specified with the ‘-target’ TARGET option.
TARGET is either ‘js’, for JavaScript, or ‘C’, which is the default if
no target language is specified.

   The recognized C file extensions are ‘.c’, ‘.C’, ‘.cc’, ‘.cp’,
‘.cpp’, ‘.CPP’, ‘.cxx’, ‘.c++’, ‘.m’, ‘.M’, and ‘.mm’.

   The extension can be omitted from a file name when the Scheme file
has a ‘.scm’, ‘.sld’ or ‘.six’ extension.  When the extension of the
Scheme file is ‘.six’ the content of the file will be parsed using the
Scheme infix syntax extension (see *note Scheme infix syntax
extension::).  Otherwise, ‘gsc’ will parse the Scheme file using the
normal Scheme prefix syntax.  Files with a C file extension must have
been previously produced by ‘gsc’ with the ‘C’ target and the ‘-c’
option, and are used by the ‘C’ target Gambit linker.

   For each Scheme file the compiler creates a file of target code,
either ‘FILE.c’ or ‘FILE.js’ for the ‘C’ and ‘js’ targets respectively.
The file’s name is the same as the Scheme file, but the extension is
changed to ‘.c’ or ‘.js’ as appropriate.  By default the file is created
in the same directory as the Scheme file.  This default can be
overridden with the compiler’s ‘-o’ option.

   The files of target code produced by the compiler serve two purposes.
They will be processed by a C compiler or JavaScript VM, and they also
contain information to be read by Gambit’s linker to generate a “link
file”.  The link file is a file of target code that collects various
linking information for a group of modules, such as the set of all
symbols and global variables used by the modules.  The linker is only
invoked when the ‘-link’ or ‘-exe’ options appear on the command line.

   Compiler options must be specified before the first file name and
after the ‘-:’ runtime option (*note Runtime options::).  If present,
the ‘-i’, ‘-f’, and ‘-v’ compiler options must come first.  The
available options are:

‘-i’
     Force interpreter mode.
‘-f’
     Do not examine the initialization file.
‘-h / -help’
     Print brief usage information on standard output and exit.
‘-v’
     Print the system version string, system time stamp, operating
     system type, and configure script options on standard output and
     exit.
‘-target TARGET’
     Select the target language.
‘-prelude EXPRESSIONS’
     Add expressions to the top of the source code being compiled.
‘-postlude EXPRESSIONS’
     Add expressions to the bottom of the source code being compiled.
‘-cc COMPILER’
     Select specific C compiler.
‘-cc-options OPTIONS’
     Add options to the command that invokes the C compiler.
‘-ld-options-prelude OPTIONS’
     Add options to the command that invokes the C linker.
‘-ld-options OPTIONS’
     Add options to the command that invokes the C linker.
‘-pkg-config PKG-CONFIG-ARGS’
     Use the ‘pkg-config’ program to determine options for the C
     compiler and C linker.
‘-pkg-config-path PKG-CONFIG-PATH’
     Add a path to the ‘PKG_CONFIG_PATH’ environment variable.
‘-warnings’
     Display warnings.
‘-verbose’
     Display a trace of the compiler’s activity.
‘-report’
     Display a global variable usage report.
‘-expansion’
     Display the source code after expansion.
‘-gvm’
     Generate a listing of the GVM code.
‘-cfg’
     Generate a control flow graph of the GVM code.
‘-dg’
     Generate a dependency graph.
‘-debug’
     Include all debugging information in the code generated.
‘-debug-location’
     Include source code location debugging information in the code
     generated.
‘-debug-source’
     Include the source code debugging information in the code
     generated.
‘-debug-environments’
     Include environment debugging information in the code generated.
‘-track-scheme’
     Generate ‘#line’ directives referring back to the Scheme code.
‘-o OUTPUT’
     Set name of output file or directory where output file(s) are
     written.
‘-dynamic’
     Compile Scheme source files to dynamically loadable object files
     (this is the default).
‘-exe’
     Compile Scheme source files to an executable program (machine code
     or script).
‘-obj’
     Compile Scheme source files to object files by invoking the C
     compiler.
‘-keep-temp’
     Keep any intermediate files that are generated.
‘-c’
     Compile Scheme source files to target code without generating a
     link file.
‘-link’
     Compile Scheme source files to target code and generate a link
     file.
‘-flat’
     Generate a flat link file instead of the default incremental link
     file.
‘-l BASE’
     Specify the link file of the base library to use for the link.
‘-module-ref MODULE-REF’
     Specify the reference of the generated module.
‘-linker-name LINKER-NAME’
     Specify the name of the low-level initialization function exported
     by the module.
‘-preload’
     Turn on ‘preload’ linker bit.
‘-nopreload’
     Turn off ‘preload’ linker bit.  Start REPL interaction.
‘-e EXPRESSIONS’
     Evaluate expressions in the interaction environment.
‘-nb-gvm-regs N’
     Specify the number of available Gambit virtual machine registers.
‘-nb-arg-regs N’
     Specify the number of procedure call parameters passed in Gambit
     virtual machine registers.
‘-compactness LEVEL’
     Specify the compactness of the generated code.

   The ‘-i’ option forces the compiler to process the remaining command
line arguments like the interpreter.

   The ‘-target’ option selects the target language of the compilation.
It is either ‘js’ for JavaScript, or ‘C’ for C (which is the default).

   The ‘-prelude’ option adds the specified expressions to the top of
the source code being compiled.  It can appear multiple times.  The main
use of this option is to supply declarations on the command line.  For
example the following invocation of the compiler will compile the file
‘bench.scm’ in unsafe mode:

     $ gsc -prelude "(declare (not safe))" bench.scm

   The ‘-postlude’ option adds the specified expressions to the bottom
of the source code being compiled.  It can appear multiple times.  The
main use of this option is to supply the expression that will start the
execution of the program.  For example:

     $ gsc -postlude "(start-bench)" bench.scm

   The ‘-cc’ option is only meaningful when the ‘C’ target is selected.
The ‘-cc’ option selects the specified C compiler for compiling the
generated C code.  When this option is used, the default C compiler
options that were determined to be needed by the configure script are
nullified because they are very likely to be invalid for the specified C
compiler.  Any options needed for this C compiler should be specified
explicitly using the ‘-cc-options’, ‘-ld-options-prelude’, and
‘-ld-options’ options.  For example:

     $ gsc -cc clang -cc-options "-O0 -bundle" bench.scm # clang on macOS
     $ gsc -cc tcc -cc-options -shared bench.scm         # tcc on linux

   The ‘-cc-options’ option is only meaningful when the ‘C’ target is
selected and a dynamically loadable object file is being generated
(neither the ‘-c’ or ‘-link’ options are used).  It can appear multiple
times.  The ‘-cc-options’ option adds the specified options to the
command that invokes the C compiler.  The main use of this option is to
specify the include path, some symbols to define or undefine, the
optimization level, or any C compiler option that is different from the
default.  For example:

     $ gsc -cc-options "-U___SINGLE_HOST -O2 -I../include" bench.scm

   The ‘-ld-options-prelude’ and ‘-ld-options’ options are only
meaningful when the ‘C’ target is selected and a dynamically loadable
object file is being generated (neither the ‘-c’ or ‘-link’ options are
used).  They can appear multiple times.  The ‘-ld-options-prelude’ and
‘-ld-options’ options add the specified options to the command that
invokes the C linker (the options in LD-OPTIONS-PRELUDE are passed to
the C linker before the input file and the options in LD-OPTIONS are
passed after).  The main use of this option is to specify additional
object files or libraries that need to be linked, or any C linker option
that is different from the default (such as the library search path and
flags to select between static and dynamic linking).  For example:

     $ gsc -ld-options "-L/usr/X11R6/lib -lX11 -dynamic" app.scm

   The ‘-pkg-config’ is only meaningful when the ‘C’ target is selected.
The ‘-pkg-config’ option will cause the ‘pkg-config’ program to be
invoked to determine the options to add to the command that invokes the
C compiler and C linker.  It can appear multiple times.  The
‘pkg-config’ program is passed the arguments in the string
PKG-CONFIG-ARGS in addition to either ‘--cflags’ or ‘--libs’.  It is
typical for PKG-CONFIG-ARGS to be the name of a system library, such as
‘"sqlite3"’, but other ‘pkg-config’ options can be specified, such as
‘"--static sqlite3"’.  The ‘-pkg-config-path’ option adds a path to the
‘PKG_CONFIG_PATH’ environment variable for use by the ‘pkg-config’
program to find ‘.pc’ files.  For example:

     $ gsc -pkg-config "x11" -pkg-config-path "/usr/share/pkgconfig" app.scm

   The ‘-warnings’ option displays on standard output all warnings that
the compiler may have.

   The ‘-verbose’ option displays on standard output a trace of the
compiler’s activity.

   The ‘-report’ option displays on standard output a global variable
usage report.  Each global variable used in the program is listed with 4
flags that indicate whether the global variable is defined, referenced,
mutated and called.

   The ‘-expansion’ option displays on standard output the source code
after expansion and inlining by the front end.

   The ‘-gvm’ option generates a listing of the intermediate code for
the “Gambit Virtual Machine” (GVM) of each Scheme file on ‘FILE.gvm’.

   The ‘-cfg’ option generates a visual representation of the control
flow graph of the intermediate code for the “Gambit Virtual Machine”
(GVM) of each Scheme file on ‘FILE.cfg’.  The file is suitable for
processing with the “dot” program.  For example, to generate the PDF
file ‘FILE.cfg.pdf’ from ‘FILE.cfg’ the following command can be used:

     $ dot -O -Tpdf FILE.cfg

   The ‘-dg’ option generates a visual representation of the dependency
graph of each Scheme file on ‘FILE.dg’.  The file is suitable for
processing with the “dot” program.  For example, to generate the PDF
file ‘FILE.dg.pdf’ from ‘FILE.dg’ the following command can be used:

     $ dot -O -Tpdf FILE.dg

   The ‘-debug’ option causes all kinds of debugging information to be
saved in the code generated.  See the documentation of the ‘debug’
declaration for details.

   The ‘-debug-location’ option causes source code location debugging
information to be saved in the code generated.  See the documentation of
the ‘debug-location’ declaration for details.

   The ‘-debug-source’ option causes source code debugging information
to be saved in the code generated.  See the documentation of the
‘debug-source’ declaration for details.

   The ‘-debug-environments’ option causes environment debugging
information to be saved in the code generated.  See the documentation of
the ‘debug-environments’ declaration for details.

   The ‘-track-scheme’ option is only meaningful when the ‘C’ target is
selected.  The ‘-track-scheme’ option causes the generation of ‘#line’
directives that refer back to the Scheme source code.  This allows the
use of a C debugger or profiler to debug Scheme code.

   The ‘-o’ option sets the filename of the output file, or the
directory in which the output file(s) generated by the compiler are
written.

   If the ‘-link’ or ‘-exe’ options appear on the command line, the
Gambit linker is invoked to generate the link file from the set of files
specified on the command line or produced by the Gambit compiler.  By
default the link file is named after the last file on the compiler’s
command line.  If the last file stripped of it’s extension is ‘LAST’
then the link file is ‘LAST_.c’ for the ‘C’ target and ‘LAST_.js’ for
the ‘js’ target.  When the ‘-c’ option is specified, the Scheme source
files are compiled to target files without invoking the linker, which is
useful for separate compilation of modules.  When the ‘-exe’ option is
specified, the generated target files and link file are combined to
produce an executable program whose name defaults to ‘LAST’ on Unix, and
‘LAST.exe’ or ‘LAST.bat’ on Windows depending on whether a machine code
executable or script is produced.  When the ‘C’ target is selected and
the ‘-obj’ option is specified, the generated C files are compiled using
the C compiler to produce object files (‘.o’ or ‘.obj’ extensions).  If
neither the ‘-link’, ‘-c’, ‘-exe’, or ‘-obj’ options appear on the
command line, the Scheme source files are compiled to dynamically
loadable object files (‘.oN’ extension).  The ‘-keep-temp’ option will
prevent the deletion of any intermediate files that are generated.  Note
that in this case the intermediate file will be generated in the same
directory as the Scheme source file even if the ‘-o’ option is used.

   The ‘-flat’ option is only meaningful when a link file is being
generated (i.e.  the ‘-link’ or ‘-exe’ options also appear on the
command line).  The ‘-flat’ option directs the Gambit linker to generate
a flat link file.  By default, the linker generates an incremental link
file (see the next section for a description of the two types of link
files).

   The ‘-l’ option is only meaningful when an incremental link file is
being generated (i.e.  the ‘-link’ or ‘-exe’ options appear on the
command line and the ‘-flat’ option is absent).  The ‘-l’ option
specifies the link file (without the ‘.c’ or ‘.js’ extension) of the
base library to use for the incremental link.  By default the link file
of the Gambit runtime library is used (i.e.  ‘~~lib/_gambit’).

   The ‘-preload’ and ‘-nopreload’ options are only meaningful when a
link file is being generated.  The ‘-preload’ option turns on the
‘preload’ linker bit for the modules that follow on the command line.
The following modules will be loaded unconditionally at program startup
and in command line order (this is the default for compatibility with
how legacy modules have been handled in the past).  The ‘-nopreload’
option turns off the ‘preload’ linker bit.  The following modules will
be loaded only to satisfy the module dependencies of the
‘##demand-module’ form.

   The ‘-’ option starts a REPL interaction.

   The ‘-e’ option evaluates the specified expressions in the
interaction environment.

   The ‘-nb-gvm-regs’ option specifies the number of Gambit virtual
machine registers that are available for the generated code.  The
default number depends on configuration options and the target but it is
typically 5.  All modules and the runtime library must be compiled with
the same setting.  This option exists mainly for experimentation by the
developers.  For example:

     $ gsc -nb-gvm-regs 10 -c bench.scm

   The ‘-nb-arg-regs’ option specifies the number of procedure call
parameters passed in Gambit virtual machine registers.  The default
number depends on configuration options and the target but it is
typically 3.  All modules and the runtime library must be compiled with
the same setting.  This option exists mainly for experimentation by the
developers.  For example:

     $ gsc -nb-arg-regs 2 -c bench.scm

   The ‘-compactness’ option selects the level of compactness of the
generated code.  The default level depends on configuration options and
the target but it is typically 5.  Levels from 0 to 5 cause the
generation of increasingly compact code with little or no impact on
execution speed.  Lower values tend to make the generated code more
humanly readable.  Above a level of 5 the compiler will trade execution
speed for saving code space.  The detailed meaning of this option
depends on the target, some targets may ignore it and some targets may
require all modules and the runtime library to be compiled with the same
compactness level.  For example:

     $ gsc -target js -compactness 0 -c bench.scm

3.4 Link files
==============

Gambit can be used to create programs and libraries of Scheme modules.
This section explains the steps required to do so and the role played by
the link files.

   In general, a program is composed of a set of Scheme modules and
modules in the target language.  Some of the modules are part of the
Gambit runtime library and the other modules are supplied by the user.
When the program is started it must setup various global tables
(including the symbol table and the global variable table) and then
sequentially execute the Scheme modules (more or less as though they
were being loaded one after another).  The information required for this
is contained in one or more “link files” generated by the Gambit linker
from the target files produced by the Gambit compiler.

   The order of execution of the Scheme modules corresponds to the order
of the modules on the command line which produced the link file.  The
order is usually important because most modules define variables and
procedures which are used by other modules (for this reason the
program’s main computation is normally started by the last module).

   When a single link file is used to contain the linking information of
all the Scheme modules it is called a “flat link file”.  Thus a program
built with a flat link file contains in its link file both information
on the user modules and on the runtime library.  This is fine if the
program is to be statically linked but is wasteful in a shared-library
context because the linking information of the runtime library can’t be
shared and will be duplicated in all programs (this linking information
typically takes hundreds of kilobytes).

   Flat link files are mainly useful to bundle multiple Scheme modules
to make a runtime library (such as the Gambit runtime library) or to
make a single file that can be loaded with the ‘load’ procedure.

   An “incremental link file” contains only the linking information that
is not already contained in a second link file (the “base” link file).
Assuming that a flat link file was produced when the runtime library was
linked, a program can be built by linking the user modules with the
runtime library’s link file, producing an incremental link file.  This
allows the creation of a shared-library which contains the modules of
the runtime library and its flat link file.  The program is dynamically
linked with this shared-library and only contains the user modules and
the incremental link file.  For small programs this approach greatly
reduces the size of the program because the incremental link file is
small.  A “hello world” program built this way can be as small as 5
Kbytes.  Note that it is perfectly fine to use an incremental link file
for statically linked programs (there is very little loss compared to a
single flat link file).

   Incremental link files may be built from other incremental link
files.  This allows the creation of shared-libraries which extend the
functionality of the Gambit runtime library.

3.4.1 Building an executable program
------------------------------------

The simplest way to create an executable program is to invoke ‘gsc’ with
the ‘-exe’ option.  The compiler will transparently perform all the
steps necessary, including compiling Scheme source files to target
files, generating the link file, and (when the ‘C’ target is selected)
compiling the C files generated to object files and creating the final
executable file using the C linker.  The following example shows how to
use the ‘C’ target to build the executable program ‘hello.exe’ which
contains the two Scheme modules ‘h.scm’ and ‘w.six’.

     $ cat h.scm
     (display "hello") (newline)
     $ cat w.six
     display("world"); newline();
     $ gsc -o hello.exe -exe h.scm w.six
     h.scm:
     /Users/feeley/gambit/doc/h.c:
     w.six:
     /Users/feeley/gambit/doc/w.c:
     /Users/feeley/gambit/doc/w_.c:
     $ ./hello.exe
     hello
     world

   The detailed steps which are performed can be viewed by setting the
‘GAMBUILD_VERBOSE’ environment variable to a nonnull value.
Alternatively, ‘gsc’’s ‘-verbose’ option can be used (it implicitly sets
the ‘GAMBUILD_VERBOSE’ environment variable).  For example:

     $ export GAMBUILD_VERBOSE=yes
     $ gsc -o hello.exe -exe h.scm w.six
     h.scm:
     /Users/feeley/gambit/doc/h.c:
     gcc  -O1    -Wno-unused -Wno-write-strings -Wdisabled-optimization
     fwrapv -fno-strict-aliasing -fno-trapping-math -fno-math-errno
     -fschedule-insns2 -foptimize-sibling-calls -fomit-frame-pointer -fPIC
     -fno-common -mpc64   -D___SINGLE_HOST  -I"/usr/local/Gambit/include"
     -c -o 'h.o'  'h.c'
     w.six:
     /Users/feeley/gambit/doc/w.c:
     gcc  -O1    -Wno-unused -Wno-write-strings -Wdisabled-optimization
     -fwrapv -fno-strict-aliasing -fno-trapping-math -fno-math-errno
     -fschedule-insns2 -foptimize-sibling-calls -fomit-frame-pointer -fPIC
     -fno-common -mpc64   -D___SINGLE_HOST  -I"/usr/local/Gambit/include"
     -c -o 'w.o'  'w.c'
     /Users/feeley/gambit/doc/w_.c:
     gcc  -O1    -Wno-unused -Wno-write-strings -Wdisabled-optimization
     -fwrapv -fno-strict-aliasing -fno-trapping-math -fno-math-errno
     -fschedule-insns2 -foptimize-sibling-calls -fomit-frame-pointer -fPIC
     -fno-common -mpc64   -D___SINGLE_HOST  -I"/usr/local/Gambit/include"
     -c -o 'w_.o'  'w_.c'
     gcc     -Wno-unused -Wno-write-strings -Wdisabled-optimization
     -fwrapv -fno-strict-aliasing -fno-trapping-math -fno-math-errno
     -fschedule-insns2 -foptimize-sibling-calls -fomit-frame-pointer -fPIC
     -fno-common -mpc64    -D___SINGLE_HOST  -I"/usr/local/Gambit/include"
     -o 'hello.exe'   'w_.o' 'h.o' 'w.o' "/usr/local/Gambit/lib/libgambit.a"

   Here is the same example using the ‘js’ target showing the creation
of a shell script invoking nodejs:

     $ export GAMBUILD_VERBOSE=yes
     $ gsc -target js -o hello.exe -exe h.scm w.six
     h.scm:
     /Users/feeley/gambit/doc/h.js:
     cat h.js > "h.o"
     w.six:
     /Users/feeley/gambit/doc/w.js:
     cat w.js > "w.o"
     /Users/feeley/gambit/doc/w_.js:
     cat w_.js > "w_.o"
     echo "#! /usr/bin/env node" > "hello.exe"
     cat w_.o h.o w.o "/usr/local/Gambit/lib/_gambit.js" >> "hello.exe"
     chmod +x "hello.exe"

   Using a single invocation of ‘gsc’ with the ‘-exe’ option is
sometimes inappropriate when the build process is more complex, for
example when the program is composed of several separately compiled
modules.  In such a case it is useful to decompose the build process
into smaller compilation steps.  The ‘hello.exe’ executable program
could have been built with the ‘C’ target by separating the generation
of C files from the C compilation and linking:

     $ gsc -c h.scm
     $ gsc -c w.six
     $ gsc -o hello.exe -exe h.c w.c

   When even finer control is desired the ‘C’ target’s build process can
be decomposed into smaller steps that invoke the C compiler and linker
explicitly.  This is described in the rest of this section.

   The ‘gsc’ compiler can be invoked to compile each Scheme module into
a C file and to create an incremental link file.  The C files and the
link file must then be compiled with a C compiler and linked (at the
object file level) with the Gambit runtime library and possibly other
libraries (such as the math library and the dynamic loading library).

   Here is for example how a program with three modules (one in C and
two in Scheme) can be built.  The content of the three source files
(‘m1.c’, ‘m2.scm’ and ‘m3.scm’) is:

     /* File: "m1.c" */
     int power_of_2 (int x) { return 1<<x; }

     ; File: "m2.scm"
     (c-declare "extern int power_of_2 ();")
     (define pow2 (c-lambda (int) int "power_of_2"))
     (define (twice x) (cons x x))

     ; File: "m3.scm"
     (write (map twice (map pow2 '(1 2 3 4)))) (newline)

   The compilation of the two Scheme source files can be done with three
invocations of ‘gsc’:

     $ gsc -c m2.scm        # create m2.c (note: .scm is optional)
     $ gsc -c m3.scm        # create m3.c (note: .scm is optional)
     $ gsc -link m2.c m3.c  # create the incremental link file m3_.c

   Alternatively, the three invocations of ‘gsc’ can be replaced by a
single invocation:

     $ gsc -link m2 m3
     m2:
     m3:

   At this point there will be 4 C files: ‘m1.c’, ‘m2.c’, ‘m3.c’, and
‘m3_.c’.  To produce an executable program these files must be compiled
with a C compiler and linked with the Gambit runtime library.  The C
compiler options needed will depend on the C compiler and the operating
system (in particular it may be necessary to add the options
‘-I/usr/local/Gambit/include -L/usr/local/Gambit/lib’ to access the
‘gambit.h’ header file and the Gambit runtime library).

   Here is an example under macOS:

     $ uname -srmp
     Darwin 20.6.0 x86_64 i386
     $ gsc -obj m1.c m2.c m3.c m3_.c
     m1.c:
     m2.c:
     m3.c:
     m3_.c:
     $ gcc m1.o m2.o m3.o m3_.o -lgambit
     $ ./a.out
     ((2 . 2) (4 . 4) (8 . 8) (16 . 16))

   Here is an example under Linux:

     $ uname -srmp
     Linux 5.10.0-9-amd64 x86_64 unknown
     $ gsc -obj m1.c m2.c m3.c m3_.c
     m1.c:
     m2.c:
     m3.c:
     m3_.c:
     $ gcc m1.o m2.o m3.o m3_.o -lgambit -lm -ldl -lutil
     $ ./a.out
     ((2 . 2) (4 . 4) (8 . 8) (16 . 16))

3.4.2 Building a loadable library
---------------------------------

To bundle multiple modules into a single object file that can be
dynamically loaded with the ‘load’ procedure, a flat link file is
needed.  The compiler’s ‘-o’ option must be used to name the C file
generated as follows.  If the dynamically loadable object file is to be
named ‘MYFILE.oN’ then the ‘-o’ option must set the name of the link
file generated to ‘MYFILE.oN.c’ (note that the ‘.c’ extension could also
be ‘.cc’, ‘.cpp’ or whatever extension is appropriate for C/C++ source
files).  The three modules of the previous example can be bundled by
generating a link file in this way:

     $ gsc -link -flat -o foo.o1.c m2 m3
     m2:
     m3:
     *** WARNING -- "cons" is not defined,
     ***            referenced in: ("m2.c")
     *** WARNING -- "map" is not defined,
     ***            referenced in: ("m3.c")
     *** WARNING -- "newline" is not defined,
     ***            referenced in: ("m3.c")
     *** WARNING -- "write" is not defined,
     ***            referenced in: ("m3.c")

   The warnings indicate that there are no definitions (‘define’s or
‘set!’s) of the variables ‘cons’, ‘map’, ‘newline’ and ‘write’ in the
set of modules being linked.  Before ‘foo.o1’ is loaded, these variables
will have to be bound; either implicitly (by the runtime library) or
explicitly.

   When compiling the C files and link file generated, the flag
‘-D___DYNAMIC’ must be passed to the C compiler and the C compiler and
linker must be told to generate a dynamically loadable shared library.

   Here is an example under macOS:

     $ uname -srmp
     Darwin 20.6.0 x86_64 i386
     $ gsc -link -flat -o foo.o1.c m2 m3 > /dev/null
     m2:
     m3:
     $ gsc -cc-options "-D___DYNAMIC" -obj m1.c m2.c m3.c foo.o1.c
     m1.c:
     m2.c:
     m3.c:
     foo.o1.c:
     $ gcc -bundle m1.o m2.o m3.o foo.o1.o -o foo.o1
     $ gsi foo.o1
     ((2 . 2) (4 . 4) (8 . 8) (16 . 16))

   Here is an example under Linux:

     $ uname -srmp
     Linux 5.10.0-9-amd64 x86_64 unknown
     $ gsc -link -flat -o foo.o1.c m2 m3 > /dev/null
     m2:
     m3:
     $ gsc -cc-options "-D___DYNAMIC" -obj m1.c m2.c m3.c foo.o1.c
     m1.c:
     m2.c:
     m3.c:
     foo.o1.c:
     $ gcc -shared m1.o m2.o m3.o foo.o1.o -o foo.o1
     $ gsi foo.o1
     ((2 . 2) (4 . 4) (8 . 8) (16 . 16))

   Here is a more complex example, under Solaris, which shows how to
build a loadable library ‘mymod.o1’ composed of the files ‘m4.scm’,
‘m5.scm’ and ‘x.c’ that links to system shared libraries (for
X-windows):

     $ uname -srmp
     SunOS ungava 5.6 Generic_105181-05 sun4m sparc SUNW,SPARCstation-20
     $ gsc -link -flat -o mymod.o1.c m4 m5
     m4:
     m5:
     *** WARNING -- "*" is not defined,
     ***            referenced in: ("m4.c")
     *** WARNING -- "+" is not defined,
     ***            referenced in: ("m5.c")
     *** WARNING -- "display" is not defined,
     ***            referenced in: ("m5.c" "m4.c")
     *** WARNING -- "newline" is not defined,
     ***            referenced in: ("m5.c" "m4.c")
     *** WARNING -- "write" is not defined,
     ***            referenced in: ("m5.c")
     $ gsc -cc-options "-D___DYNAMIC" -obj m4.c m5.c x.c mymod.o1.c
     m4.c:
     m5.c:
     x.c:
     mymod.o1.c:
     $ /usr/ccs/bin/ld -G -o mymod.o1 mymod.o1.o m4.o m5.o x.o -lX11 -lsocket
     $ gsi mymod.o1
     hello from m4
     hello from m5
     (f1 10) = 22
     $ cat m4.scm
     (define (f1 x) (* 2 (f2 x)))
     (display "hello from m4")
     (newline)

     (c-declare #<<c-declare-end
     #include "x.h"
     c-declare-end
     )
     (define x-initialize (c-lambda (char-string) bool "x_initialize"))
     (define x-display-name (c-lambda () char-string "x_display_name"))
     (define x-bell (c-lambda (int) void "x_bell"))
     $ cat m5.scm
     (define (f2 x) (+ x 1))
     (display "hello from m5")
     (newline)

     (display "(f1 10) = ")
     (write (f1 10))
     (newline)

     (x-initialize (x-display-name))
     (x-bell 50) ; sound the bell at 50%
     $ cat x.c
     #include <X11/Xlib.h>

     static Display *display;

     int x_initialize (char *display_name)
     {
       display = XOpenDisplay (display_name);
       return display != NULL;
     }

     char *x_display_name (void)
     {
       return XDisplayName (NULL);
     }

     void x_bell (int volume)
     {
       XBell (display, volume);
       XFlush (display);
     }
     $ cat x.h
     int x_initialize (char *display_name);
     char *x_display_name (void);
     void x_bell (int);

3.4.3 Building a shared-library
-------------------------------

A shared-library can be built using an incremental link file or a flat
link file.  An incremental link file is normally used when the Gambit
runtime library (or some other library) is to be extended with new
procedures.  A flat link file is mainly useful when building a “primal”
runtime library, which is a library (such as the Gambit runtime library)
that does not extend another library.  When compiling the C files and
link file generated, the flags ‘-D___LIBRARY’ and ‘-D___SHARED’ must be
passed to the C compiler.  The flag ‘-D___PRIMAL’ must also be passed to
the C compiler when a primal library is being built.

   A shared-library ‘mylib.so’ containing the two first modules of the
previous example can be built this way:

     $ uname -srmp
     Linux 5.10.0-9-amd64 x86_64 unknown
     $ gsc -link -o mylib.c m2
     $ gsc -obj -cc-options "-D___SHARED" m1.c m2.c mylib.c
     m1.c:
     m2.c:
     mylib.c:
     $ gcc -shared  m1.o m2.o mylib.o -o mylib.so

   Note that this shared-library is built using an incremental link file
(it extends the Gambit runtime library with the procedures ‘pow2’ and
‘twice’).  This shared-library can in turn be used to build an
executable program from the third module of the previous example:

     $ gsc -link -l mylib m3
     $ gsc -obj m3.c m3_.c
     m3.c:
     m3_.c:
     $ gcc m3.o m3_.o mylib.so -lgambit
     $ LD_LIBRARY_PATH=.:/usr/local/lib ./a.out
     ((2 . 2) (4 . 4) (8 . 8) (16 . 16))

3.4.4 Other compilation options
-------------------------------

The performance of the code can be increased by passing the
‘-D___SINGLE_HOST’ flag to the C compiler.  This will merge all the
procedures of a module into a single C procedure, which reduces the cost
of intra-module procedure calls.  In addition the ‘-O2’ option can be
passed to the C compiler.  For large modules, it will not be practical
to specify both ‘-O2’ and ‘-D___SINGLE_HOST’ for typical C compilers
because the compile time will be high and the C compiler might even fail
to compile the program for lack of memory.  It has been observed that
lower levels of optimization (e.g.  ‘-O1’) often give faster compilation
and also generate faster code.  It is a good idea to experiment.

   Normally C compilers will not automatically search
‘/usr/local/Gambit/include’ for header files so the flag
‘-I/usr/local/Gambit/include’ should be passed to the C compiler.
Similarly, C compilers/linkers will not automatically search
‘/usr/local/Gambit/lib’ for libraries so the flag
‘-L/usr/local/Gambit/lib’ should be passed to the C compiler/linker.
Alternatives are given in *note Accessing the system files::.

   A variety of flags are needed by some C compilers when compiling a
shared-library or a dynamically loadable library.  Some of these flags
are: ‘-shared’, ‘-call_shared’, ‘-rdynamic’, ‘-fpic’, ‘-fPIC’, ‘-Kpic’,
‘-KPIC’, ‘-pic’, ‘+z’, ‘-G’.  Check your compiler’s documentation to see
which flag you need.

3.5 Procedures specific to compiler
===================================

The Gambit Scheme compiler features the following procedures that are
not available in the Gambit Scheme interpreter.

 -- procedure: compile-file-to-target FILE [options: OPTIONS] [output:
          OUTPUT] [expression: EXPRESSION]

     The FILE parameter must be a string.  If EXPRESSION is not
     specified, FILE must name an existing file containing Scheme source
     code.  The extension can be omitted from FILE when the Scheme file
     has a ‘.scm’, ‘.sld’ or ‘.six’ extension.  By default, this
     procedure compiles the source file into a file containing C code.
     A different target language can be selected in the OPTIONS.  The
     generated file is named after FILE with the extension replaced with
     ‘.c’ or ‘.js’, as appropriate for the target selected.  The name of
     the generated file can also be specified directly with the OUTPUT
     parameter.  If OUTPUT is a string naming a directory then the
     generated file is created in that directory.  Otherwise the name of
     the generated file is OUTPUT.

     Compilation options are specified through the OPTIONS parameter
     which must be an association list.  Any combination of the
     following options can be used: ‘target’, ‘verbose’, ‘report’,
     ‘expansion’, ‘gvm’, ‘debug’, ‘module-ref’, and ‘linker-name’.

     When EXPRESSION is specified, the FILE parameter is not open or
     read.  Instead, EXPRESSION is used as though it was the content of
     the file.  This makes it possible to compile source code without
     having to create a file to contain the code.  Note that FILE is
     used in error messages and to determine the output file name if
     OUTPUT is not specified.

     When the compilation is successful, ‘compile-file-to-target’
     returns the name of the file generated.  When there is a
     compilation error, ‘#f’ is returned.

          $ cat h.scm
          (display "hello") (newline)
          $ gsc
          Gambit v4.9.4

          > (compile-file-to-target "h")
          "/Users/feeley/gambit/doc/h.c"

 -- procedure: compile-file FILE [options: OPTIONS] [output: OUTPUT]
          [base: BASE] [expression: EXPRESSION] [cc-options: CC-OPTIONS]
          [ld-options-prelude: LD-OPTIONS-PRELUDE] [ld-options:
          LD-OPTIONS]

     The FILE, OPTIONS, OUTPUT, and EXPRESSION parameters have the same
     meaning as for the ‘compile-file-to-target’ procedure, except that
     FILE may be a Scheme source file or a file possibly generated by
     the Gambit Scheme compiler (for example with the
     ‘compile-file-to-target’ procedure).  The CC-OPTIONS parameter is a
     string containing the options to pass to the C compiler and the
     LD-OPTIONS-PRELUDE and LD-OPTIONS parameters are strings containing
     the options to pass to the C linker (the options in
     LD-OPTIONS-PRELUDE are passed to the C linker before the input file
     and the options in LD-OPTIONS are passed after).

     The ‘compile-file’ procedure compiles the source file FILE into an
     object file, which is either a file dynamically loadable using the
     ‘load’ procedure, or a C linkable object file destined to be linked
     with the C linker (for example to create a standalone executable
     program).  The presence of the ‘obj’ option in OPTIONS will cause
     the creation of a C linkable object file and therefore the options
     LD-OPTIONS-PRELUDE and LD-OPTIONS are ignored, otherwise a
     dynamically loadable file is created.  In both cases, if FILE is a
     Scheme source file, the compiler first compiles FILE to a C file
     which is created in the same directory as FILE regardless of the
     OUTPUT parameter.  Then the C file is compiled with the C compiler.

     When the compilation is successful, ‘compile-file’ returns the name
     of the object file generated.  When there is a compilation error,
     ‘#f’ is returned.

     The name of the object file can be specified with the OUTPUT
     parameter.  If OUTPUT is a string naming a directory then the
     object file is created in that directory.  Otherwise the name of
     the object file is OUTPUT.

     In the case of a dynamically loadable object file, by default the
     object file is named after FILE with the extension replaced with
     ‘.oN’, where N is a positive integer that acts as a version number.
     The next available version number is generated automatically by
     ‘compile-file’.

     When dynamically loaded object files are loaded using the ‘load’
     procedure, the ‘.oN’ extension can be specified (to select a
     particular version) or omitted (to load the file with a ‘.oN’
     extension with the highest N consecutively from 1).  When the ‘.oN’
     extension is not specified and older versions are no longer needed,
     all versions must be deleted and the compilation must be repeated
     (this is necessary because the file name, including the extension,
     is used to name some of the exported symbols of the object file).

     Note that dynamically loadable object files can only be generated
     on host operating systems that support dynamic loading.

          $ cat h.scm
          (display "hello") (newline)
          $ gsc
          Gambit v4.9.4

          > (compile-file "h")
          "/Users/feeley/gambit/doc/h.o1"
          > (load "h")
          hello
          "/Users/feeley/gambit/doc/h.o1"
          > (compile-file-to-target "h" output: "h.o99.c")
          "/Users/feeley/gambit/doc/h.o99.c"
          > (compile-file "h.o99.c")
          "/Users/feeley/gambit/doc/h.o99"
          > (load "h.o99")
          hello
          "/Users/feeley/gambit/doc/h.o99"
          > (compile-file-to-target "h")
          "/Users/feeley/gambit/doc/h.c"
          > (compile-file "h.c" options: '(obj))
          "/Users/feeley/gambit/doc/h.o"

 -- procedure: link-incremental MODULE-LIST [output: OUTPUT]
          [linker-name: LINKER-NAME] [base: BASE] [warnings?: WARNINGS?]

     The first parameter must be a non empty list of strings naming
     Scheme modules to link (the file extension may be omitted).  An
     incremental link file is generated for the modules specified in
     MODULE-LIST.  By default the link file generated is named
     ‘LAST_.EXT’, where LAST is the name of the last module, without the
     file extension, and EXT is the appropriate extension for the
     target.  The name of the generated link file can be specified with
     the OUTPUT parameter.  If OUTPUT is a string naming a directory
     then the link file is created in that directory.  Otherwise the
     name of the link file is OUTPUT.

     The base link file is specified by the BASE parameter, which must
     be a string.  By default the base link file is the Gambit runtime
     library link file ‘~~lib/_gambit’ (with extension appropriate for
     the target).  However, when BASE is supplied it is the name of the
     base link file (the file extension may be omitted).

     The WARNINGS? parameter controls whether warnings are generated for
     undefined references.

     The following example shows how to build the executable program
     ‘hello’ which contains the two Scheme modules ‘h.scm’ and ‘w.six’.

          $ uname -srmp
          Darwin 8.1.0 Power Macintosh powerpc
          $ cat h.scm
          (display "hello") (newline)
          $ cat w.six
          display("world"); newline();
          $ gsc
          Gambit v4.9.4

          > (compile-file-to-target "h")
          "/Users/feeley/gambit/doc/h.c"
          > (compile-file-to-target "w")
          "/Users/feeley/gambit/doc/w.c"
          > (link-incremental '("h" "w") output: "hello.c")
          "/Users/feeley/gambit/doc/hello_.c"
          > ,q
          $ gsc -obj h.c w.c hello.c
          h.c:
          w.c:
          hello.c:
          $ gcc h.o w.o hello.o -lgambit -o hello
          $ ./hello
          hello
          world

 -- procedure: link-flat MODULE-LIST [output: OUTPUT] [linker-name:
          LINKER-NAME] [warnings?: WARNINGS?]

     The first parameter must be a non empty list of strings naming
     Scheme modules to link (the file extension may be omitted).  The
     first string must be the name of a Scheme module or the name of a
     link file and the remaining strings must name Scheme modules.  A
     flat link file is generated for the modules specified in
     MODULE-LIST.  By default the link file generated is named
     ‘LAST_.EXT’, where LAST is the name of the last module, without the
     file extension, and EXT is the appropriate extension for the
     target.  The name of the generated link file can be specified with
     the OUTPUT parameter.  If OUTPUT is a string naming a directory
     then the link file is created in that directory.  Otherwise the
     name of the link file is OUTPUT.  If a dynamically loadable object
     file is produced from the link file ‘OUTPUT’, then the name of the
     dynamically loadable object file must be ‘OUTPUT’ stripped of its
     file extension.

     The WARNINGS? parameter controls whether warnings are generated for
     undefined references.

     The following example shows how to build the dynamically loadable
     object file ‘lib.o1’ which contains the two Scheme modules ‘m6.scm’
     and ‘m7.scm’.

          $ uname -srmp
          Darwin 8.1.0 Power Macintosh powerpc
          $ cat m6.scm
          (define (f x) (g (* x x)))
          $ cat m7.scm
          (define (g y) (+ n y))
          $ gsc
          Gambit v4.9.4

          > (compile-file-to-target "m6")
          "/Users/feeley/gambit/doc/m6.c"
          > (compile-file-to-target "m7")
          "/Users/feeley/gambit/doc/m7.c"
          > (link-flat '("m6" "m7") output: "lib.o1.c")
          *** WARNING -- "*" is not defined,
          ***            referenced in: ("m6.c")
          *** WARNING -- "+" is not defined,
          ***            referenced in: ("m7.c")
          *** WARNING -- "n" is not defined,
          ***            referenced in: ("m7.c")
          "/Users/feeley/gambit/doc/lib.o1.c"
          > ,q
          $ gcc -bundle -D___DYNAMIC m6.c m7.c lib.o1.c -o lib.o1
          $ gsc
          Gambit v4.9.4

          > (load "lib")
          *** WARNING -- Variable "n" used in module "m7" is undefined
          "/Users/feeley/gambit/doc/lib.o1"
          > (define n 10)
          > (f 5)
          35
          > ,q

     The warnings indicate that there are no definitions (‘define’s or
     ‘set!’s) of the variables ‘*’, ‘+’ and ‘n’ in the modules contained
     in the library.  Before the library is used, these variables will
     have to be bound; either implicitly (by the runtime library) or
     explicitly.

4 Runtime options
*****************

Both ‘gsi’ and ‘gsc’ as well as executable programs compiled and linked
using ‘gsc’ take a ‘-:’ option which supplies parameters to the runtime
system.  This option must appear first on the command line.  The colon
is followed by a comma separated list of options with no intervening
spaces.  The available options are:

‘min-heap=’SIZE or the shorthand ‘m’SIZE
     Set minimum heap size.

‘max-heap=’SIZE or the shorthand ‘h’SIZE
     Set maximum heap size.

‘live-ratio=’RATIO or the shorthand ‘l’RATIO
     Set the ratio of heap that is live after a garbage collection.

‘gambit’ or the (deprecated) shorthand ‘S’
     Select Gambit Scheme mode.  This is the default mode.

‘r5rs’ or the (deprecated) shorthand ‘s’
     Select R5RS Scheme mode.

‘r7rs’
     Select R7RS Scheme mode.

‘debug’[‘=’[OPT...]] or the shorthand ‘d’[OPT...]
     Set debugging options.

‘~~’NAME‘=’DIRECTORY
     Override the NAME installation directory.

‘add-arg=’ARGUMENT or the shorthand ‘+’ARGUMENT
     Add ARGUMENT to the command line before other arguments.

‘io-settings=’[IO...] or the shorthand ‘i’[IO...]
     Set general I/O settings.

‘file-settings=’[IO...] or the shorthand ‘f’[IO...]
     Set general file I/O settings.

‘stdio-settings=’[IO...] or the shorthand ‘-’[IO...]
     Set general stdio settings.

‘0’[IO...]
     Set stdin settings.

‘1’[IO...]
     Set stdout settings.

‘2’[IO...]
     Set stderr settings.

‘terminal-settings=’[IO...] or the shorthand ‘t’[IO...]
     Set terminal I/O settings.

‘search=’[DIR]
     Set or reset module search order.

‘whitelist=’[SOURCE]
     Set or reset the whitelist of trusted sources for automatic
     installation of hosted modules.

‘ask-install=’WHEN
     Set automatic installation confirmation mode.

   The ‘min-heap=’SIZE and ‘max-heap=’SIZE options set limits on the
size of the heap.  The SIZE is an integer that may be followed by ‘G’
(gigabytes), ‘M’ (megabytes), or ‘K’ or nothing (kilobytes).  The heap
will not shrink lower than the minimum heap size which defaults to 0.
The heap will not grow larger than the maximum heap size if it is set
(by default the heap may grow until the virtual memory is exhausted).

   The ‘live-ratio=’RATIO option sets the percentage of the heap that
will be occupied with live objects after the heap is resized at the end
of a garbage collection.  RATIO is an integer between 1 and 100
inclusively indicating the desired percentage.  The garbage collector
resizes the heap to reach this percentage occupation (roughly), within
the limits of the ‘min-heap’ and ‘max-heap’ options.  By default, the
percentage is 50.

   The ‘gambit’, ‘r5rs’ and ‘r7rs’ options configure the runtime system
to conform to Gambit Scheme, R5RS Scheme and R7RS Scheme respectively.
The reader is case-insensitive in ‘r5rs’ mode, and is case-sensitive in
‘r7rs’ and ‘gambit’ modes.  The reader supports keywords only in
‘gambit’ mode, which is the default mode.

   The ‘debug=’OPT,...  option sets various debugging options.  The
equal sign is followed by a sequence of letters indicating suboptions.

‘p’
     Uncaught exceptions will be treated as “errors” in the primordial
     thread only.

‘a’
     Uncaught exceptions will be treated as “errors” in all threads.

‘r’
     When an “error” occurs a new REPL will be started.

‘s’
     When an “error” occurs a new REPL will be started.  Moreover the
     program starts in single-stepping mode.

‘q’
     When an “error” occurs the program will terminate with a nonzero
     exit status.

‘R’
     When a user interrupt occurs a new REPL will be started.  User
     interrupts are typically obtained by typing <^C>.  Note that with
     some system configurations <^C> abruptly terminates the process.
     For example, under Microsoft Windows, <^C> works fine with the
     standard console but with the MSYS terminal window it terminates
     the process.

‘D’
     When a user interrupt occurs it will be deferred until the
     parameter object ‘current-user-interrupt-handler’ is set or bound.

‘Q’
     When a user interrupt occurs the program will terminate with a
     nonzero exit status.

LEVEL
     The verbosity level is set to LEVEL, a digit from 0 to 9.  At level
     0 the runtime system will not display error messages and warnings.
     At level 1 and above error messages and warnings are displayed.  At
     level 2 and above a backtrace is displayed.  At level 3 and above
     variable bindings are displayed in the backtrace.  At level 5 and
     above garbage collection reports are displayed during program
     execution.

‘c’
     The REPL interaction channel will be the console.

‘-’
     The REPL interaction channel will be standard input and standard
     output.

‘+’
     The REPL interaction channel will be standard input and standard
     output and standard error.

‘@’[HOST][‘:’PORT]
     When a REPL is started by a thread a connection will be established
     with the address HOST:PORT and that will be the REPL’s interaction
     channel.  The default HOST is 127.0.0.1 and the default PORT is
     44556.

‘$’[INTF][‘:’PORT]
     The runtime system will open a socket to listen on port number PORT
     for incoming connections on the network interface with address
     INTF.  The default INTF is 127.0.0.1 and the default PORT is 44555.

   The default debugging options are equivalent to ‘debug=pqQ1-’ (i.e.
an uncaught exception in the primordial thread terminates the program
after displaying an error message).  When the option ‘debug’ is used
without suboptions it is equivalent to ‘debug=prR1-’ (i.e.  a new REPL
is started only when an uncaught exception occurs in the primordial
thread).  When ‘gsi’ and ‘gsc’ are running the main REPL, the debugging
options are changed to cause errors in the primordial thread and user
interrupts to start a nested REPL.

   The ‘~~’NAME‘=’DIRECTORY option overrides the setting of the NAME
installation directory.  If NAME is empty, it will override the central
installation directory.

   The ‘add-arg=’ARGUMENT option adds the text that follows to the
command line before other arguments.

   The option ‘io-settings=’[IO...]  sets the default I/O settings of
all types of ports.  The option ‘file-settings=’[IO...]  sets the
default I/O settings for ports associated to files.  The option
‘stdio-settings=’[IO...]  sets the default I/O settings for ports
associated to stdio (but finer control is possible with ‘0’[IO...],
‘1’[IO...], and ‘2’[IO...]  that set the I/O settings of stdin, stdout,
and stderr respectively).  The option ‘terminal-settings=’[IO...]
overrides the default I/O settings for ports associated to terminals.
The default character encoding, end-of-line encoding and buffering can
be set.  Moreover, for terminals the line-editing feature can be enabled
or disabled.  Each IO is a one or two letter code as follows:

‘A’
     ASCII character encoding.
‘1’
     ISO-8859-1 character encoding.
‘2’
     UCS-2 character encoding.
‘4’
     UCS-4 character encoding.
‘6’
     UTF-16 character encoding.
‘8’
     UTF-8 character encoding.
‘U’
     UTF character encoding with fallback to UTF-8 on input if no BOM is
     present.
‘UA’
     UTF character encoding with fallback to ASCII on input if no BOM is
     present.
‘U1’
     UTF character encoding with fallback to ISO-8859-1 on input if no
     BOM is present.
‘U6’
     UTF character encoding with fallback to UTF-16 on input if no BOM
     is present.
‘U8’
     UTF character encoding with fallback to UTF-8 on input if no BOM is
     present.
‘L’
     If the ‘LC_ALL’ or ‘LC_CTYPE’ or ‘LANG’ environment variables end
     with ‘.UTF-8’ or ‘.ISO-8859-1’ or ‘.LATIN-1’ (or a variation) set
     the character encoding accordingly.
‘c’
     End-of-line is encoded as CR (carriage-return).
‘l’
     End-of-line is encoded as LF (linefeed)
‘cl’
     End-of-line is encoded as CR-LF.
‘u’
     Unbuffered I/O.
‘n’
     Line buffered I/O (‘n’ for “at newline”).
‘f’
     Fully buffered I/O.
‘r’
     Illegal character encoding is treated as an error (exception
     raised).
‘R’
     Silently replace illegal character encodings with Unicode character
     #xfffd (replacement character).
‘e’
     Enable line-editing (applies to terminals only).
‘E’
     Disable line-editing (applies to terminals only).

   The ‘search=’[DIR] option adds DIR to the head of the list of module
search order directories, unless DIR is empty, in which case it is set
to the empty list.  The initial setting of the list of module search
order directories is ‘~~lib’ followed by ‘~~userlib’.

   When a hosted module can’t be found in the directories on the list of
module search order directories it will be automatically installed if it
is from a source on the whitelist of trusted sources, which initially
contains only ‘github.com/gambit’.  The ‘whitelist=’[SOURCE] option adds
SOURCE to the whitelist, unless SOURCE is empty in which case the
whitelist will be set to the empty list (no source is trusted).

   The ‘ask-install=’WHEN option sets the automatic installation mode
confirmation mode to WHEN, which is one of ‘always’, ‘repl’, and
‘never’.  When a hosted module can’t be found in the directories on the
list of module search order directories and it is from a source not on
the whitelist the runtime system will ask for installation confirmation
when WHEN is ‘always’, or when a REPL has already been started for the
current thread and WHEN is ‘repl’.  In the ‘never’ mode the runtime
system will not install the module automatically.  The default mode is
‘repl’.

   When a program’s execution starts, the runtime system obtains the
runtime options by processing in turn various sources of runtime
options: the defaults, the environment variable ‘GAMBOPT’, the script
line of the source code, and, unless the program is an interpreted
script, the first command line argument of the program.  Any runtime
option can be overriden by a subsequent source of runtime options.  It
is sometimes useful to prevent overriding the runtime options of the
script line.  This can be achieved by starting the script line runtime
options with ‘-:,’.  In this case the environment variable ‘GAMBOPT’ is
ignored, and the first command line argument of the program is not used
for runtime options (it is treated like a normal command line argument
even if it starts with ‘-:’).

   For example:

     $ export GAMBOPT=debug=0,~~=~/my-gambit2
     $ gsi -e '(pretty-print (path-expand "~~")) (/ 1 0)'
     "/Users/feeley/my-gambit2/"
     $ echo $?
     70
     $ gsi -:debug=1 -e '(pretty-print (path-expand "~~")) (/ 1 0)'
     "/Users/feeley/my-gambit2/"
     *** ERROR IN (string)@1.35 -- Divide by zero
     (/ 1 0)

5 Debugging
***********

5.1 Debugging model
===================

The evaluation of an expression may stop before it is completed for the
following reasons:

  a. An evaluation error has occured, such as attempting to divide by
     zero.

  b. The user has interrupted the evaluation (usually by typing <^C>).

  c. A breakpoint has been reached or ‘(step)’ was evaluated.

  d. Single-stepping mode is enabled.

   When an evaluation stops, a message is displayed indicating the
reason and location where the evaluation was stopped.  The location
information includes, if known, the name of the procedure where the
evaluation was stopped and the source code location in the format
‘STREAM@LINE.COLUMN’, where STREAM is either a string naming a file or a
symbol within parentheses, such as ‘(console)’.

   A “nested REPL” is then initiated in the context of the point of
execution where the evaluation was stopped.  The nested REPL’s
continuation and evaluation environment are the same as the point where
the evaluation was stopped.  For example when evaluating the expression
‘(let ((y (- 1 1))) (* (/ x y) 2))’, a “divide by zero” error is
reported and the nested REPL’s continuation is the one that takes the
result and multiplies it by two.  The REPL’s lexical environment
includes the lexical variable ‘y’.  This allows the inspection of the
evaluation context (i.e.  the lexical and dynamic environments and
continuation), which is particularly useful to determine the exact
location and cause of an error.

   The prompt of nested REPLs includes the nesting level; ‘1>’ is the
prompt at the first nesting level, ‘2>’ at the second nesting level, and
so on.  An end of file (usually <^D>) will cause the current REPL to be
terminated and the enclosing REPL (one nesting level less) to be
resumed.

   At any time the user can examine the frames in the REPL’s
continuation, which is useful to determine which chain of procedure
calls lead to an error.  A backtrace that lists the chain of active
continuation frames in the REPL’s continuation can be obtained with the
‘,b’ command.  The frames are numbered from 0, that is frame 0 is the
most recent frame of the continuation where execution stopped, frame 1
is the parent frame of frame 0, and so on.  It is also possible to move
the REPL to a specific parent continuation (i.e.  a specific frame of
the continuation where execution stopped) with the ‘,N’, ‘,N+’, ‘,N-’,
‘,+’, ‘,-’, ‘,++’, and ‘,--’ commands.  When the frame number of the
frame being examined is not zero, it is shown in the prompt after the
nesting level, for example ‘1\5>’ is the prompt when the REPL nesting
level is 1 and the frame number is 5.

   Expressions entered at a nested REPL are evaluated in the environment
(both lexical and dynamic) of the continuation frame currently being
examined if that frame was created by interpreted Scheme code.  If the
frame was created by compiled Scheme code then expressions get evaluated
in the global interaction environment.  This feature may be used in
interpreted code to fetch the value of a variable in the current frame
or to change its value with ‘set!’.  Note that some special forms
(‘define’ in particular) can only be evaluated in the global interaction
environment.

5.2 Debugging commands
======================

In addition to expressions, the REPL accepts the following special
“comma” commands:

‘,? and ,help’
     Give a summary of the REPL commands.

‘,(h SUBJECT)’
     This command will show the section of the Gambit manual with the
     definition of the procedure or special form SUBJECT, which must be
     a symbol.  For example ‘,(h time)’ will show the section
     documenting the ‘time’ special form.  Please see the ‘help’
     procedure for additional information.

‘,h’
     This command will show the section of the Gambit manual with the
     definition of the procedure which raised the exception for which
     this REPL was started.

‘,q’
     Terminate the process with exit status 0.  This is equivalent to
     calling ‘(exit 0)’.

‘,qt’
     Terminate the current thread (note that terminating the primordial
     thread terminates the process).

‘,t’
     Return to the outermost REPL, also known as the “top-level REPL”.

‘,d’
     Leave the current REPL and resume the enclosing REPL. This command
     does nothing in the top-level REPL.

‘,(c EXPR)’
     Leave the current REPL and continue the computation that initiated
     the REPL with a specific value.  This command can only be used to
     continue a computation that signaled an error.  The expression EXPR
     is evaluated in the current context and the resulting value is
     returned as the value of the expression which signaled the error.
     For example, if the evaluation of the expression ‘(* (/ x y) 2)’
     signaled an error because ‘y’ is zero, then in the nested REPL a
     ‘,(c (+ 4 y))’ will resume the computation of ‘(* (/ x y) 2)’ as
     though the value of ‘(/ x y)’ was 4.  This command must be used
     carefully because the context where the error occured may rely on
     the result being of a particular type.  For instance a ‘,(c #f)’ in
     the previous example will cause ‘*’ to signal a type error (this
     problem is the most troublesome when debugging Scheme code that was
     compiled with type checking turned off so be careful).

‘,c’
     Leave the current REPL and continue the computation that initiated
     the REPL. This command can only be used to continue a computation
     that was stopped due to a user interrupt, breakpoint or a
     single-step.

‘,s’
     Leave the current REPL and continue the computation that initiated
     the REPL in single-stepping mode.  The computation will perform an
     evaluation step (as defined by ‘step-level-set!’) and then stop,
     causing a nested REPL to be entered.  Just before the evaluation
     step is performed, a line is displayed (in the same format as
     ‘trace’) which indicates the expression that is being evaluated.
     If the evaluation step produces a result, the result is also
     displayed on another line.  A nested REPL is then entered after
     displaying a message which describes the next step of the
     computation.  This command can only be used to continue a
     computation that was stopped due to a user interrupt, breakpoint or
     a single-step.

‘,l’
     This command is similar to ‘,s’ except that it “leaps” over
     procedure calls, that is procedure calls are treated like a single
     step.  Single-stepping mode will resume when the procedure call
     returns, or if and when the execution of the called procedure
     encounters a breakpoint.

‘,N’
     Move to frame number N of the continuation.  After changing the
     current frame, a one-line summary of the frame is displayed as if
     the ‘,y’ command was entered.

‘,N+’
     Move forward by N frames in the chain of continuation frames (i.e.
     towards older continuation frames).  After changing the current
     frame, a one-line summary of the frame is displayed as if the ‘,y’
     command was entered.

‘,N-’
     Move backward by N frames in the chain of continuation frames (i.e.
     towards more recent continuation frames).  After changing the
     current frame, a one-line summary of the frame is displayed as if
     the ‘,y’ command was entered.

‘,+’
     Equivalent to ‘,1+’.

‘,-’
     Equivalent to ‘,1-’.

‘,++’
     Equivalent to ‘,N+’ where N is the number of continuation frames
     displayed at the head of a backtrace.

‘,--’
     Equivalent to ‘,N-’ where N is the number of continuation frames
     displayed at the head of a backtrace.

‘,y’
     Display a one-line summary of the current frame.  The information
     is displayed in four fields.  The first field is the frame number.
     The second field is the procedure that created the frame or
     ‘(interaction)’ if the frame was created by an expression entered
     at the REPL. The remaining fields describe the subproblem
     associated with the frame, that is the expression whose value is
     being computed.  The third field is the location of the
     subproblem’s source code and the fourth field is a reproduction of
     the source code, possibly truncated to fit on the line.  The last
     two fields may be missing if that information is not available.  In
     particular, the third field is missing when the frame was created
     by a user call to the ‘eval’ procedure or by a compiled procedure
     not compiled with the declaration ‘debug-location’, and the last
     field is missing when the frame was created by a compiled procedure
     not compiled with the declaration ‘debug-source’.

‘,b’
     Display a backtrace summarizing each frame in the chain of
     continuation frames starting with the current frame.  For each
     frame, the same information as for the ‘,y’ command is displayed
     (except that location information is displayed in the format
     ‘STREAM@LINE:COLUMN’).  If there are more than 15 frames in the
     chain of continuation frames, some of the middle frames will be
     omitted.

‘,be’
     Like the ‘,b’ command but also display the environment.

‘,bed’
     Like the ‘,be’ command but also display the dynamic environment.

‘,(b EXPR)’
     Display the backtrace of EXPR’s value, X, which is obtained by
     evaluating EXPR in the current frame.  X must be a continuation or
     a thread.  When X is a continuation, the frames in that
     continuation are displayed.  When X is a thread, the backtrace of
     the current continuation of that thread is displayed.

‘,(be EXPR)’
     Like the ‘,(b EXPR)’ command but also display the environment.

‘,(bed EXPR)’
     Like the ‘,(be EXPR)’ command but also display the dynamic
     environment.

‘,i’
     Pretty print the procedure that created the current frame or
     ‘(interaction)’ if the frame was created by an expression entered
     at the REPL. Compiled procedures will only be pretty printed when
     they are compiled with the declaration ‘debug-source’.

‘,e’
     Display the environment which is accessible from the current frame.
     The lexical environment is displayed, followed by the dynamic
     environment if the parameter object
     ‘repl-display-dynamic-environment?’ is not false.  Global lexical
     variables are not displayed.  Moreover the frame must have been
     created by interpreted code or code compiled with the declaration
     ‘debug-environments’.  Due to space safety considerations and
     compiler optimizations, some of the lexical variable bindings may
     be missing.  Lexical variable bindings are displayed using the
     format ‘VARIABLE = EXPRESSION’ (when VARIABLE is mutable) or
     ‘VARIABLE == EXPRESSION’ (when VARIABLE is immutable, which may
     happen in compiled code due to compiler optimization) and
     dynamically-bound parameter bindings are displayed using the format
     ‘(PARAMETER) = EXPRESSION’.  Note that EXPRESSION can be a
     self-evaluating expression (number, string, boolean, character,
     ...), a quoted expression, a lambda expression or a global variable
     (the last two cases, which are only used when the value of the
     variable or parameter is a procedure, simplifies the debugging of
     higher-order procedures).  A PARAMETER can be a quoted expression
     or a global variable.  Lexical bindings are displayed in inverse
     binding order (most deeply nested first) and shadowed variables are
     included in the list.

‘,ed’
     Like the ‘,e’ command but the dynamic environment is always
     displayed.

‘,(e EXPR)’
     Display the environment of EXPR’s value, X, which is obtained by
     evaluating EXPR in the current frame.  X must be a continuation, a
     thread, a procedure, or a nonnegative integer.  When X is a
     continuation, the environment at that point in the code is
     displayed.  When X is a thread, the environment of the current
     continuation of that thread is displayed.  When X is a procedure,
     the lexical environment where X was created is combined with the
     current continuation and this combined environment is displayed.
     When X is an integer, the environment at frame number X of the
     continuation is displayed.

‘,(ed EXPR)’
     Like the ‘,(e EXPR)’ command but the dynamic environment is always
     displayed.

‘,st’
     Display the state of the threads in the current thread’s thread
     group.  A thread can be: uninitialized, initialized, active, and
     terminated (normally or abnormally).  Active threads can be
     running, sleeping and waiting on a synchronization object (mutex,
     condition variable or port) possibly with a timeout.

‘,(st EXPR)’
     Display the state of a specific thread or thread group.  The value
     of EXPR must be a thread or thread group.

‘,(v EXPR)’
     Start a new REPL visiting EXPR’s value, X, which is obtained by
     evaluating EXPR in the current frame.  X must be a continuation, a
     thread, a procedure, or a nonnegative integer.  When X is a
     continuation, the new REPL’s continuation is X and evaluations are
     done in the environment at that point in the code.  When X is a
     thread, the thread is interrupted and the new REPL’s continuation
     is the point where the thread was interrupted.  When X is a
     procedure, the lexical environment where X was created is combined
     with the current continuation and evaluations are done in this
     combined environment.  When X is an integer, the REPL is started in
     frame number X of the continuation.

5.3 Debugging example
=====================

Here is a sample interaction with ‘gsi’:

     $ gsi
     Gambit v4.9.4

     > (define (invsqr x) (/ 1 (expt x 2)))
     > (define (mymap fn lst)
         (define (mm in)
           (if (null? in)
               '()
               (cons (fn (car in)) (mm (cdr in)))))
         (mm lst))
     > (mymap invsqr '(5 2 hello 9 1))
     *** ERROR IN invsqr, (console)@1.25 -- (Argument 1) NUMBER expected
     (expt 'hello 2)
     1> ,i
     #<procedure #2 invsqr> =
     (lambda (x) (/ 1 (expt x 2)))
     1> ,e
     x = 'hello
     1> ,b
     0  invsqr                    (console)@1:25          (expt x 2)
     1  #<procedure #4>           (console)@6:17          (fn (car in))
     2  #<procedure #4>           (console)@6:31          (mm (cdr in))
     3  #<procedure #4>           (console)@6:31          (mm (cdr in))
     4  (interaction)             (console)@8:1           (mymap invsqr '(5 2 hel...
     1> ,+
     1  #<procedure #4>           (console)@6.17          (fn (car in))
     1\1> (pp #4)
     (lambda (in) (if (null? in) '() (cons (fn (car in)) (mm (cdr in)))))
     1\1> ,e
     in = '(hello 9 1)
     mm = (lambda (in) (if (null? in) '() (cons (fn (car in)) (mm (cdr in)))))
     fn = invsqr
     lst = '(5 2 hello 9 1)
     1\1> ,(e mm)
     mm = (lambda (in) (if (null? in) '() (cons (fn (car in)) (mm (cdr in)))))
     fn = invsqr
     lst = '(5 2 hello 9 1)
     1\1> fn
     #<procedure #2 invsqr>
     1\1> (pp fn)
     (lambda (x) (/ 1 (expt x 2)))
     1\1> ,+
     2  #<procedure #4>           (console)@6.31          (mm (cdr in))
     1\2> ,e
     in = '(2 hello 9 1)
     mm = (lambda (in) (if (null? in) '() (cons (fn (car in)) (mm (cdr in)))))
     fn = invsqr
     lst = '(5 2 hello 9 1)
     1\2> ,(c (list 3 4 5))
     (1/25 1/4 3 4 5)
     > ,q

5.4 Procedures related to debugging
===================================

 -- procedure: help [SUBJECT]
 -- procedure: help-browser [NEW-VALUE]

     The ‘help’ procedure displays the section of the Gambit manual with
     the definition of the procedure or special form SUBJECT, which must
     be a procedure or symbol.  For example the call ‘(help gensym)’
     will show the section documenting the ‘gensym’ procedure and the
     call ‘(help 'time)’ will show the section documenting the ‘time’
     special form.  When the SUBJECT is absent, the documentation of the
     ‘help’ procedure is shown.  The ‘help’ procedure returns the void
     object.

     The parameter object ‘help-browser’ is bound to a string naming the
     external program that is used by the ‘help’ procedure to view the
     documentation.  Initially it is bound to the empty string.  In
     normal circumstances when ‘help-browser’ is bound to an empty
     string the ‘help’ procedure runs the script ‘~~bin/gambdoc.bat’
     which searches for a suitable web browser to open the documentation
     in HTML format.  Unless the system was built with the command
     ‘configure --enable-help-browser=...’, the text-only browser ‘lynx’
     (see <http://lynx.isc.org/>) will be used by default if it is
     available.  We highly recommend that you install this browser if
     you are interested in viewing the documentation within the console
     in which the REPL is running.  You can exit ‘lynx’ conveniently by
     typing an end of file (usually <^D>).

     For example:

          > (help-browser "firefox") ; use firefox instead of lynx
          > (help 'gensym)
          > (help gensym) ; OK because gensym is a procedure
          > (help 'time)
          > (help time) ; not OK because time is a special form
          *** ERROR IN (console)@5.7 -- Macro name can't be used as a variable: time
          >

 -- procedure: apropos [SUBSTRING [PORT]]

     The ‘apropos’ procedure writes to the port PORT a report of all the
     global variables whose name contains SUBSTRING, a string or symbol.
     If SUBSTRING is not specified the report contains all the global
     variables.  If it is not specified, PORT defaults to the
     interaction channel (i.e.  the output will appear at the REPL). The
     ‘apropos’ procedure returns the void object.

     The global variables are grouped into namespaces.  The empty
     namespace, if it is relevant, is last.  This reduces the likelihood
     it will scroll off the screen if there are several global variables
     in other namespaces, which are typically less interesting.

     Note that with the ‘apropos’ procedure it is possible to reveal the
     existence of procedures of the runtime system and modules that are
     not intended to be called by user code.  These procedures often
     avoid type checking their arguments or must be called in a specific
     context, so calling them incorrectly may crash the system.  On the
     other hand it also allows discovering the existence of certain
     functionalities that may have gone unnoticed.

     For example:

          > (apropos "cons")
          "##" namespace:
            10^-constants, cons, cons*, cons*-aux, console-port,
            constant-expression-value, constant-expression?,
            cprc-quasi-cons, deconstruct-call,
            define-type-construct-constant, degen-quasi-cons,
            gen-quasi-cons, quasi-cons, stdio/console-repl-channel,
            void-constant?, xcons
          empty namespace:
            cons, cons*, console-port, xcons
          > (import (srfi 69))
          > (apropos "table?")
          "##" namespace:
            gc-hash-table?, mutable?, readtable?, table?
          "srfi/69#" namespace:
            hash-table?
          empty namespace:
            readtable?, table?
          > (apropos "srfi/69#")
          "srfi/69#" namespace:
            ||, alist->hash-table, hash, hash-by-identity,
            hash-table->alist, hash-table-copy, hash-table-delete!,
            hash-table-equivalence-function, hash-table-exists?,
            hash-table-fold, hash-table-hash-function,
            hash-table-keys, hash-table-merge!, hash-table-ref,
            hash-table-ref/default, hash-table-set!, hash-table-size,
            hash-table-update!, hash-table-update!/default,
            hash-table-values, hash-table-walk, hash-table?,
            make-hash-table, string-ci-hash, string-hash

 -- procedure: repl-result-history-ref I
 -- procedure: repl-result-history-max-length-set! N

     The REPL keeps a history of the last few results printed by the
     REPL. The call ‘(repl-result-history-ref I)’ returns the Ith
     previous result (the last for I=0, the next to last for I=1, etc).
     By default the REPL result history remembers up to 3 results.  The
     maximal length of the history can be set to N between 0 and 10 by a
     call to ‘(repl-result-history-max-length-set! N)’.

     For convenience the reader defines an abbreviation for calling
     ‘repl-result-history-ref’.  Tokens formed by a sequence of one or
     more hash signs, such as ‘‘#’’, ‘‘##’’, etc, are expanded by the
     reader into the list ‘(repl-result-history-ref I)’, where I is the
     number of hash signs minus 1.  In other words, ‘‘#’’ will return
     the last result printed by the REPL, ‘‘##’’ will return the next to
     last, etc.

     For example:

          > (map (lambda (x) (* x x)) '(1 2 3))
          (1 4 9)
          > (reverse #)
          (9 4 1)
          > (append # ##)
          (9 4 1 1 4 9)
          > 1
          1
          > 1
          1
          > (+ # ##)
          2
          > (+ # ##)
          3
          > (+ # ##)
          5
          > ####
          *** ERROR IN (console)@9.1 -- (Argument 1) Out of range
          (repl-result-history-ref 3)
          1>

 -- procedure: trace PROC...
 -- procedure: untrace PROC...

     The ‘trace’ procedure starts tracing calls to the specified
     procedures.  When a traced procedure is called, a line containing
     the procedure and its arguments is displayed (using the procedure
     call expression syntax).  The line is indented with a sequence of
     vertical bars which indicate the nesting depth of the procedure’s
     continuation.  After the vertical bars is a greater-than sign which
     indicates that the evaluation of the call is starting.

     When a traced procedure returns a result, it is displayed with the
     same indentation as the call but without the greater-than sign.
     This makes it easy to match calls and results (the result of a
     given call is the value at the same indentation as the greater-than
     sign).  If a traced procedure P1 performs a tail call to a traced
     procedure P2, then P2 will use the same indentation as P1.  This
     makes it easy to spot tail calls.  The special handling for tail
     calls is needed to preserve the space complexity of the program
     (i.e.  tail calls are implemented as required by Scheme even when
     they involve traced procedures).

     The ‘untrace’ procedure stops tracing calls to the specified
     procedures.  When no argument is passed to the ‘trace’ procedure,
     the list of procedures currently being traced is returned.  The
     void object is returned by the ‘trace’ procedure when it is passed
     one or more arguments.  When no argument is passed to the ‘untrace’
     procedure stops all tracing and returns the void object.  A
     compiled procedure may be traced but only if it is bound to a
     global variable.

     For example:

          > (define (fact n) (if (< n 2) 1 (* n (fact (- n 1)))))
          > (trace fact)
          > (fact 5)
          | > (fact 5)
          | | > (fact 4)
          | | | > (fact 3)
          | | | | > (fact 2)
          | | | | | > (fact 1)
          | | | | | 1
          | | | | 2
          | | | 6
          | | 24
          | 120
          120
          > (trace -)
          *** WARNING -- Rebinding global variable "-" to an interpreted procedure
          > (define (fact-iter n r) (if (< n 2) r (fact-iter (- n 1) (* n r))))
          > (trace fact-iter)
          > (fact-iter 5 1)
          | > (fact-iter 5 1)
          | | > (- 5 1)
          | | 4
          | > (fact-iter 4 5)
          | | > (- 4 1)
          | | 3
          | > (fact-iter 3 20)
          | | > (- 3 1)
          | | 2
          | > (fact-iter 2 60)
          | | > (- 2 1)
          | | 1
          | > (fact-iter 1 120)
          | 120
          120
          > (trace)
          (#<procedure #2 fact-iter> #<procedure #3 -> #<procedure #4 fact>)
          > (untrace)
          > (fact 5)
          120

 -- procedure: step
 -- procedure: step-level-set! LEVEL

     The ‘step’ procedure enables single-stepping mode.  After the call
     to ‘step’ the computation will stop just before the interpreter
     executes the next evaluation step (as defined by
     ‘step-level-set!’).  A nested REPL is then started.  Note that
     because single-stepping is stopped by the REPL whenever the prompt
     is displayed it is pointless to enter ‘(step)’ by itself.  On the
     other hand entering ‘(begin (step) EXPR)’ will evaluate EXPR in
     single-stepping mode.

     The procedure ‘step-level-set!’ sets the stepping level which
     determines the granularity of the evaluation steps when
     single-stepping is enabled.  The stepping level LEVEL must be an
     exact integer in the range 0 to 7.  At a level of 0, the
     interpreter ignores single-stepping mode.  At higher levels the
     interpreter stops the computation just before it performs the
     following operations, depending on the stepping level:

       1. procedure call

       2. ‘delay’ special form and operations at lower levels

       3. ‘lambda’ special form and operations at lower levels

       4. ‘define’ special form and operations at lower levels

       5. ‘set!’ special form and operations at lower levels

       6. variable reference and operations at lower levels

       7. constant reference and operations at lower levels

     The default stepping level is 7.

     For example:

          > (define (fact n) (if (< n 2) 1 (* n (fact (- n 1)))))
          > (step-level-set! 1)
          > (begin (step) (fact 5))
          *** STOPPED IN (console)@3.15
          1> ,s
          | > (fact 5)
          *** STOPPED IN fact, (console)@1.22
          1> ,s
          | | > (< n 2)
          | | #f
          *** STOPPED IN fact, (console)@1.43
          1> ,s
          | | > (- n 1)
          | | 4
          *** STOPPED IN fact, (console)@1.37
          1> ,s
          | | > (fact (- n 1))
          *** STOPPED IN fact, (console)@1.22
          1> ,s
          | | | > (< n 2)
          | | | #f
          *** STOPPED IN fact, (console)@1.43
          1> ,s
          | | | > (- n 1)
          | | | 3
          *** STOPPED IN fact, (console)@1.37
          1> ,l
          | | | > (fact (- n 1))
          *** STOPPED IN fact, (console)@1.22
          1> ,l
          | | > (* n (fact (- n 1)))
          | | 24
          *** STOPPED IN fact, (console)@1.32
          1> ,l
          | > (* n (fact (- n 1)))
          | 120
          120

 -- procedure: break PROC...
 -- procedure: unbreak PROC...

     The ‘break’ procedure places a breakpoint on each of the specified
     procedures.  When a procedure is called that has a breakpoint, the
     interpreter will enable single-stepping mode (as if ‘step’ had been
     called).  This typically causes the computation to stop soon inside
     the procedure if the stepping level is high enough.

     The ‘unbreak’ procedure removes the breakpoints on the specified
     procedures.  With no argument, ‘break’ returns the list of
     procedures currently containing breakpoints.  The void object is
     returned by ‘break’ if it is passed one or more arguments.  With no
     argument ‘unbreak’ removes all the breakpoints and returns the void
     object.  A breakpoint can be placed on a compiled procedure but
     only if it is bound to a global variable.

     For example:

          > (define (double x) (+ x x))
          > (define (triple y) (- (double (double y)) y))
          > (define (f z) (* (triple z) 10))
          > (break double)
          > (break -)
          *** WARNING -- Rebinding global variable "-" to an interpreted procedure
          > (f 5)
          *** STOPPED IN double, (console)@1.21
          1> ,b
          0  double                    (console)@1:21          +
          1  triple                    (console)@2:31          (double y)
          2  f                         (console)@3:18          (triple z)
          3  (interaction)             (console)@6:1           (f 5)
          1> ,e
          x = 5
          1> ,c
          *** STOPPED IN double, (console)@1.21
          1> ,c
          *** STOPPED IN f, (console)@3.29
          1> ,c
          150
          > (break)
          (#<procedure #3 -> #<procedure #4 double>)
          > (unbreak)
          > (f 5)
          150

 -- procedure: generate-proper-tail-calls [NEW-VALUE]

     [Note: this procedure is DEPRECATED and will be removed in a future
     version of Gambit.  Use the ‘proper-tail-calls’ declaration
     instead.]

     The parameter object ‘generate-proper-tail-calls’ is bound to a
     boolean value controlling how the interpreter handles tail calls.
     When it is bound to ‘#f’ the interpreter will treat tail calls like
     nontail calls, that is a new continuation will be created for the
     call.  This setting is useful for debugging, because when a
     primitive signals an error the location information will point to
     the call site of the primitive even if this primitive was called
     with a tail call.  The initial value of this parameter object is
     ‘#t’, which means that a tail call will reuse the continuation of
     the calling function.

     This parameter object only affects code that is subsequently
     processed by ‘load’ or ‘eval’, or entered at the REPL.

     For example:

          > (generate-proper-tail-calls)
          #t
          > (let loop ((i 1)) (if (< i 10) (loop (* i 2)) oops))
          *** ERROR IN #<procedure #2>, (console)@2.47 -- Unbound variable: oops
          1> ,b
          0  #<procedure #2>           (console)@2:47          oops
          1  (interaction)             (console)@2:1           ((letrec ((loop (lambda...
          1> ,t
          > (generate-proper-tail-calls #f)
          > (let loop ((i 1)) (if (< i 10) (loop (* i 2)) oops))
          *** ERROR IN #<procedure #3>, (console)@6.47 -- Unbound variable: oops
          1> ,b
          0  #<procedure #3>           (console)@6:47          oops
          1  #<procedure #3>           (console)@6:32          (loop (* i 2))
          2  #<procedure #3>           (console)@6:32          (loop (* i 2))
          3  #<procedure #3>           (console)@6:32          (loop (* i 2))
          4  #<procedure #3>           (console)@6:32          (loop (* i 2))
          5  (interaction)             (console)@6:1           ((letrec ((loop (lambda...


 -- procedure: display-environment-set! DISPLAY?

     [Note: this procedure is DEPRECATED and will be removed in a future
     version of Gambit.  Use the parameter object
     ‘repl-display-environment?’ instead.]

     This procedure sets a flag that controls the automatic display of
     the environment by the REPL. If DISPLAY? is true, the environment
     is displayed by the REPL before the prompt.  The default setting is
     not to display the environment.

 -- procedure: repl-display-environment? DISPLAY?

     The parameter object ‘repl-display-environment?’ is bound to a
     boolean value that controls the automatic display of the
     environment by the REPL. If DISPLAY? is true, the environment is
     displayed by the REPL before the prompt.  This is particularly
     useful in single-stepping mode.  The default setting is not to
     display the environment.

 -- procedure: display-dynamic-environment? DISPLAY?

     The parameter object ‘display-dynamic-environment?’ is bound to a
     boolean value that controls wether the dynamic environment is
     displayed when the environment is displayed.  The default setting
     is not to display the dynamic environment.

 -- procedure: pretty-print OBJ [PORT]

     This procedure pretty-prints OBJ on the port PORT.  If it is not
     specified, PORT defaults to the current output-port.

     For example:

          > (pretty-print
              (let* ((x '(1 2 3 4)) (y (list x x x))) (list y y y)))
          (((1 2 3 4) (1 2 3 4) (1 2 3 4))
           ((1 2 3 4) (1 2 3 4) (1 2 3 4))
           ((1 2 3 4) (1 2 3 4) (1 2 3 4)))

 -- procedure: pp OBJ [PORT]

     This procedure pretty-prints OBJ on the port PORT.  When OBJ is a
     procedure created by the interpreter or a procedure created by code
     compiled with the declaration ‘debug-source’, the procedure’s
     source code is displayed.  If it is not specified, PORT defaults to
     the interaction channel (i.e.  the output will appear at the REPL).

     For example:

          > (define (f g) (+ (time (g 100)) (time (g 1000))))
          > (pp f)
          (lambda (g)
            (+ (##time (lambda () (g 100)) '(g 100))
               (##time (lambda () (g 1000)) '(g 1000))))

 -- procedure: gc-report-set! REPORT?

     This procedure controls the generation of reports during garbage
     collections.  If the argument is true, a brief report of memory
     usage is generated after every garbage collection.  It contains:
     the time taken for this garbage collection, the amount of memory
     allocated in megabytes since the program was started, the size of
     the heap in megabytes, the heap memory in megabytes occupied by
     live data, the proportion of the heap occupied by live data, and
     the number of bytes occupied by movable and nonmovable objects.

5.5 Console line-editing
========================

The console implements a simple Scheme-friendly line-editing
user-interface that is enabled by default.  It offers parentheses
balancing, a history of previous commands, symbol completion, and
several emacs-compatible keyboard commands.  The user’s input is
displayed in a bold font and the output produced by the system is in a
plain font.  The history of previous commands is saved in the file
‘~/.gambit_history’.  It is restored when a REPL is started.

   Symbol completion is triggered with the tab key.  When the cursor is
after a sequence of characters that can form a symbol, typing the tab
key will search the symbol table for the first symbol (in alphabetical
order) that begins with that sequence and insert that symbol.  Typing
the tab key in succession will cycle through all symbols with that
prefix.  When all possible symbols have been shown or there are no
possible completions, the text reverts to the uncompleted symbol and the
bell is rung.

   Here are the keyboard commands available (where the ‘‘M-’’ prefix
means the escape key is typed and the ‘‘C-’’ prefix means the control
key is pressed):

‘C-d’
     Generate an end-of-file when the line is empty, otherwise delete
     character at cursor.

‘delete or backspace’
     Delete character before cursor.

‘M-C-d’
     Delete word forward and keep a copy of this text on the clipboard.

‘M-delete’
     Delete word backward and keep a copy of this text on the clipboard.

‘M-backspace’
     Delete S-expression backward and keep a copy of this text on the
     clipboard.

‘C-a’
     Move cursor to beginning of line.

‘C-e’
     Move cursor to end of line.

‘C-b or left-arrow’
     Move cursor left one character.

‘M-b’
     Move cursor left one word.

‘M-C-b or ‘M-’left-arrow’
     Move cursor left one S-expression.

‘C-f or right-arrow’
     Move cursor right one character.

‘M-f’
     Move cursor right one word.

‘M-C-f or ‘M-’right-arrow’
     Move cursor right one S-expression.

‘C-p or ‘M-p’ or up-arrow’
     Move to previous line in history.

‘C-n or ‘M-n’ or down-arrow’
     Move to next line in history.

‘C-t’
     Transpose character at cursor with previous character.

‘M-t’
     Transpose word after cursor with previous word.

‘M-C-t’
     Transpose S-expression after cursor with previous S-expression.

‘C-l’
     Clear console and redraw line being edited.

‘C-nul’
     Set the mark to the cursor.

‘C-w’
     Delete the text between the cursor and the mark and keep a copy of
     this text on the clipboard.

‘C-k’
     Delete the text from the cursor to the end of the line and keep a
     copy of this text on the clipboard.

‘C-y’
     Paste the text that is on the clipboard.

‘F8’
     Same as typing ‘#||#,c;’ (REPL command to continue the
     computation).

‘F9’
     Same as typing ‘#||#,-;’ (REPL command to move to newer frame).

‘F10’
     Same as typing ‘#||#,+;’ (REPL command to move to older frame).

‘F11’
     Same as typing ‘#||#,s;’ (REPL command to step the computation).

‘F12’
     Same as typing ‘#||#,l;’ (REPL command to leap the computation).

   On macOS, depending on your configuration, you may have to press the
‘fn’ key to access the function key ‘F12’ and the ‘option’ key to access
the other function keys.

   On Microsoft Windows the clipboard is the system clipboard.  This
allows text to be copied and pasted between the program and other
applications.  On other operating systems the clipboard is internal to
the program (it is not integrated with the operating system).

5.6 Emacs interface
===================

Gambit comes with the Emacs package ‘gambit.el’ which provides a nice
environment for running Gambit from within the Emacs editor.  This
package filters the standard output of the Gambit process and when it
intercepts a location information (in the format ‘STREAM@LINE.COLUMN’
where STREAM is either ‘(stdin)’ when the expression was obtained from
standard input, ‘(console)’ when the expression was obtained from the
console, or a string naming a file) it opens a window to highlight the
corresponding expression.

   To use this package, make sure the file ‘gambit.el’ is accessible
from your load-path and that the following lines are in your ‘.emacs’
file:

     (autoload 'gambit-inferior-mode "gambit" "Hook Gambit mode into cmuscheme.")
     (autoload 'gambit-mode "gambit" "Hook Gambit mode into scheme.")
     (add-hook 'inferior-scheme-mode-hook (function gambit-inferior-mode))
     (add-hook 'scheme-mode-hook (function gambit-mode))
     (setq scheme-program-name "gsi -:debug=-")

   Alternatively, if you don’t mind always loading this package, you can
simply add this line to your ‘.emacs’ file:

     (require 'gambit)

   You can then start an inferior Gambit process by typing ‘M-x
run-scheme’.  The commands provided in ‘cmuscheme’ mode will be
available in the Gambit interaction buffer (i.e.  ‘*scheme*’) and in
buffers attached to Scheme source files.  Here is a list of the most
useful commands (for a complete list type ‘C-h m’ in the Gambit
interaction buffer):
‘C-x C-e’
     Evaluate the expression which is before the cursor (the expression
     will be copied to the Gambit interaction buffer).
‘C-c C-z’
     Switch to Gambit interaction buffer.
‘C-c C-l’
     Load a file (file attached to current buffer is default) using
     ‘(load FILE)’.
‘C-c C-k’
     Compile a file (file attached to current buffer is default) using
     ‘(compile-file FILE)’.

   The file ‘gambit.el’ provides these additional commands:

‘F8 or C-c c’
     Continue the computation (same as typing ‘#||#,c;’ to the REPL).
‘F9 or C-c ]’
     Move to newer frame (same as typing ‘#||#,-;’ to the REPL).
‘F10 or C-c [’
     Move to older frame (same as typing ‘#||#,+;’ to the REPL).
‘F11 or C-c s’
     Step the computation (same as typing ‘#||#,s;’ to the REPL).
‘F12 or C-c l’
     Leap the computation (same as typing ‘#||#,l;’ to the REPL).
‘C-c _’
     Removes the last window that was opened to highlight an expression.

   The two keystroke version of these commands can be shortened to
‘M-c’, ‘M-[’, ‘M-]’, ‘M-s’, ‘M-l’, and ‘M-_’ respectively by adding this
line to your ‘.emacs’ file:

     (setq gambit-repl-command-prefix "\e")

   This is more convenient to type than the two keystroke ‘C-c’ based
sequences but the purist may not like this because it does not follow
normal Emacs conventions.

   Here is what a typical ‘.emacs’ file will look like:

     (setq load-path ; add directory containing gambit.el
       (cons "/usr/local/Gambit/share/emacs/site-lisp"
             load-path))
     (setq scheme-program-name "/tmp/gsi -:debug=-") ; if gsi not in executable path
     (setq gambit-highlight-color "gray") ; if you don't like the default
     (setq gambit-repl-command-prefix "\e") ; if you want M-c, M-s, etc
     (require 'gambit)

5.7 GUIDE
=========

The implementation and documentation for GUIDE, the Gambit Universal
IDE, are not yet complete.

6 Scheme extensions
*******************

6.1 Extensions to standard procedures
=====================================

 -- procedure: transcript-on FILE
 -- procedure: transcript-off

     These procedures do nothing.

 -- procedure: call-with-current-continuation PROC
 -- procedure: call/cc PROC

     The procedure ‘call-with-current-continuation’ is bound to the
     global variables ‘call-with-current-continuation’ and ‘call/cc’.

6.2 Extensions to standard special forms
========================================

 -- special form: lambda lambda-formals body
 -- special form: define (variable define-formals) body

          lambda-formals = ‘(’ formal-argument-list ‘)’ |
          r4rs-lambda-formals
          define-formals = formal-argument-list | r4rs-define-formals
          formal-argument-list = dsssl-formal-argument-list |
          rest-at-end-formal-argument-list
          dsssl-formal-argument-list = reqs opts rest keys
          rest-at-end-formal-argument-list = reqs opts keys rest | reqs
          opts keys ‘.’ rest-formal-argument
          reqs = required-formal-argument*
          required-formal-argument = variable
          opts = ‘#!optional’ optional-formal-argument* | empty
          optional-formal-argument = variable | ‘(’ variable initializer
          ‘)’
          rest = ‘#!rest’ rest-formal-argument | empty
          rest-formal-argument = variable
          keys = ‘#!key’ keyword-formal-argument* | empty
          keyword-formal-argument = variable | ‘(’ variable initializer
          ‘)’
          initializer = expression
          r4rs-lambda-formals = ‘(’ variable* ‘)’ | ‘(’ variable+ ‘.’
          variable ‘)’ | variable
          r4rs-define-formals = variable* | variable* ‘.’ variable

     These forms are extended versions of the ‘lambda’ and ‘define’
     special forms of standard Scheme.  They allow the use of optional
     formal arguments, either positional or named, and support the
     syntax and semantics of the DSSSL standard.

     When the procedure introduced by a ‘lambda’ (or ‘define’) is
     applied to a list of actual arguments, the formal and actual
     arguments are processed as specified in the R4RS if the
     lambda-formals (or define-formals) is a r4rs-lambda-formals (or
     r4rs-define-formals).

     If the formal-argument-list matches dsssl-formal-argument-list or
     extended-formal-argument-list they are processed as follows:

       a. Variables in required-formal-arguments are bound to successive
          actual arguments starting with the first actual argument.  It
          shall be an error if there are fewer actual arguments than
          required-formal-arguments.

       b. Next variables in optional-formal-arguments are bound to
          remaining actual arguments.  If there are fewer remaining
          actual arguments than optional-formal-arguments, then the
          variables are bound to the result of evaluating initializer,
          if one was specified, and otherwise to ‘#f’.  The initializer
          is evaluated in an environment in which all previous formal
          arguments have been bound.

       c. If ‘#!key’ does not appear in the formal-argument-list and
          there is no rest-formal-argument then it shall be an error if
          there are any remaining actual arguments.

       d. If ‘#!key’ does not appear in the formal-argument-list and
          there is a rest-formal-argument then the rest-formal-argument
          is bound to a list of all remaining actual arguments.

       e. If ‘#!key’ appears in the formal-argument-list and there is no
          rest-formal-argument then there shall be an even number of
          remaining actual arguments.  These are interpreted as a series
          of pairs, where the first member of each pair is a keyword
          specifying the argument name, and the second is the
          corresponding value.  It shall be an error if the first member
          of a pair is not a keyword.  It shall be an error if the
          argument name is not the same as a variable in a
          keyword-formal-argument.  If the same argument name occurs
          more than once in the list of actual arguments, then the first
          value is used.  If there is no actual argument for a
          particular keyword-formal-argument, then the variable is bound
          to the result of evaluating initializer if one was specified,
          and otherwise to ‘#f’.  The initializer is evaluated in an
          environment in which all previous formal arguments have been
          bound.

       f. If ‘#!key’ appears in the formal-argument-list and there is a
          rest-formal-argument before the ‘#!key’ then there may be an
          even or odd number of remaining actual arguments and the
          rest-formal-argument is bound to a list of all remaining
          actual arguments.  Then, these remaining actual arguments are
          scanned from left to right in pairs, stopping at the first
          pair whose first element is not a keyword.  Each pair whose
          first element is a keyword matching the name of a
          keyword-formal-argument gives the value (i.e.  the second
          element of the pair) of the corresponding formal argument.  If
          the same argument name occurs more than once in the list of
          actual arguments, then the first value is used.  If there is
          no actual argument for a particular keyword-formal-argument,
          then the variable is bound to the result of evaluating
          initializer if one was specified, and otherwise to ‘#f’.  The
          initializer is evaluated in an environment in which all
          previous formal arguments have been bound.

       g. If ‘#!key’ appears in the formal-argument-list and there is a
          rest-formal-argument after the ‘#!key’ then there may be an
          even or odd number of remaining actual arguments.  The
          remaining actual arguments are scanned from left to right in
          pairs, stopping at the first pair whose first element is not a
          keyword.  Each pair shall have as its first element a keyword
          matching the name of a keyword-formal-argument; the second
          element gives the value of the corresponding formal argument.
          If the same argument name occurs more than once in the list of
          actual arguments, then the first value is used.  If there is
          no actual argument for a particular keyword-formal-argument,
          then the variable is bound to the result of evaluating
          initializer if one was specified, and otherwise to ‘#f’.  The
          initializer is evaluated in an environment in which all
          previous formal arguments have been bound.  Finally, the
          rest-formal-argument is bound to the list of the actual
          arguments that were not scanned (i.e.  after the last
          keyword/value pair).

     In all cases it is an error for a variable to appear more than once
     in a formal-argument-list.

     Note that this specification is compatible with the DSSSL language
     standard (i.e.  a correct DSSSL program will have the same
     semantics when run with Gambit).

     It is unspecified whether variables receive their value by binding
     or by assignment.  Currently the compiler and interpreter use
     different methods, which can lead to different semantics if
     ‘call-with-current-continuation’ is used in an initializer.  Note
     that this is irrelevant for DSSSL programs because
     ‘call-with-current-continuation’ does not exist in DSSSL.

     For example:

          > ((lambda (#!rest x) x) 1 2 3)
          (1 2 3)
          > (define (f a #!optional b) (list a b))
          > (define (g a #!optional (b a) #!key (k (* a b))) (list a b k))
          > (define (h1 a #!rest r #!key k) (list a k r))
          > (define (h2 a #!key k #!rest r) (list a k r))
          > (f 1)
          (1 #f)
          > (f 1 2)
          (1 2)
          > (g 3)
          (3 3 9)
          > (g 3 4)
          (3 4 12)
          > (g 3 4 k: 5)
          (3 4 5)
          > (g 3 4 k: 5 k: 6)
          (3 4 5)
          > (h1 7)
          (7 #f ())
          > (h1 7 k: 8 9)
          (7 8 (k: 8 9))
          > (h1 7 k: 8 z: 9)
          (7 8 (k: 8 z: 9))
          > (h2 7)
          (7 #f ())
          > (h2 7 k: 8 9)
          (7 8 (9))
          > (h2 7 k: 8 z: 9)
          *** ERROR IN (console)@17.1 -- Unknown keyword argument passed to procedure
          (h2 7 k: 8 z: 9)

6.3 Miscellaneous extensions
============================

 -- procedure: subvector VECTOR START END

     This procedure is the vector analog of the ‘substring’ procedure.
     It returns a newly allocated vector formed from the elements of the
     vector VECTOR beginning with index START (inclusive) and ending
     with index END (exclusive).

     For example:

          > (subvector '#(a b c d e f) 3 5)
          #(d e)

 -- procedure: vector-copy VECTOR [START [END]]

     This procedure is like the procedure ‘subvector’ except the
     parameter START defaults to 0 and the parameter END defaults to the
     length of the vector VECTOR.  Note that the elements are not
     recursively copied.

     For example:

          > (define v1 '#(a b c d e f))
          > (define v2 (vector-copy v1))
          > v2
          #(a b c d e f)
          > (eq? v1 v2)
          #f
          > (vector-copy v1 3)
          #(d e f)
          > (vector-copy v1 3 5)
          #(d e)

 -- procedure: vector-copy! DEST-VECTOR DEST-START VECTOR [START [END]]

     This procedure mutates the vector DEST-VECTOR.  It copies the
     elements of the vector VECTOR beginning with index START
     (inclusive) and ending with index END (exclusive) to the vector
     DEST-VECTOR at index DEST-START.  The parameters START and END
     default respectively to 0 and the length of the vector VECTOR.  It
     is an error to copy more elements than will fit in the tail of the
     vector DEST-VECTOR starting at index DEST-START.  Note that the
     elements are not recursively copied.

     For example:

          > (define v1 (vector 10 11 12 13 14 15))
          > (define v2 (vector 20 21 22 23))
          > (vector-copy! v1 1 v2)
          > v1
          #(10 20 21 22 23 15)
          > (vector-copy! v1 1 v2 3)
          > v1
          #(10 23 21 22 23 15)
          > (vector-copy! v1 1 v2 1 3)
          > v1
          #(10 21 22 22 23 15)

 -- procedure: vector-append VECTOR...

     This procedure is the vector analog of the ‘string-append’
     procedure.  It returns a newly allocated vector whose elements form
     the concatenation of the given vectors.

     For example:

          > (define v '#(1 2 3))
          > (vector-append v v v)
          #(1 2 3 1 2 3 1 2 3)

 -- procedure: vector-concatenate LST [SEPARATOR]

     This procedure returns a newly allocated vector whose elements form
     the concatenation of all the vectors in the list LST.  If the
     optional vector SEPARATOR argument is specified, it will be added
     between all the elements of LST.  Without the SEPARATOR argument
     the result is the same as ‘(apply vector-append LST)’.

     For example:

          > (define v '#(1 2 3))
          > (vector-concatenate (list v v v))
          #(1 2 3 1 2 3 1 2 3)
          > (vector-concatenate (list v v v) '#(88 99))
          #(1 2 3 88 99 1 2 3 88 99 1 2 3)

 -- procedure: subvector-fill! VECTOR START END FILL

     This procedure is like ‘vector-fill!’, but fills a selected part of
     the given vector.  It sets the elements of the vector VECTOR,
     beginning with index START (inclusive) and ending with index END
     (exclusive) to FILL.  The value returned is unspecified.

     For example:

          > (define v (vector 'a 'b 'c 'd 'e 'f))
          > (subvector-fill! v 3 5 'x)
          > v
          #(a b c x x f)

 -- procedure: subvector-move! SRC-VECTOR SRC-START SRC-END DST-VECTOR
          DST-START

     This procedure replaces part of the contents of vector DST-VECTOR
     with part of the contents of vector SRC-VECTOR.  It copies elements
     from SRC-VECTOR, beginning with index SRC-START (inclusive) and
     ending with index SRC-END (exclusive) to DST-VECTOR beginning with
     index DST-START (inclusive).  The value returned is unspecified.

     For example:

          > (define v1 '#(1 2 3 4 5 6))
          > (define v2 (vector 'a 'b 'c 'd 'e 'f))
          > (subvector-move! v1 3 5 v2 1)
          > v2
          #(a 4 5 d e f)

 -- procedure: vector-shrink! VECTOR K

     This procedure shortens the vector VECTOR so that its new size is
     K.  The value returned is unspecified.

     For example:

          > (define v (vector 'a 'b 'c 'd 'e 'f))
          > v
          #(a b c d e f)
          > (vector-shrink! v 3)
          > v
          #(a b c)

 -- procedure: vector-cas! VECTOR K NEW-VALUE OLD-VALUE

     The procedure ‘vector-cas!’ performs an atomic compare-and-swap
     operation on the element of vector VECTOR at index K.  If the
     element’s value is ‘eq?’ to OLD-VALUE then the element is changed
     to NEW-VALUE, otherwise the value does not change.  Regardless what
     happened, the element’s value prior to any change is returned.  It
     is thus possible to detect a change by an explicit ‘eq?’ test of
     the result.

     For example:

          > (define v (vector 'a))
          > (eq? 'foo (vector-cas! v 0 'b 'foo))
          #f
          > v
          #(a)
          > (eq? 'a (vector-cas! v 0 'b 'a))
          #t
          > v
          #(b)

 -- procedure: vector-inc! VECTOR K [STEP]

     The procedure ‘vector-inc!’ performs an atomic incrementation on
     the element of vector VECTOR at index K, which must be a fixnum.
     The parameter STEP defaults to 1 and it is the fixnum value that is
     added (with wraparound) to the element.  The procedure returns the
     value of the element prior to the incrementation.

     For example:

          > (define v (vector 100))
          > (vector-inc! v 0)
          100
          > (vector-inc! v 0)
          101
          > (vector-inc! v 0 5)
          102
          > v
          #(107)

 -- procedure: vector-set VECTOR K OBJ

     The procedure ‘vector-set’ returns a new copy of the vector VECTOR
     with the element at index K replaced with OBJ.

     For example:

          > (define v1 (vector 10 11 12 13))
          > (define v2 (vector-set v1 2 99))
          > v2
          #(10 11 99 13)
          > (eq? v1 v2)
          #f

 -- procedure: string-set STRING K CHAR

     The procedure ‘string-set’ returns a new copy of the string STRING
     with the character at index K replaced with CHAR.

     For example:

          > (define s1 (string #\a #\b #\c #\d))
          > (define s2 (string-set s1 2 #\.))
          > s2
          "ab.d"
          > (eq? s1 s2)
          #f

 -- procedure: string-concatenate LST [SEPARATOR]

     This procedure returns a newly allocated string which is the
     concatenation of all the strings in the list LST.  If the optional
     string SEPARATOR argument is specified, it will be added between
     all the elements of LST.  Without the SEPARATOR argument the result
     is the same as ‘(apply string-append LST)’.

     For example:

          > (define s "abc")
          > (string-concatenate (list s s s))
          "abcabcabc"
          > (string-concatenate (list s s s) ",")
          "abc,abc,abc"

 -- procedure: substring-fill! STRING START END FILL

     This procedure is like ‘string-fill!’, but fills a selected part of
     the given string.  It sets the elements of the string STRING,
     beginning with index START (inclusive) and ending with index END
     (exclusive) to FILL.  The value returned is unspecified.

     For example:

          > (define s (string #\a #\b #\c #\d #\e #\f))
          > (substring-fill! s 3 5 #\x)
          > s
          "abcxxf"

 -- procedure: substring-move! SRC-STRING SRC-START SRC-END DST-STRING
          DST-START

     This procedure replaces part of the contents of string DST-STRING
     with part of the contents of string SRC-STRING.  It copies elements
     from SRC-STRING, beginning with index SRC-START (inclusive) and
     ending with index SRC-END (exclusive) to DST-STRING beginning with
     index DST-START (inclusive).  The value returned is unspecified.

     For example:

          > (define s1 "123456")
          > (define s2 (string #\a #\b #\c #\d #\e #\f))
          > (substring-move! s1 3 5 s2 1)
          > s2
          "a45def"

 -- procedure: string-shrink! STRING K

     This procedure shortens the string STRING so that its new size is
     K.  The value returned is unspecified.

     For example:

          > (define s (string #\a #\b #\c #\d #\e #\f))
          > s
          "abcdef"
          > (string-shrink! s 3)
          > s
          "abc"

 -- procedure: box OBJ
 -- procedure: box? OBJ
 -- procedure: unbox BOX
 -- procedure: set-box! BOX OBJ

     These procedures implement the “box” data type.  A box is a cell
     containing a single mutable field.  The lexical syntax of a box
     containing the object OBJ is ‘#&OBJ’ (*note Box syntax::).

     The procedure ‘box’ returns a new box object whose content is
     initialized to OBJ.  The procedure ‘box?’ returns ‘#t’ if OBJ is a
     box, and otherwise returns ‘#f’.  The procedure ‘unbox’ returns the
     content of the box BOX.  The procedure ‘set-box!’ changes the
     content of the box BOX to OBJ.  The procedure ‘set-box!’ returns an
     unspecified value.

     For example:

          > (define b (box 0))
          > b
          #&0
          > (define (inc!) (set-box! b (+ (unbox b) 1)))
          > (inc!)
          > b
          #&1
          > (unbox b)
          1

 -- procedure: keyword? OBJ
 -- procedure: keyword->string KEYWORD
 -- procedure: string->keyword STRING

     These procedures implement the “keyword” data type.  Keywords are
     similar to symbols but are self evaluating and distinct from the
     symbol data type.  The lexical syntax of keywords is specified in
     *note Keyword syntax::.

     The procedure ‘keyword?’ returns ‘#t’ if OBJ is a keyword, and
     otherwise returns ‘#f’.  The procedure ‘keyword->string’ returns
     the name of KEYWORD as a string.  The procedure ‘string->keyword’
     returns the keyword whose name is STRING.

     For example:

          > (keyword? 'color)
          #f
          > (keyword? color:)
          #t
          > (keyword->string color:)
          "color"
          > (string->keyword "color")
          color:

 -- procedure: gensym [PREFIX]

     This procedure returns a new “uninterned symbol”.  Uninterned
     symbols are guaranteed to be distinct from the symbols generated by
     the procedures ‘read’ and ‘string->symbol’.  The symbol PREFIX is
     the prefix used to generate the new symbol’s name.  If it is not
     specified, the prefix defaults to ‘g’.

     For example:

          > (gensym)
          #:g0
          > (gensym)
          #:g1
          > (gensym 'star-trek-)
          #:star-trek-2

 -- procedure: string->uninterned-symbol NAME [HASH]
 -- procedure: uninterned-symbol? OBJ

     The procedure ‘string->uninterned-symbol’ returns a new uninterned
     symbol whose name is NAME and hash is HASH.  The name must be a
     string and the hash must be a nonnegative fixnum.

     The procedure ‘uninterned-symbol?’ returns ‘#t’ when OBJ is a
     symbol that is uninterned and ‘#f’ otherwise.

     For example:

          > (uninterned-symbol? (gensym))
          #t
          > (string->uninterned-symbol "foo")
          #:foo:
          > (uninterned-symbol? (string->uninterned-symbol "foo"))
          #t
          > (uninterned-symbol? 'hello)
          #f
          > (uninterned-symbol? 123)
          #f

 -- procedure: string->uninterned-keyword NAME [HASH]
 -- procedure: uninterned-keyword? OBJ

     The procedure ‘string->uninterned-keyword’ returns a new uninterned
     keyword whose name is NAME and hash is HASH.  The name must be a
     string and the hash must be a nonnegative fixnum.

     The procedure ‘uninterned-keyword?’ returns ‘#t’ when OBJ is a
     keyword that is uninterned and ‘#f’ otherwise.

     For example:

          > (string->uninterned-keyword "foo")
          #:foo:
          > (uninterned-keyword? (string->uninterned-keyword "foo"))
          #t
          > (uninterned-keyword? hello:)
          #f
          > (uninterned-keyword? 123)
          #f

 -- procedure: identity OBJ

     This procedure returns OBJ.

 -- procedure: void

     This procedure returns the void object.  The read-eval-print loop
     prints nothing when the result is the void object.

 -- procedure: eval EXPR [ENV]

     The first parameter is a datum representing an expression.  The
     ‘eval’ procedure evaluates this expression in the global
     interaction environment and returns the result.  If present, the
     second parameter is ignored (it is provided for compatibility with
     R5RS).

     For example:

          > (eval '(+ 1 2))
          3
          > ((eval 'car) '(1 2))
          1
          > (eval '(define x 5))
          > x
          5

 -- special form: define-macro (name define-formals) body

     Define name as a macro special form which expands into body.  This
     form can only appear where a ‘define’ form is acceptable.  Macros
     are lexically scoped.  The scope of a local macro definition
     extends from the definition to the end of the body of the
     surrounding binding construct.  Macros defined at the top level of
     a Scheme module are only visible in that module.  To have access to
     the macro definitions contained in a file, that file must be
     included either directly using the ‘include’ special form or
     indirectly with the ‘import’ special form.  Macros which are
     visible from the REPL are also visible during the compilation of
     Scheme source files.

     For example:

          (define-macro (unless test . body)
            `(if ,test #f (begin ,@body)))

          (define-macro (push var #!optional val)
            `(set! ,var (cons ,val ,var)))

     To examine the code into which a macro expands you can use the
     compiler’s ‘-expansion’ option or the ‘pp’ procedure.  For example:

          > (define-macro (push var #!optional val)
              `(set! ,var (cons ,val ,var)))
          > (pp (lambda () (push stack 1) (push stack) (push stack 3)))
          (lambda ()
            (set! stack (cons 1 stack))
            (set! stack (cons #f stack))
            (set! stack (cons 3 stack)))

 -- special form: define-syntax name expander

     Define name as a macro special form whose expansion is specified by
     expander.  This form is available only when the runtime option
     ‘-:s’ is used.  This option causes the loading of the
     ‘~~lib/syntax-case’ support library, which is the Hieb and Dybvig
     portable ‘syntax-case’ implementation which has been ported to the
     Gambit interpreter and compiler.  Note that this implementation of
     ‘syntax-case’ does not support special forms that are specific to
     Gambit.

     For example:

          $ gsi -:s
          Gambit v4.9.4

          > (define-syntax unless
              (syntax-rules ()
                ((unless test body ...)
                 (if test #f (begin body ...)))))
          > (let ((test 111)) (unless (= 1 2) (list test test)))
          (111 111)
          > (pp (lambda () (let ((test 111)) (unless (= 1 2) (list test test)))))
          (lambda () ((lambda (%%test14) (if (= 1 2) #f (list %%test14 %%test14))) 111))
          > (unless #f (pp xxx))
          *** ERROR IN (console)@7.16 -- Unbound variable: xxx

 -- procedure: compilation-target

     This procedure can only be executed during the phase of the Scheme
     code’s processing (compilation) that corresponds to macro
     expansion.  Calls to this procedure are typically contained in
     macro definitions but they can also be contained in procedures that
     are called from a macro definition’s body directly or indirectly.

     The result returned by the ‘compilation-target’ procedure gives an
     indication of the target language of the compilation.  This can be
     used to write macros that depend on the type of compilation and the
     target language.

     When the result is the symbol ‘T’ the macro expansion is in the
     context of compiling to the target language ‘T’, e.g.  ‘C’, ‘js’,
     etc.  When the result is a single element list ‘(T)’ the macro
     expansion is for the interpreter which itself was compiled for the
     target language ‘T’, e.g.  ‘(C)’, ‘(js)’, etc.

     For example:

          $ cat ct.scm
          (define (level-0)
            (string-append "0: " (object->string (compilation-target))))

          (define-macro (test)

            (define (level-1)
              (string-append "1: " (object->string (compilation-target))))

            (define-macro (level-2)
              (string-append "2: " (object->string (compilation-target))))

            `(list ,(level-1) ,(level-2)))

          (pp (test))

          (pp (level-0)) ;; run time exception
          $ gsi ct.scm
          ("1: (C)" "2: (C)")
          *** ERROR IN level-0, "ct.scm"@2.40 -- Not in compilation context
          (compilation-target)
          $ gsc -target js -exe ct.scm
          $ ./ct
          ("1: js" "2: (C)")
          *** ERROR IN level-0 -- Not in compilation context
          (compilation-target)

     Regardless of whether ‘ct.scm’ is being processed by the
     interpreter or the compiler, the body of the ‘level-0’ procedure is
     not in a compilation context and in the body of the ‘level-2’ macro
     the compilation target is ‘(C)’ indicating that the macro expansion
     is being done for interpretation.

     During the execution of the ‘level-1’ procedure, the compilation
     target will correspond to what is processing ‘ct.scm’ (interpreter
     or compiler).

     Note that the compilation target can also be tested by the
     ‘cond-expand’ special form.

 -- special form: cond-expand CE-CLAUSE ...

     The ‘cond-expand’ expression type provides a way to statically
     expand different expressions depending on the presence or absence
     of a set of features.  A CE-CLAUSE takes the following form:

          (FEATURE-REQUIREMENT EXPRESSION ...)

     The last clause can be an “else clause,” which has the form

          (else EXPRESSION)

     A FEATURE-REQUIREMENT takes one of the following forms:

        • ‘FEATURE-IDENTIFIER’

        • ‘(library LIBRARY-NAME)’

        • ‘(and FEATURE-REQUIREMENT ...)’

        • ‘(or FEATURE-REQUIREMENT ...)’

        • ‘(not FEATURE-REQUIREMENT)’

        • ‘(compilation-target TARGET ...)’

     The runtime system maintains a list of feature identifiers which
     are present, as well as a list of libraries which can be imported.
     The value of a FEATURE-REQUIREMENT is determined by replacing each
     ‘FEATURE-IDENTIFIER’ and ‘(library LIBRARY-NAME)’ on the runtime
     system’s lists with ‘#t’.  Similarly, ‘#t’ replaces each
     ‘(compilation-target TARGET ...)’ for which one of the TARGET
     matches the expansion time value of ‘(compilation-target)’, with a
     ‘TARGET’ of ‘(_)’ matching any single element list (i.e.  the
     interpreter).  All other ‘FEATURE-IDENTIFIER’, ‘(library
     LIBRARY-NAME)’, and ‘(compilation-target TARGET ...)’ are replaced
     with ‘#f’.  The resulting expression is then evaluated as a Scheme
     boolean expression under the normal interpretation of ‘and’, ‘or’,
     and ‘not’.

     A ‘cond-expand’ is then expanded by evaluating the
     FEATURE-REQUIREMENTs of successive CE-CLAUSEs in order until one of
     them returns ‘#t’.  When a true clause is found, the corresponding
     EXPRESSIONs are expanded to a ‘begin’, and the remaining clauses
     are ignored.  If none of the FEATURE-REQUIREMENTs evaluate to ‘#t’,
     then if there is an else clause, its EXPRESSIONs are included.
     Otherwise, an expansion time error is raised.  Unlike ‘cond’,
     ‘cond-expand’ does not depend on the value of any variables.

     The feature identifier ‘gambit’ is always true when the
     ‘cond-expand’ is expanded by the Gambit interpreter or compiler.

     For example:

          > (cond-expand (foobar 111) (gambit 222) (else 333))
          222
          > (cond-expand ((compilation-target js) 111) (else 222))
          222
          > (cond-expand ((compilation-target (_)) 111) (else 222))
          111

 -- special form: define-cond-expand-feature FEATURE-IDENTIFIER ...

     The ‘define-cond-expand-feature’ form can be used to add the
     feature identifiers FEATURE-IDENTIFIER ... to the list of features
     maintained by the runtime system.  These features are usable for
     the expansion of following ‘cond-expand’ forms in the same file of
     source code, and the processing of other files and REPL
     interactions.

     For example:

          > (cond-expand (foobar 111) (gambit 222) (else 333))
          222
          > (define-cond-expand-feature foobar)
          > (cond-expand (foobar 111) (gambit 222) (else 333))
          111

 -- special form: declare declaration...

     This form introduces declarations to be used by the compiler
     (currently the interpreter ignores the declarations).  This form
     can only appear where a ‘define’ form is acceptable.  Declarations
     are lexically scoped in the same way as macros.  The following
     declarations are accepted by the compiler:

     ‘(DIALECT)’
          Use the given dialect’s semantics.  DIALECT can be:
          ‘ieee-scheme’, ‘r4rs-scheme’, ‘r5rs-scheme’ or
          ‘gambit-scheme’.

     ‘(STRATEGY)’
          Select block compilation or separate compilation.  In block
          compilation, the compiler assumes that global variables
          defined in the current file that are not mutated in the file
          will never be mutated.  STRATEGY can be: ‘block’ or
          ‘separate’.

     ‘([not] inline)’
          Allow (or disallow) inlining of user procedures.

     ‘([not] inline-primitives PRIMITIVE...)’
          The given primitives should (or should not) be inlined if
          possible (all primitives if none specified).

     ‘(inlining-limit N)’
          Select the degree to which the compiler inlines user
          procedures.  N is the upper-bound, in percent, on code
          expansion that will result from inlining.  Thus, a value of
          300 indicates that the size of the program will not grow by
          more than 300 percent (i.e.  it will be at most 4 times the
          size of the original).  A value of 0 disables inlining.  The
          size of a program is the total number of subexpressions it
          contains (i.e.  the size of an expression is one plus the size
          of its immediate subexpressions).  The following conditions
          must hold for a procedure to be inlined: inlining the
          procedure must not cause the size of the call site to grow
          more than specified by the inlining limit, the site of
          definition (the ‘define’ or ‘lambda’) and the call site must
          be declared as ‘(inline)’, and the compiler must be able to
          find the definition of the procedure referred to at the call
          site (if the procedure is bound to a global variable, the
          definition site must have a ‘(block)’ declaration).  Note that
          inlining usually causes much less code expansion than
          specified by the inlining limit (an expansion around 10% is
          common for N=370).

     ‘(allocation-limit N)’
          Indicate the maximum size of objects allocated with
          ‘make-vector’, ‘make-string’, ‘make-u8vector’, etc.  Knowing
          the maximum size allows the compiler to inline calls to these
          allocators for small allocations.  This is only supported by
          the C target and only up to a size that is allowed for
          _movable objects_ (typically on the order of 1-2 KB). When N
          is an exact nonnegative integer it is the upper-bound on the
          number of elements of the allocated objects.  When N is ‘#t’ a
          dynamic test of the size is done.  When N is ‘#f’ the
          allocation operation is not inlined.

     ‘([not] lambda-lift)’
          Lambda-lift (or don’t lambda-lift) locally defined procedures.

     ‘([not] constant-fold)’
          Allow (or disallow) constant-folding of primitive procedures.

     ‘([not] standard-bindings VAR...)’
          The given global variables are known (or not known) to be
          equal to the value defined for them in the dialect (all
          variables defined in the standard if none specified).

     ‘([not] extended-bindings VAR...)’
          The given global variables are known (or not known) to be
          equal to the value defined for them in the runtime system (all
          variables defined in the runtime if none specified).

     ‘([not] run-time-bindings VAR...)’
          The given global variables will be tested at run time to see
          if they are equal to the value defined for them in the runtime
          system (all variables defined in the runtime if none
          specified).

     ‘([not] safe)’
          Generate (or don’t generate) code that will prevent fatal
          errors at run time.  Note that in ‘safe’ mode certain semantic
          errors will not be checked as long as they can’t crash the
          system.  For example the primitive ‘char=?’ may disregard the
          type of its arguments in ‘safe’ as well as ‘not safe’ mode.

     ‘([not] interrupts-enabled)’
          Generate (or don’t generate) interrupt checks.  Interrupt
          checks are used to detect user interrupts and also to check
          for stack overflows.  Interrupt checking should not be turned
          off casually.

     ‘([not] poll-on-return)’
          Generate (or don’t generate) interrupt checks on procedure
          returns (when interrupt checking is enabled).  This
          declaration has no effect on the behavior of interrupt
          checking on procedure calls, which is needed to guarantee that
          stack overflows are handled properly.

     ‘([not] debug)’
          Enable (or disable) the generation of debugging information.
          The kind of debugging information that is generated depends on
          the declarations ‘debug-location’, ‘debug-source’, and
          ‘debug-environments’.  If any of the command line options
          ‘-debug’, ‘-debug-location’, ‘-debug-source’ and
          ‘-debug-environments’ are present, the ‘debug’ declaration is
          initially enabled, otherwise it is initially disabled.  When
          all kinds of debugging information are generated there is a
          substantial increase in the C compilation time and the size of
          the generated code.  When compiling a 3000 line Scheme file it
          was observed that the total compilation time is 500% longer
          and the executable code is 150% bigger.

     ‘([not] debug-location)’
          Select (or deselect) source code location debugging
          information.  When this declaration and the ‘debug’
          declaration are in effect, run time error messages indicate
          the location of the error in the source code file.  If any of
          the command line options ‘-debug-source’ and
          ‘-debug-environments’ are present and ‘-debug-location’ is
          absent, the ‘debug-location’ declaration is initially
          disabled, otherwise it is initially enabled.  When compiling a
          3000 line Scheme file it was observed that the total
          compilation time is 200% longer and the executable code is 60%
          bigger.

     ‘([not] debug-source)’
          Select (or deselect) source code debugging information.  When
          this declaration and the ‘debug’ declaration are in effect,
          run time error messages indicate the source code, the
          backtraces are more precise, and the ‘pp’ procedure will
          display the source code of compiled procedures.  If any of the
          command line options ‘-debug-location’ and
          ‘-debug-environments’ are present and ‘-debug-source’ is
          absent, the ‘debug-source’ declaration is initially disabled,
          otherwise it is initially enabled.  When compiling a 3000 line
          Scheme file it was observed that the total compilation time is
          90% longer and the executable code is 90% bigger.

     ‘([not] debug-environments)’
          Select (or deselect) environment debugging information.  When
          this declaration and the ‘debug’ declaration are in effect,
          the debugger will have access to the environments of the
          continuations.  In other words the local variables defined in
          compiled procedures (and not optimized away by the compiler)
          will be shown by the ‘,e’ REPL command.  If any of the command
          line options ‘-debug-location’ and ‘-debug-source’ are present
          and ‘-debug-environments’ is absent, the ‘debug-environments’
          declaration is initially disabled, otherwise it is initially
          enabled.  When compiling a 3000 line Scheme file it was
          observed that the total compilation time is 70% longer and the
          executable code is 40% bigger.

     ‘([not] proper-tail-calls)’
          Generate (or don’t generate) proper tail calls.  When proper
          tail calls are turned off, tail calls are handled like
          non-tail calls, that is a continuation frame will be created
          for all calls regardless of their kind.  This is useful for
          debugging because the caller of a procedure will be visible in
          the backtrace produced by the REPL’s ‘,b’ command even when
          the call is a tail call.  Be advised that this does cause
          stack space to be consumed for tail calls which may cause the
          stack to overflow when performing long iterations with tail
          calls (whether they are expressed with a ‘letrec’, named
          ‘let’, ‘do’, or other form).

     ‘([not] generative-lambda)’
          Force (or don’t force) the creation of fresh closures when
          evaluating lambda-expressions.  A fresh closure is always
          created when a lambda-expression has at least one free
          variable (that has not been eliminated by dead-code
          elimination or other compiler optimization) or when the
          generative-lambda declaration is turned on.  When a
          lambda-expression has no free variables and the
          generative-lambda declaration is turned off, the value of the
          lambda-expression may be the same procedure (in the sense of
          ‘eq?’).

     ‘([not] optimize-dead-local-variables)’
          Remove (or preserve) the dead local variables in the
          environment.  Preserving the dead local variables is useful
          for debugging because continuations will contain the dead
          variables.  Thus, if the code is also compiled with the
          declaration ‘debug-environments’ the ‘,e’, ‘,ed’, ‘,be’, and
          ‘,bed’ REPL commands will display the dead variables.  On the
          other hand, preserving the dead local variables may change the
          space complexity of the program (i.e.  some of the data that
          would normally be reclaimed by the garbage collector will not
          be).  Note that due to other compiler optimizations some dead
          local variables may be removed regardless of this declaration.

     ‘([not] optimize-dead-definitions VAR...)’
          Remove (or preserve) the dead toplevel definitions of the
          given global variables (all global variables if none
          specified).  A toplevel definition is dead if it is not
          referenced by toplevel expressions of the program or toplevel
          definitions that aren’t dead (regardless of the evaluation of
          its expression causing a side-effect).  When a module is
          separately compiled and some of its definitions are only used
          by other modules, this declaration must be used with care to
          keep definitions that are used by other modules, for example
          if ‘foo’ is referenced in another module the following
          declaration should be used: ‘(declare (not
          optimize-dead-definitions foo))’.

     ‘(NUMBER-TYPE PRIMITIVE...)’
          Numeric arguments and result of the specified primitives are
          known to be of the given type (all primitives if none
          specified).  NUMBER-TYPE can be: ‘generic’, ‘fixnum’, or
          ‘flonum’.

     ‘(MOSTLY-NUMBER-TYPE PRIMITIVE...)’
          Numeric arguments and result of the specified primitives are
          expected to be most often of the given type (all primitives if
          none specified).  MOSTLY-NUMBER-TYPE can be: ‘mostly-generic’,
          ‘mostly-fixnum’, ‘mostly-fixnum-flonum’, ‘mostly-flonum’, or
          ‘mostly-flonum-fixnum’.

     The default declarations used by the compiler are equivalent to:

          (declare
            (gambit-scheme)
            (separate)
            (inline)
            (inline-primitives)
            (inlining-limit 370)
            (allocation-limit #t)
            (constant-fold)
            (lambda-lift)
            (not standard-bindings)
            (not extended-bindings)
            (run-time-bindings)
            (safe)
            (interrupts-enabled)
            (not poll-on-return)
            (not debug)           ;; depends on debugging command line options
            (debug-location)      ;; depends on debugging command line options
            (debug-source)        ;; depends on debugging command line options
            (debug-environments)  ;; depends on debugging command line options
            (proper-tail-calls)
            (not generative-lambda)
            (optimize-dead-local-variables)
            (not optimize-dead-definitions)
            (generic)
            (mostly-fixnum-flonum)
          )

     These declarations are compatible with the semantics of R5RS Scheme
     and includes a few procedures from R6RS (mainly fixnum specific and
     flonum specific procedures).  Typically used declarations that
     enhance performance, at the cost of violating the R5RS Scheme
     semantics, are: ‘(standard-bindings)’, ‘(block)’, ‘(not safe)’ and
     ‘(fixnum)’.

 -- procedure: continuation? OBJ
 -- procedure: continuation-capture PROC
 -- procedure: continuation-graft CONT PROC OBJ...
 -- procedure: continuation-return CONT OBJ...

     These procedures provide access to internal first-class
     continuations which are represented using continuation objects
     distinct from procedures.

     The procedure ‘continuation?’ returns ‘#t’ when OBJ is a
     continuation object and ‘#f’ otherwise.

     The procedure ‘continuation-capture’ is similar to the ‘call/cc’
     procedure but it represents the continuation with a continuation
     object.  The PROC parameter must be a procedure accepting a single
     argument.  The procedure ‘continuation-capture’ reifies its
     continuation and calls PROC with the corresponding continuation
     object as its sole argument.  Like for ‘call/cc’, the implicit
     continuation of the call to PROC is the implicit continuation of
     the call to ‘continuation-capture’.

     The procedure ‘continuation-graft’ performs a procedure call to the
     procedure PROC with arguments OBJ... and the implicit continuation
     corresponding to the continuation object CONT.  The current
     continuation of the call to procedure ‘continuation-graft’ is
     ignored.

     The procedure ‘continuation-return’ invokes the implicit
     continuation corresponding to the continuation object CONT with the
     result(s) OBJ....  This procedure can be easily defined in terms of
     ‘continuation-graft’:

          (define (continuation-return cont . objs)
            (continuation-graft cont apply values objs))

     For example:

          > (define x #f)
          > (define p (make-parameter 11))
          > (pp (parameterize ((p 22))
                  (cons 33 (continuation-capture
                            (lambda (c) (set! x c) 44)))))
          (33 . 44)
          > x
          #<continuation #2>
          > (continuation-return x 55)
          (33 . 55)
          > (continuation-graft x (lambda () (expt 2 10)))
          (33 . 1024)
          > (continuation-graft x expt 2 10)
          (33 . 1024)
          > (continuation-graft x (lambda () (p)))
          (33 . 22)
          > (define (map-sqrt1 lst)
              (call/cc
               (lambda (k)
                 (map (lambda (x)
                        (if (< x 0)
                            (k 'error)
                            (sqrt x)))
                      lst))))
          > (map-sqrt1 '(1 4 9))
          (1 2 3)
          > (map-sqrt1 '(1 -1 9))
          error
          > (define (map-sqrt2 lst)
              (continuation-capture
               (lambda (c)
                 (map (lambda (x)
                        (if (< x 0)
                            (continuation-return c 'error)
                            (sqrt x)))
                      lst))))
          > (map-sqrt2 '(1 4 9))
          (1 2 3)
          > (map-sqrt2 '(1 -1 9))
          error

 -- procedure: display-exception EXC [PORT]
 -- procedure: display-exception-in-context EXC CONT [PORT]
 -- procedure: display-procedure-environment PROC [PORT]
 -- procedure: display-continuation-environment CONT [PORT]
 -- procedure: display-continuation-dynamic-environment CONT [PORT]

 -- procedure: display-continuation-backtrace CONT [PORT [DISPLAY-ENV?
          [ALL-FRAMES? [MAX-HEAD [MAX-TAIL [DEPTH]]]]]]

     The procedure ‘display-continuation-backtrace’ displays the frames
     of the continuation corresponding to the continuation object CONT
     on the port PORT.  If it is not specified, PORT defaults to the
     current output-port.  The frames are displayed in the same format
     as the REPL’s ‘,b’ command.

     The parameter DISPLAY-ENV?, which defaults to ‘#f’, controls if the
     frames are displayed with its environment (the variables accessible
     and their bindings).

     The parameter ALL-FRAMES?, which defaults to ‘#f’, controls which
     frames are displayed.  Some frames of ancillary importance, such as
     internal frames created by the interpreter, are not displayed when
     ALL-FRAMES? is ‘#f’.  Otherwise all frames are displayed.

     The parameters MAX-HEAD and MAX-TAIL, which default to 10 and 4
     respectively, control how many frames are displayed at the head and
     tail of the continuation.

     The parameter DEPTH, which defaults to 0, causes the frame numbers
     to be offset by that value.

     For example:

          > (define x #f)
          > (define (fib n)
              (if (< n 2)
                  (continuation-capture
                   (lambda (c) (set! x c) 1))
                  (+ (fib (- n 1))
                     (fib (- n 2)))))
          > (fib 10)
          89
          > (display-continuation-backtrace x)
          0  fib             (console)@7:12     (fib (- n 2))
          1  fib             (console)@7:12     (fib (- n 2))
          2  fib             (console)@7:12     (fib (- n 2))
          3  fib             (console)@7:12     (fib (- n 2))
          4  fib             (console)@7:12     (fib (- n 2))
          5  (interaction)   (console)@8:1      (fib 10)
          #f
          > (display-continuation-backtrace x (current-output-port) #f #t)
          0  fib             (console)@7:12     (fib (- n 2))
          1  fib             (console)@6:9      (+ (fib (- n 1)) (fib (- ...
          2  fib             (console)@7:12     (fib (- n 2))
          3  fib             (console)@6:9      (+ (fib (- n 1)) (fib (- ...
          4  fib             (console)@7:12     (fib (- n 2))
          5  fib             (console)@6:9      (+ (fib (- n 1)) (fib (- ...
          6  fib             (console)@7:12     (fib (- n 2))
          7  fib             (console)@6:9      (+ (fib (- n 1)) (fib (- ...
          8  fib             (console)@7:12     (fib (- n 2))
          9  fib             (console)@6:9      (+ (fib (- n 1)) (fib (- ...
          ...
          13 ##with-no-result-expected-toplevel
          14 ##repl-debug
          15 ##repl-debug-main
          16 ##kernel-handlers
          #f
          > (display-continuation-backtrace x (current-output-port) #t #f)
          0  fib             (console)@7:12     (fib (- n 2))
                  n = 2
          1  fib             (console)@7:12     (fib (- n 2))
                  n = 4
          2  fib             (console)@7:12     (fib (- n 2))
                  n = 6
          3  fib             (console)@7:12     (fib (- n 2))
                  n = 8
          4  fib             (console)@7:12     (fib (- n 2))
                  n = 10
          5  (interaction)   (console)@8:1      (fib 10)
          #f
          > (display-continuation-backtrace x (current-output-port) #f #f 2 1 100)
          100 fib            (console)@7:12     (fib (- n 2))
          101 fib            (console)@7:12     (fib (- n 2))
          ...
          105 (interaction)  (console)@8:1      (fib 10)
          #f

6.4 Undocumented extensions
===========================

The procedures in this section are not yet documented.

 -- procedure: make-thread-group [NAME [THREAD-GROUP]]
 -- procedure: thread-group? OBJ
 -- procedure: thread-group-name THREAD-GROUP
 -- procedure: thread-group-parent THREAD-GROUP
 -- procedure: thread-group-resume! THREAD-GROUP
 -- procedure: thread-group-suspend! THREAD-GROUP
 -- procedure: thread-group-terminate! THREAD-GROUP
 -- procedure: thread-group->thread-group-list THREAD-GROUP
 -- procedure: thread-group->thread-group-vector THREAD-GROUP
 -- procedure: thread-group->thread-list THREAD-GROUP
 -- procedure: thread-group->thread-vector THREAD-GROUP
 -- procedure: thread-group-specific THREAD-GROUP
 -- procedure: thread-group-specific-set! THREAD-GROUP OBJ

 -- procedure: thread-state THREAD
 -- procedure: thread-state-uninitialized? THREAD-STATE
 -- procedure: thread-state-initialized? THREAD-STATE
 -- procedure: thread-state-running? THREAD-STATE
 -- procedure: thread-state-running-processor THREAD-STATE
 -- procedure: thread-state-waiting? THREAD-STATE
 -- procedure: thread-state-waiting-for THREAD-STATE
 -- procedure: thread-state-waiting-timeout THREAD-STATE
 -- procedure: thread-state-normally-terminated? THREAD-STATE
 -- procedure: thread-state-normally-terminated-result THREAD-STATE
 -- procedure: thread-state-abnormally-terminated? THREAD-STATE
 -- procedure: thread-state-abnormally-terminated-reason THREAD-STATE
 -- procedure: top [TIMEOUT [THREAD-GROUP [PORT]]]

 -- procedure: thread-interrupt! THREAD [THUNK]

 -- procedure: thread-suspend! THREAD
 -- procedure: thread-resume! THREAD

 -- procedure: thread-thread-group THREAD

 -- special form: define-type-of-thread name field...

 -- procedure: thread-init! THREAD THUNK [NAME [THREAD-GROUP]]

 -- procedure: initialized-thread-exception? OBJ
 -- procedure: initialized-thread-exception-procedure EXC
 -- procedure: initialized-thread-exception-arguments EXC

 -- procedure: uninitialized-thread-exception? OBJ
 -- procedure: uninitialized-thread-exception-procedure EXC
 -- procedure: uninitialized-thread-exception-arguments EXC

 -- procedure: inactive-thread-exception? OBJ
 -- procedure: inactive-thread-exception-procedure EXC
 -- procedure: inactive-thread-exception-arguments EXC

 -- procedure: rpc-remote-error-exception? OBJ
 -- procedure: rpc-remote-error-exception-procedure EXC
 -- procedure: rpc-remote-error-exception-arguments EXC
 -- procedure: rpc-remote-error-exception-message EXC

 -- procedure: invalid-utf8-encoding-exception? OBJ
 -- procedure: invalid-utf8-encoding-exception-procedure EXC
 -- procedure: invalid-utf8-encoding-exception-arguments EXC

 -- procedure: module-not-found-exception? OBJ
 -- procedure: module-not-found-exception-procedure EXC
 -- procedure: module-not-found-exception-arguments EXC

 -- procedure: processor? OBJ
 -- procedure: current-processor
 -- procedure: processor-id PROCESSOR

 -- procedure: timeout->time TIMEOUT

 -- procedure: current-second

 -- procedure: current-jiffy

 -- procedure: jiffies-per-second

 -- procedure: get-environment-variable NAME

 -- procedure: get-environment-variables

 -- procedure: executable-path

 -- procedure: command-name
 -- procedure: command-args

 -- procedure: script-file
 -- procedure: script-directory

 -- procedure: open-dummy

 -- procedure: port-settings-set! PORT SETTINGS

 -- procedure: port-io-exception-handler-set! PORT HANDLER

 -- procedure: input-port-bytes-buffered PORT

 -- procedure: input-port-characters-buffered PORT

 -- procedure: nonempty-input-port-character-buffer-exception? OBJ
 -- procedure: nonempty-input-port-character-buffer-exception-arguments
          EXC
 -- procedure: nonempty-input-port-character-buffer-exception-procedure
          EXC

 -- procedure: repl-input-port
 -- procedure: repl-output-port
 -- procedure: repl-error-port
 -- procedure: console-port

 -- procedure: current-user-interrupt-handler [HANDLER]
 -- procedure: default-user-interrupt-handler
 -- procedure: defer-user-interrupts

 -- procedure: primordial-exception-handler EXC

 -- procedure: err-code->string CODE

 -- procedure: foreign? OBJ
 -- procedure: foreign-tags FOREIGN
 -- procedure: foreign-address FOREIGN
 -- procedure: foreign-release! FOREIGN
 -- procedure: foreign-released? FOREIGN

 -- procedure: invalid-hash-number-exception? OBJ
 -- procedure: invalid-hash-number-exception-procedure EXC
 -- procedure: invalid-hash-number-exception-arguments EXC

 -- procedure: tcp-client-local-socket-info TCP-CLIENT-PORT
 -- procedure: tcp-client-peer-socket-info TCP-CLIENT-PORT
 -- procedure: tcp-client-self-socket-info TCP-CLIENT-PORT

 -- procedure: tcp-server-socket-info TCP-SERVER-PORT

 -- procedure: socket-info? OBJ
 -- procedure: socket-info-address SOCKET-INFO
 -- procedure: socket-info-family SOCKET-INFO
 -- procedure: socket-info-port-number SOCKET-INFO

 -- procedure: system-version
 -- procedure: system-version-string

 -- procedure: system-type
 -- procedure: system-type-string
 -- procedure: configure-command-string

 -- procedure: system-stamp

 -- special form: future EXPR
 -- procedure: touch OBJ

 -- procedure: tty? OBJ
 -- procedure: tty-history TTY
 -- procedure: tty-history-set! TTY HISTORY
 -- procedure: tty-history-max-length-set! TTY N
 -- procedure: tty-paren-balance-duration-set! TTY DURATION
 -- procedure: tty-text-attributes-set! TTY ATTRIBUTES
 -- procedure: tty-mode-set! TTY MODE
 -- procedure: tty-type-set! TTY TYPE

 -- procedure: with-input-from-port PORT THUNK
 -- procedure: with-output-to-port PORT THUNK

 -- procedure: input-port-char-position PORT
 -- procedure: output-port-char-position PORT

 -- procedure: open-event-queue SELECTOR

 -- procedure: main ...

 -- procedure: dead-end

 -- procedure: poll-point

 -- special form: define-record-type ...
 -- special form: define-type ...

 -- special form: this-source-file

 -- special form: receive ...

 -- special form: define-values ...

 -- special form: define-module-alias ...

 -- special form: r7rs-guard ...

 -- special form: case-lambda ...
 -- special form: syntax-case ...
 -- special form: syntax ...

 -- procedure: datum->syntax OBJ
 -- procedure: syntax->datum STX
 -- procedure: syntax->list STX
 -- procedure: syntax->vector STX

 -- procedure: length+ CLIST

 -- procedure: car+cdr PAIR

 -- procedure: first PAIR
 -- procedure: second PAIR
 -- procedure: third PAIR
 -- procedure: fourth PAIR
 -- procedure: fifth PAIR
 -- procedure: sixth PAIR
 -- procedure: seventh PAIR
 -- procedure: eighth PAIR
 -- procedure: ninth PAIR
 -- procedure: tenth PAIR

 -- procedure: not-pair? X

 -- procedure: null-list? LIST

 -- procedure: proper-list? S

 -- procedure: circular-list? S

 -- procedure: dotted-list? S

 -- procedure: filter PRED LIST
 -- procedure: remove PRED LIST
 -- procedure: remq ELEM LIST

 -- procedure: concatenate LIST-OF-LISTS [SEPARATOR]
 -- procedure: concatenate! LIST-OF-LISTS

 -- procedure: list= ELT= LIST ...

 -- procedure: list-set LIST K VAL
 -- procedure: list-set! LIST K VAL

 -- procedure: fold PROC BASE LIST ...
 -- procedure: fold-right PROC BASE LIST ...

 -- procedure: iota COUNT [START [STEP]]

 -- procedure: circular-list X Y...

 -- procedure: cons* X Y...

 -- procedure: list-copy LIST

 -- procedure: list-tabulate N INIT-PROC

 -- procedure: make-list N [FILL]

 -- procedure: reverse! LIST

 -- procedure: append-reverse LIST TAIL
 -- procedure: append-reverse! LIST TAIL

 -- procedure: xcons D A

 -- procedure: take X I
 -- procedure: drop X I

 -- procedure: last PAIR
 -- procedure: last-pair PAIR

 -- procedure: list-sort PROC LIST
 -- procedure: list-sort! PROC LIST

 -- procedure: finite? X
 -- procedure: infinite? X
 -- procedure: nan? X

 -- procedure: acosh X

 -- procedure: asinh X

 -- procedure: atanh X

 -- procedure: cosh X

 -- procedure: sinh X

 -- procedure: tanh X

 -- procedure: conjugate X

 -- procedure: bits BOOL...
 -- procedure: bits->list I [LEN]
 -- procedure: list->bits LIST
 -- procedure: bits->vector I [LEN]
 -- procedure: vector->bits VECTOR

 -- special form: six.infix DATUM
 -- undefined: six.!
 -- special form: six.!x X
 -- special form: six.&x X
 -- special form: six.**x X
 -- special form: six.*x X
 -- special form: six.++x X
 -- special form: six.+x X
 -- special form: six.--x X
 -- special form: six.-x X
 -- special form: six.arrow EXPR IDENT
 -- undefined: six.asyncx X
 -- undefined: six.awaitx X
 -- undefined: six.break
 -- special form: six.call FUNC ARG...
 -- undefined: six.case
 -- undefined: six.clause
 -- special form: six.compound STATEMENT...
 -- special form: six.cons X Y
 -- undefined: six.continue
 -- special form: six.define-procedure IDENT PROC
 -- special form: six.define-variable IDENT TYPE DIMS INIT
 -- special form: six.do-while STAT EXPR
 -- special form: six.dot EXPR IDENT
 -- special form: six.for STAT1 EXPR2 EXPR3 STAT2
 -- undefined: six.goto EXPR
 -- undefined: six.from-import EXPR1 EXPR2
 -- undefined: six.from-import-* EXPR
 -- special form: six.identifier IDENT
 -- special form: six.if EXPR STAT1 [STAT2]
 -- undefined: six.import EXPR
 -- special form: six.index EXPR1 EXPR2
 -- undefined: six.label IDENT STAT
 -- special form: six.list X Y
 -- special form: six.literal VALUE
 -- procedure: six.make-array INIT DIM...
 -- special form: six.new IDENT ARG...
 -- special form: six.null
 -- special form: six.procedure TYPE PARAMS STAT
 -- special form: six.procedure-body STAT...
 -- undefined: six.return
 -- undefined: six.switch
 -- undefined: six.typeofx X
 -- special form: six.while EXPR STAT...
 -- special form: six.x!==y X Y
 -- special form: six.x!=y X Y
 -- special form: six.x%=y X Y
 -- special form: six.x%y X Y
 -- special form: six.x&&y X Y
 -- special form: six.x&=y X Y
 -- special form: six.x&y X Y
 -- special form: six.x**=y X Y
 -- special form: six.x**y X Y
 -- special form: six.x*=y X Y
 -- special form: six.x*y X Y
 -- special form: six.x@=y X Y
 -- special form: six.x@y X Y
 -- special form: six.x++ X
 -- special form: six.x+=y X Y
 -- special form: six.x+y X Y
 -- special form: |six.x,y| X Y
 -- special form: six.x-- X
 -- special form: six.x-=y X Y
 -- special form: six.x-y X Y
 -- special form: six.x//=y X Y
 -- special form: six.x//y X Y
 -- special form: six.x/=y X Y
 -- special form: six.x/y X Y
 -- undefined: six.x:-y X Y
 -- special form: six.x:=y X Y
 -- special form: six.x:y X Y
 -- special form: six.x<<=y X Y
 -- special form: six.x<<y X Y
 -- special form: six.x<=y X Y
 -- special form: six.x<y X Y
 -- special form: six.x===y X Y
 -- special form: six.x==y X Y
 -- special form: six.x=y X Y
 -- special form: six.x>=y X Y
 -- special form: six.x>>>=y X Y
 -- special form: six.x>>>y X Y
 -- special form: six.x>>=y X Y
 -- special form: six.x>>y X Y
 -- special form: six.x>y X Y
 -- special form: six.x?y:z X Y Z
 -- special form: six.x^=y X Y
 -- special form: six.x^y X Y
 -- special form: |six.x\|=y| X Y
 -- special form: |six.x\|y| X Y
 -- special form: |six.x\|\|y| X Y
 -- special form: six.xandy X Y
 -- undefined: six.xinstanceofy X Y
 -- special form: six.xiny X Y
 -- special form: six.xisy X Y
 -- special form: six.notx X Y
 -- special form: six.xory X Y
 -- special form: six.~x X
 -- undefined: six.yieldx X

 -- procedure: readtable-comment-handler READTABLE
 -- procedure: readtable-comment-handler-set READTABLE NEW-VALUE

 -- procedure: open-output-bytevector [U8VECTOR-OR-SETTINGS]

7 Modules
*********

Gambit supports multiple modularization approaches and constructs:
legacy modules, primitive modules and R7RS compatible modules.  These
are described in that order, which corresponds to increased abstraction
level.  Unless there is a need for detailed control over the modules, it
is best to use the R7RS compatible module system for the development of
new code.

7.1 Legacy Modules
==================

The legacy way of modularizing code, which was popular up to R5RS, is
still supported by Gambit.  It consists of using the ‘load’ procedure
and the ‘include’ form.  We discuss it first to introduce some useful
terms and explain the shortcomings of this modularization approach.

 -- procedure: load PATH

     The ‘load’ procedure’s PATH argument, a string, specifies the
     location in the file system of a file to load.  Loading a file
     executes the code contained in the file, which is either source
     code or compiled code (a dynamically loadable “object file” created
     by the Gambit Scheme compiler, see the procedure ‘compile-file’).
     When PATH has no extension the ‘load’ procedure first attempts to
     load the file with no extension as a Scheme source file.  If that
     file doesn’t exist it will search for both a source file and an
     object file.  The object file’s path is obtained by adding to PATH
     a ‘.oN’ extension with the highest consecutive version number
     starting with 1.  The source file’s path is obtained by adding to
     PATH the file extensions ‘.sld’, ‘.scm’ and ‘.six’ (the first found
     is the source file).  If both a source file and an object file
     exist, then the one with the latest modification time is loaded.
     Otherwise the file that is found is loaded.  When PATH has an
     extension, the ‘load’ procedure will only attempt to load the file
     with that specific extension.  After executing the code contained
     in the file, the ‘load’ procedure returns the path of the file that
     was loaded.

     When a source code file is loaded its extension is used to
     determine how it is parsed, unless the file’s first line is a
     special script line (see *note Scheme scripts::).  When the
     extension is different from ‘.six’ the content of the file is
     parsed using the normal Scheme prefix syntax.  When the extension
     is ‘.six’ the content of the file is parsed using the Scheme infix
     syntax extension (see *note Scheme infix syntax extension::).

     Due to operating system limitations, loading a given ‘.oN’ object
     file more than once in the same process is not supported.  It is
     possible however to recompile the source code file to create a new
     ‘.oM’ object file with M > N and load that object file.

     For example:

          $ cat my-mod.scm
          (define (double x) (* x 2))
          (display "my-mod has finished loading!!!\n")
          $ gsi
          Gambit v4.9.4

          > (load "my-mod")
          my-mod has finished loading!!!
          "/Users/feeley/gambit/doc/my-mod.scm"
          > (double 21)
          42
          > (load "my-mod.scm")
          my-mod has finished loading!!!
          "/Users/feeley/gambit/doc/my-mod.scm"
          > ,q
          $ gsc my-mod
          $ gsi
          Gambit v4.9.4

          > (load "my-mod")
          my-mod has finished loading!!!
          "/Users/feeley/gambit/doc/my-mod.o1"
          > (double 21)
          42
          > (load "my-mod")
          *** ERROR IN (console)@3.1 -- Can't load a given object file more than once
          (load "my-mod")
          1>

     Note that any macro definition in the loaded file is local to the
     file and is not visible from the REPL or other files that loaded
     this file.  The ‘include’ form can be used to access the macros
     defined in another file.

 -- special form: include PATH
 -- special form: ##include PATH

     The path argument must be a string specifying the location of an
     existing file containing Scheme source code.  Relative paths are
     relative to the file that contains the ‘include’ form.  The
     ‘include’ special form splices the content of the specified source
     file.  This form can only appear where a ‘define’ form is
     acceptable, i.e.  at top level or in the body of a binding form.

     For example:

          $ cat my-defs.scm
          (define-macro (double x) `(* ,x 2))
          (define (quad y) (double (double y)))
          (display "howdy!\n")
          $ cat my-includer.scm
          (define (f x)
            (include "my-defs.scm")
            (+ 1 (quad x)))
          $ gsi
          Gambit v4.9.4

          > (load "my-includer")
          "/Users/feeley/udem-dlteam/gambit/my-includer.scm"
          > (f 10)
          howdy!
          41
          > (f 20)
          howdy!
          81

   With legacy modularization, the code that implements the module’s
functionality is put in a source code file and this module is accessed
by other code by using a ‘load’ or ‘include’ of that file.  Here is an
example of an ‘angle0’ module that is used by an ‘app0’ main program:

     ;;;---------------------------------------- file: angle0/angle0.scm
     (define factor (/ (atan 1) 45))
     (define (deg->rad x) (* x factor))
     (define (rad->deg x) (/ x factor))

     ;;;---------------------------------------- file: app0.scm
     (load "angle0/angle0.scm")   ;; or (include "angle0/angle0.scm")
     (println "90 degrees is " (deg->rad 90) " radians")

     ;; run with:  gsi app0.scm

   This modularization approach has a number of issues:

   • It hinders code sharing among different programs and users because
     a shared module’s location in the filesystem must be known to all
     modules loading or including it.  In the above example the path
     ‘"angle0/angle0.scm"’ is relative so the ‘load’ procedure will
     resolve the path incorrectly if the program executes
     ‘(current-directory "...")’ before calling ‘load’.

   • When a module is needed by more than one other module there will be
     code duplication, redundant evaluation/compilation, and probably
     incorrect execution if the module has side effects that should only
     happen once (displaying a message, opening a database on the
     filesystem, initializing the module’s state, etc).  Moreover, when
     the module has been compiled to an object file it can’t be loaded
     more than once.

   • All the definitions of a module will be put in the global
     environment (including top level macro definitions when using a top
     level ‘include’ but not when using ‘load’).  This pollutes the
     global environment with definitions that were not intended to be
     exported by the module’s designer, such as the variable ‘factor’ in
     the above example that is only meant to be used by the ‘deg->rad’
     and ‘rad->deg’ procedures.  Other modules may also need a ‘factor’
     variable internally, for instance to convert distances from the
     metric to the english system.  Nothing prevents such accidental
     clashes.

7.2 Primitive Modules
=====================

7.2.1 ‘##demand-module’ and ‘##supply-module’ forms
---------------------------------------------------

The ‘##demand-module’ form offers a way to avoid the issues of multiple
loading and filesystem localization of modules.  The sole parameter of
this form is an (unevaluated) symbol that identifies the module on which
the module containing the ‘##demand-module’ depends.  When a module ‘A’
contains a ‘(##demand-module B)’, Gambit’s runtime system will ensure
that module ‘B’ is loaded before module ‘A’ is loaded.  It also
registers the module in a cache when it is loaded so that it is loaded
exactly once.  In other words the ‘##demand-module’ form expresses the
requirement that the current module needs the functionality of another
module.  A module can contain multiple uses of ‘##demand-module’ and
possibly more than once for a given module.  The ‘##demand-module’ form
can appear anywhere a ‘define’ can appear.  There is also a related
‘##supply-module’ form that should appear in the module to declare the
module’s identity.

   Gambit’s runtime system searches for modules in various directories,
by default in ‘~~lib’ then in ‘~~userlib’ (which maps to
‘~/.gambit_userlib’ by default).  These two directories are where
builtin modules and user installed modules are located respectively.
The source code for a module ‘M’ is searched, in each of the “module
search order” directories, first in ‘M/M.EXT’ and then in ‘M.EXT’, where
.EXT is one of the acceptable Scheme source code file extensions
(‘.sld’, ‘.scm’, ‘.six’, etc).  The list of module search order
directories can be extended with the ‘-:search=’DIR runtime option or by
a command line argument to ‘gsi’ and ‘gsc’ that ends with a path
separator or a ‘.’.

   With ‘##demand-module’ and ‘##supply-module’ the previous example can
be rewritten like this:

     ;;;---------------------------------------- file: angle1/angle1.scm
     (##supply-module angle1) ;; declare that this is the module angle1
     (define factor (/ (atan 1) 45))
     (define (deg->rad x) (* x factor))
     (define (rad->deg x) (/ x factor))

     ;;;---------------------------------------- file: app1.scm
     (##demand-module angle1) ;; declare dependency on module angle1
     (println "90 degrees is " (deg->rad 90) " radians")

     ;; run with either:  gsi . app1.scm
     ;;              or:  gsi -:search=. app1.scm
     ;;
     ;; or install the angle1 module to avoid the . and -:search=.

7.2.2 ‘##namespace’ and ‘##import’ forms
----------------------------------------

 -- special form: namespace ...
 -- special form: ##namespace ...
 -- special form: import MODULE-REF
 -- special form: ##import MODULE-REF

   The ‘##namespace’ form offers a way to avoid name clashes by
specifying a mapping between identifiers.  The mapping it specifies has
the same scope as a macro definition: it applies to the rest of a source
code file if it is at top level, or applies to the rest of the body of a
binding form if it is used in the body of a binding form.  The call
‘(##namespace ("foo#" a b))’ specifies that a reference to ‘a’ becomes
‘foo#a’ and a reference to ‘b’ becomes ‘foo#b’.  Here ‘foo#’ is the
namespace.  Finer control over the mapping is possible by using aliases
as in ‘(##namespace ("foo#" (a bar) b))’ which maps ‘a’ to ‘foo#bar’ and
‘b’ to ‘foo#b’.  Multiple namespace specifications can appear in the
body of the ‘##namespace’ form.  When no identifiers are specified, the
mapping maps all identifiers not containing ‘#’ to the namespace.  For
example in the scope of ‘(##namespace ("foo#"))’ the reference ‘x’ maps
to ‘foo#x’ and the reference ‘bar#x’ remains unchanged.

   Given that modules are identified by a unique symbol, the global
names defined by a module ‘M’ can be put in the namespace ‘M#’ to avoid
name clashes with other modules.  The source code of module ‘M’ and the
modules depending on ‘M’ can explicitly prefix the global names defined
by ‘M’ with ‘M#’ or use a ‘##namespace’ form to make this prefixing
implicit.  By convention the namespace definition for the identifiers
exported by module ‘M’ is specified in the source code file ‘M#.scm’ in
the same directory as the ‘M.scm’ file.

   Using this convention and the ‘include’ and ‘##namespace’ forms, the
previous example can be rewritten like this:

     ;;;---------------------------------------- file: angle2/angle2#.scm
     (##namespace ("angle2#" deg->rad rad->deg))

     ;;;---------------------------------------- file: angle2/angle2.scm
     (include "angle2#.scm")
     (##namespace ("angle2#" factor))
     (##supply-module angle2)
     (define factor (/ (atan 1) 45))
     (define (deg->rad x) (* x factor))
     (define (rad->deg x) (/ x factor))

     ;;;---------------------------------------- file: app2.scm
     (include "angle2/angle2#.scm")
     (##demand-module angle2)
     (println "90 degrees is " (deg->rad 90) " radians")

   Note that the parameters of the two ‘include’ forms are different,
but this is correct because the paths are relative to the file
containing the ‘include’ form.  However the module localization problem
has been reintroduced for the file ‘angle2/angle2#.scm’.

   This problem can be solved using the ‘##import’ form that combines
the semantics of the ‘include’ and ‘##demand-module’ forms.  The call
‘(##import M)’ will use the module search order directories to locate
the source code file of module ‘M’ and will expand to an ‘include’ of
the “hash” file ‘M#.EXT’ if it exists in the same directory, and a
‘(##demand-module M)’.

   In addition, a builtin module ‘gambit’ exists that contains all the
global names exported by the runtime library.  The ‘gambit’ module’s
“hash” file ‘gambit#.scm’ contains a ‘##namespace’ form that lists all
the names exported by the runtime library in an empty namespace:

     ;;;---------------------------------------- file: ~~lib/gambit#.scm
     (##namespace ("" define if quote set! cons car cdr + - * / ;; etc

   Using the ‘gambit’ module and the ‘##import’ form, the previous
example can be rewritten like this:

     ;;;---------------------------------------- file: angle3/angle3#.scm
     (##namespace ("angle3#" deg->rad rad->deg))

     ;;;---------------------------------------- file: angle3/angle3.scm
     (##namespace ("angle3#")) ;; map all identifiers to angle3# namespace
     (##import gambit)         ;; except those defined by Gambit's RTS
     (##supply-module angle3)
     (define factor (/ (atan 1) 45))
     (define (deg->rad x) (* x factor))
     (define (rad->deg x) (/ x factor))

     ;;;---------------------------------------- file: app3.scm
     (##import angle3)
     (println "90 degrees is " (deg->rad 90) " radians")

   In this example the ‘(##import angle3)’ takes care of the namespace
mapping and the loading of ‘angle3.scm’ because it is equivalent to:

     (begin
      (##include "angle3/angle3#.scm")
      (##demand-module angle3))

7.2.3 Macros
------------

In addition to procedures, a module ‘M’ may export macros.  The file
‘M#.scm’ is the designated place to put exported macro definitions.
These macro definitions will essentially be copied at the point where
the ‘##import’ of the module is done.  Macros that are needed strictly
for the implementation of a module may be defined in the file ‘M.scm’
and these macro definitions will not be visible elsewhere.  Note that
the macros defined with ‘define-macro’ are not hygienic, so the macro
definition writer should take care to explicitly indicate what
identifiers resolve to using fully qualified identifiers (i.e.
containing a ‘#’ sign).

   To explain these issues, lets extend our example module in the
following ways.  First we want the module to export the macros ‘sind’
and ‘asind’ that are like the ‘sin’ and ‘asin’ procedures but use
degrees instead of radians.  Note that it would be a better design for
‘sind’ and ‘asind’ to be procedures, but we’ll implement them as macros
for the sake of the example.  Second we want the procedures ‘deg->rad’
and ‘rad->deg’ to check that their argument is a real number using a
‘check-real’ macro.

   In a setting where name clashes are not an issue these macros can be
defined as follows:

     (define-macro (sind x) `(sin (deg->rad ,x)))
     (define-macro (asind x) `(rad->deg (asin ,x)))
     (define-macro (check-real x y)
       `(if (real? ,x) ,y (error "not real!")))

   Name clashes will occur when the locations where these macros are
called are in the scope of new bindings for ‘sin’, ‘deg->rad’, ‘if’,
‘error’, etc which are identifiers used in the expanded code.  A name
clash can also happen for the name ‘define-macro’ itself.  To remove the
possibility of name clashes the ‘##namespace’ form and fully qualified
identifiers can be used.  All the Gambit special forms, such as ‘let’,
‘if’, and ‘define-macro’, have a fully qualified version (‘##let’,
‘##if’, and ‘##define-macro’).  Gambit predefined procedures, such as
‘sin’, ‘real?’, and ‘error’, don’t necessarily have a fully qualified
version (some do and some don’t) but an empty namespace definition in a
‘##let’ form can be used to avoid the clash, i.e.  ‘(##let ()
(##namespace ("") sin))’ refers to the global variable ‘sin’ whatever
scope it is in.  With these forms our example can be written like this:

     ;;;---------------------------------------- file: angle4/angle4#.scm
     (##namespace ("angle4#" deg->rad rad->deg))
     (##define-macro (sind x) `((##let () (##namespace ("")) sin)
                                (angle4#deg->rad ,x)))
     (##define-macro (asind x) `(angle4#rad->deg
                                 ((##let () (##namespace ("")) asin) ,x)))

     ;;;---------------------------------------- file: angle4/angle4.scm
     (##namespace ("angle4#")) ;; map all identifiers to angle4# namespace
     (##import gambit)         ;; except those defined by Gambit's RTS
     (##supply-module angle4)
     (##define-macro (check-real x y)
       `(##if ((##let () (##namespace ("")) real?) ,x)
              ,y
              ((##let () (##namespace ("")) error) "not real!")))
     (define factor (/ (atan 1) 45))
     (define (deg->rad x) (check-real x (* x factor)))
     (define (rad->deg x) (check-real x (/ x factor)))

     ;;;---------------------------------------- file: app4.scm
     (##import angle4)
     (println "90 degrees is " (deg->rad 90) " radians")
     (println "sind(90) is " (sind 90))

7.3 Primitive Procedures
========================

Identifiers with a ‘##’ prefix are not valid identifiers according to
RnRS. This means that code containing ‘##’ prefixed identifiers cannot
be processed by and shared with other Scheme implementations.  They are
hard to read by people that aren’t used to that extension.  Moreover the
code lacks abstraction and safety because using ‘##car’ rather than
‘car’ has a specific meaning: avoiding type checks.  Consequently it is
hard to "turn on" safe execution of the code when it needs to be
debugged.  Many parts of the runtime library are expressed at a low
level of abstraction (with ‘##’ prefixed identifiers) even when not
required.

   For those reasons ‘##’ prefixed identifiers should be used sparingly
in new code, and existing code should gradually be rewritten to avoid
them.  The primitive operations which are used to build higher-level
operations are all defined as procedures with a ‘##’ prefix.

   The file ‘~~lib/_gambit#.scm’ contains the definition of the
‘primitive’ macro whose purpose is to abstract from the ‘##’ prefix.
The call ‘(primitive foo)’ is equivalent to ‘##foo’ and ‘(primitive (foo
a b))’ is equivalent to ‘(##foo a b)’.  The file ‘~~lib/_gambit#.scm’
also contains the definition of the ‘standard’ macro whose purpose is
similar, but forces the use of the empty namespace.  The call ‘(standard
+)’ is equivalent to ‘(##let () (##namespace ("" +)) +)’ and ‘(standard
(+ a b))’ is equivalent to ‘((##let () (##namespace ("" +)) +) a b)’.
Code that uses the ‘primitive’ and ‘standard’ macros can be ported to
other Scheme implementations by defining implementation specific
‘primitive’ and ‘standard’ macros that implement the appropriate mapping
for that implementation.

   The file ‘~~lib/_gambit#.scm’ also contains definitions for the
‘define-procedure’ and ‘define-primitive’ macros.  The ‘primitive’ and
‘standard’ macros work in tandem with the ‘define-procedure’ and
‘define-primitive’ macros and the ‘~~lib/gambit/prim/prim#.scm’ file and
‘(gambit prim)’ library.  The file ‘~~lib/gambit/prim/prim#.scm’
contains namespace declarations that map operations exported by the
runtime library without a ‘##’ prefix to their ‘##’ prefixed names if
this preserves the meaning of the operation but possibly (and usually)
with no type checking.  The ‘(gambit prim)’ library is similar but in
the form of a R7RS library.  For example the following code:

     (include "~~lib/gambit/prim/prim#.scm")
     (define (foo x) (square (car x)))
     (println (foo (bar 0.5)))
     (pp "hello")

   is equivalent to this code:

     (##define (foo x) (##square (##car x)))
     (##println (foo (bar 0.5)))
     (##unimplemented#pp "hello")

   The namespace declarations in ‘~~lib/gambit/prim/prim#.scm’ have
caused a mapping of ‘square’ to ‘##square’, ‘car’ to ‘##car’ and
‘println’ to ‘##println’ because those primitives perform the same
operations (when the code has no errors).  Note that ‘foo’ and ‘bar’
have remained the same, because they are not procedures exported by the
runtime library, and ‘pp’ has been mapped to ‘##unimplemented#pp’
because ‘pp’ is a procedure exported by the runtime library but ‘##pp’
is not defined.  Having ‘unimplemented’ in the name helps catch
situations where the programmer expected a primitive operation to exist
but this isn’t the case.

   The ‘define-procedure’ macro does two things.  It supports type
annotations in the parameter list and it inserts a ‘(include
"~~lib/gambit/prim/prim#.scm")’ in the body so that primitive operations
can be used without the ‘##’ prefix.  Type checking and automatic
forcing of promise arguments are also added implicitly.  The macro
‘define-primitive’ is similar, but the procedure defined is implicitly
prefixed with ‘##’.

   So all of these things work together to abstract away from the
concept of _primitive_ operations.  _Primitives_ are implemented using
procedures with a ‘##’ prefix, but other Scheme implementations could do
it differently.

   Finally, there’s the ‘(declare-safe-define-procedure <bool>)’ macro
that can be used to enable/disable the mapping of names exported by the
runtime library to the corresponding primitives.  This is useful to
enable type checks in the code.  For example the following definition:

     (define-procedure (foo (x vector))
       (vector-ref x 5))

   which expands to

     (define (foo x)
       (macro-check-vector x '(1 . x) (foo x)
         (##vector-ref x 5)))

   which expands to

     (define (foo x)
         (if (##vector? x)
             (##vector-ref x 5)
             (##fail-check-vector '(1 . x) foo x)))

   If the code is in the scope of a ‘(declare-safe-define-procedure #t)’
then it is ‘vector-ref’ that is called instead of ‘##vector-ref’ which
will both check that X is a vector (redundantly) and that the index is
in range.  However, the use of ‘##vector-ref’ can be forced by writing
the code with an explicit use of the ‘primitive’ macro:

     (define-procedure (foo (x vector))
       (primitive (vector-ref x 5)))

   The expectation is that the ‘primitive’ special form will be used
sparingly.  Searching the source code for the pattern ‘"(primitive"’ is
a good way to find potentially unsafe code.

7.3.1 Type specifiers
---------------------

Here is a list of the available type specifiers for a ‘define-procedure’
parameter X and the associated constraint on the value of X.

   Note that there is no direct way for checking for a "list" or "list
of elements of type T". A procedure taking a list parameter will likely
iterate on the list’s pairs going from cdr to cdr until a non-pair is
found.  Then a check for the empty list with
‘(macro-check-proper-list-null lst <arg-id> (<procedure-name> <args>...)
<body>)’ will check that the parameter is a proper list (i.e.  that it
ends with the empty list).

7.3.1.1 Basic types (other than numbers)
........................................

‘boolean’
     X is a boolean
‘char’
     X is a character
‘pair’
     X is a pair
‘procedure’
     X is a procedure
‘string’
     X is a string
‘symbol’
     X is a symbol
‘vector’
     X is a vector

7.3.1.2 Numbers
...............

‘number’
     X is a number (possibly complex, rational, etc)

‘real’
     X is a real number (any number except complex)

‘fixnum’
     X is a fixnum and -2^(W-3) <= X <= 2^(W-3) - 1
‘(fixnum-range lo hi)’
     X is a fixnum and lo <= X < hi
‘(fixnum-range-incl lo hi)’
     X is a fixnum and lo <= X <= hi

‘index’
     X is a fixnum and 0 <= X
‘(index-range lo hi)’
     X is a fixnum and 0 <= lo <= X < hi
‘(index-range-incl lo hi)’
     X is a fixnum and 0 <= lo <= X <= hi

‘exact-signed-int8’
     X is an exact integer, -128 <= X <= 127
‘exact-signed-int16’
     X is an exact integer n, -32768 <= X <= 32767
‘exact-signed-int32’
     X is an exact integer n, -2^31 <= X <= 2^31 - 1
‘exact-signed-int64’
     X is an exact integer n, -2^63 <= X <= 2^63 - 1
‘exact-unsigned-int8’
     X is an exact integer n, 0 <= X <= 255
‘exact-unsigned-int16’
     X is an exact integer n, 0 <= X <= 65535
‘exact-unsigned-int32’
     X is an exact integer n, 0 <= X <= 2^32 - 1
‘exact-unsigned-int64’
     X is an exact integer n, 0 <= X <= 2^64 - 1

‘flonum’
     X is a flonum, exception mentions FLONUM
‘inexact-real’
     X is a flonum, exception mentions Inexact REAL
‘inexact-real-list’
     X is a flonum, exception mentions Inexact REAL LIST

7.3.1.3 Time types
..................

‘time’
     X is a time object
‘absrel-time’
     X is a real or a time object
‘absrel-time-or-false’
     X is ‘#f’ or a real or a time object

7.3.1.4 Ports
.............

‘port’
     X is a port (input, output, or input-output)
‘input-port’
     X is an input port
‘output-port’
     X is an output port

‘object-input-port’
     X is an object input port
‘object-output-port’
     X is an object output port
‘vector-input-port’
     X is a vector input port
‘vector-output-port’
     X is a vector output port

‘character-input-port’
     X is a character input port
‘character-output-port’
     X is a character output port
‘string-input-port’
     X is a string input port
‘string-output-port’
     X is a string output port

‘byte-port’
     X is a byte port (input, output, or input-output)
‘byte-input-port’
     X is a byte input port
‘byte-output-port’
     X is a byte output port
‘u8vector-input-port’
     X is a u8vector input port u8vector-output-port x is a u8vector
     output port

‘device-input-port’
     X is a device intput port
‘device-output-port’
     X is a device output port
‘process-port’
     X is a process port

‘tcp-client-port’
     X is a tcp-client port
‘tcp-server-port’
     X is a tcp-server port

‘udp-port’
     X is a udp port
‘udp-input-port’
     X is a udp input port
‘udp-output-port’
     X is a udp output port

‘tty-port’
     X is a tty port

7.3.1.5 List and vector variants of above
.........................................

‘list’
     no type checking (a non-null non-pair object is in fact a
     degenerate dotted list), exception mentions LIST
‘proper-list’
     no type checking (code traversing the list must check for a
     proper-list), exception mentions PROPER LIST
‘proper-list-null’
     X is the empty list, exception mentions PROPER LIST
‘proper-or-circular-list’
     no type checking (code traversing the list must check for a
     proper-list or circular-list), exception mentions PROPER or
     CIRCULAR LIST
‘proper-or-circular-list-null’
     X is the empty list, exception mentions PROPER LIST
‘char-list’
     X is a character, exception mentions CHARACTER LIST
‘char-vector’
     X is a character, exception mentions CHARACTER VECTOR
‘pair-list’
     X is a pair, exception mentions PAIR LIST
‘exact-unsigned-int8-list-exact-unsigned-int8’
     X is an exact-unsigned-int8, exception mentions INTEGER LIST
‘exact-unsigned-int16-list-exact-unsigned-int16’
     X is an exact-unsigned-int16, exception mentions INTEGER LIST
‘exact-unsigned-int32-list-exact-unsigned-int32’
     X is an exact-unsigned-int32, exception mentions INTEGER LIST
‘exact-unsigned-int64-list-exact-unsigned-int64’
     X is an exact-unsigned-int64, exception mentions INTEGER LIST
‘exact-signed-int8-list-exact-signed-int8’
     X is an exact-signed-int8, exception mentions INTEGER LIST
‘exact-signed-int16-list-exact-signed-int16’
     X is an exact-signed-int16, exception mentions INTEGER LIST
‘exact-signed-int32-list-exact-signed-int32’
     X is an exact-signed-int32, exception mentions INTEGER LIST
‘exact-signed-int64-list-exact-signed-int64’
     X is an exact-signed-int64, exception mentions INTEGER LIST

7.3.1.6 Gambit types
....................

‘error-exception’
     X is an error-exception object
‘box’
     X is a box
‘condvar’
     X is a condition variable
‘f32vector’
     X is a f32vector
‘f64vector’
     X is a f64vector
‘foreign’
     X is a foreign object
‘keyword’
     X is a keyword
‘mutex’
     X is a mutex
‘processor’
     X is a processor object
‘s16vector’
     X is a s16vector
‘s32vector’
     X is a s32vector
‘s64vector’
     X is a s64vector
‘s8vector’
     X is a s8vector
‘table’
     X is a table
‘tgroup’
     X is a thread group
‘thread’
     X is a thread
‘u16vector’
     X is a u16vector
‘u32vector’
     X is a u32vector
‘u64vector’
     X is a u64vector
‘u8vector’
     X is a u8vector
‘will’
     X is a will
‘continuation’
     X is a continuation object
‘random-source’
     X is a random-source object
‘readtable’
     X is a readtable
‘type’
     X is a structure type descriptor
‘mutable’
     X is a mutable object

7.3.1.7 Others
..............

‘initialized-thread’
‘not-initialized-thread’
‘not-started-thread’
‘not-started-thread-given-initialized’
‘string-or-ip-address’
‘string-or-nonnegative-fixnum’

7.4 R7RS Compatible Modules
===========================

The R7RS Scheme standard specifies a modularization approach based on
the concept of library.  A library is defined using the ‘define-library’
form.  This form is implemented as a macro that expands into the
constructs used by primitive modules, in particular a ‘##namespace’
declaration with a namespace derived from the library’s name so that all
variables defined by the library are in that namespace.  With the
‘define-library’ form the ‘angle3’ module example given previously can
be written like this:

     ;;;---------------------------------------- file: angle3.sld
     (define-library (angle3)

       (export deg->rad rad->deg)

       (import (scheme base)
               (scheme inexact))

       (begin
         (define factor (/ (atan 1) 45))
         (define (deg->rad x) (* x factor))
         (define (rad->deg x) (/ x factor))))

   For this library the expansion of the ‘define-library’ form will
contain a ‘##namespace’ declaration that causes the definition of the
global variables ‘angle3#factor’, ‘angle3#deg->rad’, and
‘angle3#rad->deg’.  Meanwhile an ‘(import (angle3))’ in another library
will generate a ‘##namespace’ declaration that maps uses of ‘deg->rad’
and ‘rad->deg’ to the global variables ‘angle3#deg->rad’ and
‘angle3#rad->deg’ respectively (note that the unexported global variable
‘factor’ is not included in the generated ‘##namespace’ declaration).

   For more complex libraries whose code is split into multiple files it
is convenient to put all the files in a dedicated subdirectory.  This is
the preferred filesystem structure for a library but the runtime system
supports both styles.  The previous module could be structured like this
instead:

     ;;;---------------------------------------- file: angle3/angle3.sld
     (define-library (angle3)

       (export deg->rad rad->deg)

       (import (scheme base)
               (scheme inexact))

       (include "angle3.scm")) ;; path is relative to angle3.sld file

     ;;;---------------------------------------- file: angle3/angle3.scm
     (define factor (/ (atan 1) 45))
     (define (deg->rad x) (* x factor))
     (define (rad->deg x) (/ x factor))

7.4.1 Identifying libraries
---------------------------

Each library is given a name so that it can be referred to in various
contexts, most notably in ‘import’ forms and the interpreter’s and
compiler’s command line.  The R7RS defines a library name as a list
whose members are identifiers and exact non-negative integers, for
example ‘(widget)’, ‘(_hamt)’, ‘(scheme base)’, and ‘(srfi 64)’.

   The system maps these R7RS library names to module identifiers that
are symbols formed by concatenating the parts of the library name
separated with ‘/’.  The library name and module name are
interchangeable.  Consequently, ‘(import srfi/64)’ and ‘(import _hamt)’
are respectively equivalent to ‘(import (srfi 64))’ and ‘(import
(_hamt))’.  Using the module name to identify libraries on the command
line is convenient as it avoids having to escape parentheses and spaces.

7.4.2 The ‘define-library’ form
-------------------------------

 -- special form: define-library NAME DECLARATION ...

   In a library definition NAME specifies the name of the library and
DECLARATION is one of:

     (export <EXPORT SPEC> ...)
     (import <IMPORT SET> ...)
     (begin <COMMAND OR DEFINITION> ...)
     (include <FILENAME> ...)
     (include-ci <FILENAME> ...)
     (include-library-declarations <FILENAME> ...)
     (cond-expand <COND EXPAND FEATURES> ...)
     (namespace <NAMESPACE>)
     (cc-options <OPTIONS> ...)
     (ld-options <OPTIONS> ...)
     (ld-options-prelude <OPTIONS> ...)
     (pkg-config <OPTIONS> ...)
     (pkg-config-path <PATH> ...)

7.4.3 ‘(export <EXPORT SPEC> ...)’
----------------------------------

An ‘export’ declaration specifies a list of identifiers which can be
made visible to other libraries or programs.  An <EXPORT SPEC> takes one
of the following forms:

     <IDENTIFIER>
     (rename <IDENTIFIER>1 <IDENTIFIER>2)

   In an <EXPORT SPEC>, an <IDENTIFIER> names a single binding (variable
or macro) defined within or imported into the library, where the
external name for the export is the same as the name of the binding
within the library.  A ‘rename’ spec exports the binding defined within
or imported into the library and named by <IDENTIFIER>1 in each
‘(<IDENTIFIER>1 <IDENTIFIER>2)’ pairing, using <IDENTIFIER>2 as the
external name.

7.4.4 ‘(import <IMPORT SET> ...)’
---------------------------------

A library declares a dependency to another library with the ‘import’
declaration.  The ‘(import <IMPORT SET> ...)’ form identifies the
imported library or libraries.

   Each <IMPORT SET> names a set of bindings from a library and possibly
specifies local names for the imported bindings.  An <IMPORT SET> takes
one of the following forms:

     <LIBRARY NAME>
     (only <IMPORT SET> <IDENTIFIER> ...)
     (except <IMPORT SET> <IDENTIFIER> ...)
     (prefix <IMPORT SET> <IDENTIFIER>)
     (rename <IMPORT SET> (<IDENTIFIER>1 <IDENTIFIER>2) ...)

   In the first form, all of the identifiers in the named library’s
‘export’ clauses are imported with the same names (or the exported names
if exported with ‘rename’).  The additional <IMPORT SET> forms modify
this set as follows:

   • ‘only’ produces a subset of the given <IMPORT SET> including only
     the listed identifiers (after any renaming).  It is an error if any
     of the listed identifiers are not found in the original set.

   • ‘except’ produces a subset of the given <IMPORT SET>, excluding the
     listed identifiers (after any renaming).  It is an error if any of
     the listed identifiers are not found in the original set.

   • ‘rename’ modifies the given <IMPORT SET>, replacing each instance
     of <IDENTIFIER>1 with <IDENTIFIER>2.  It is an error if any of the
     listed <IDENTIFIER>1s are not found in the original set.

   • ‘prefix’ automatically renames all identifiers in the given <IMPORT
     SET>, prefixing each with the specified <IDENTIFIER>.

   It is an error to import the same identifier more than once with
different bindings, or to redefine or mutate an imported binding with a
definition or with ‘set!’, or to refer to an identifier before it is
imported.

   As an extension to the R7RS syntax it is allowed for a <LIBRARY NAME>
to contain a trailing ‘@VERSION’ when the library is hosted in a ‘git’
repository.  The VERSION must match a tag of that repository and it
indicates the specific library version required.  For example, ‘(import
(github.com/gambit hello @1.0))’ or equivalently ‘(import
github.com/gambit/hello@1.0)’.  Note that the version specifier is not
separated with a ‘/’ in the module name.

   Another extension to the R7RS syntax when the library is hosted in a
‘git’ repository is the use of dots before the name of the library to
indicate a relative reference within the repository.  The number of dots
indicates the number of parent hops.  For example, in the library
‘(github.com/gambit hello demo)’ an ‘(import (.. hello))’ will resolve
to the ‘(github.com/gambit hello)’ library.  A relative library
reference should not contain an explicit ‘@VERSION’ because the VERSION
is implicitly the same as the referring module.

7.4.5 ‘(begin <COMMAND OR DEFINITION> ...)’, ‘(include <FILENAME> ...)’, and ‘(include-ci <FILENAME> ...)’
----------------------------------------------------------------------------------------------------------

The ‘begin’, ‘include’, and ‘include-ci’ declarations are used to
specify the body of the library.  They have the same syntax and
semantics as the corresponding expression types.  This form of ‘begin’
is analogous to, but not the same as, the two types of ‘begin’
expressions.

7.4.6 ‘(include-library-declarations <FILENAME> ...)’
-----------------------------------------------------

The ‘include-library-declarations’ declaration is similar to ‘include’
except that the contents of the file are spliced directly into the
current library definition.  This can be used, for example, to share the
same ‘export’ declaration among multiple libraries as a simple form of
library interface.

7.4.7 ‘(cond-expand <COND EXPAND FEATURES> ...)’
------------------------------------------------

The ‘cond-expand’ declaration has the same syntax and semantics as the
‘cond-expand’ expression type, except that it expands to spliced-in
library declarations rather than expressions enclosed in ‘begin’.

7.4.8 Extensions to the R7RS library declarations
-------------------------------------------------

The ‘(namespace <NAMESPACE>)’ declaration allows overriding the
namespace used for the library.  This is mainly useful for system
libraries to prevent namespace prefixing using a ‘(namespace "")’
declaration.

   The remaining declarations are relevant to the ‘C’ target and ignored
otherwise.  They provide information, in the form of strings, to be
passed to the compiler options of the same name when this library is
compiled:

   • ‘(cc-options <OPTIONS> ...)’

   • ‘(ld-options <OPTIONS> ...)’

   • ‘(ld-options-prelude <OPTIONS> ...)’

   • ‘(pkg-config <OPTIONS> ...)’

   • ‘(pkg-config-path <PATH> ...)’

   For example, a library could force the C compiler to generate machine
code for i386 with:

     (define-library (foo)
       (export bar)
       (import (scheme base))
       (cc-options "-march=i386") ;; request compilation for i386
       (begin (define (bar) 42)))

7.5 Installing Modules
======================

When a module is imported, the processing of the ‘import’ form must
locate and read the source code of the module at macro expansion time to
determine which names are exported and to what they are mapped.  The
list of module search directories (‘~~lib’ followed by ‘~~userlib’ by
default) is searched to find the module’s source code.  At execution
time the same search algorithm is used to locate and load the module,
either in source code form or compiled form.  The ‘~~lib’ directory is
where the system’s builtin modules are put when Gambit is installed.
The ‘~~userlib’ directory is a convenient place where other modules can
be installed by the user because locating them does not require
extending the list of module search directories.

 -- procedure: module-search-order-reset!
 -- procedure: module-search-order-add! DIR

     The list of module search directories can be modified using the
     procedures ‘module-search-order-reset!’ and
     ‘module-search-order-add!’ that respectively clear the list and
     extend the list with the directory DIR which must be a string.  The
     list can also be extended by using the ‘-:search=’DIR runtime
     option or by a command line argument to ‘gsi’ and ‘gsc’ that ends
     with a path separator or a ‘.’ (*note ##demand-module and
     ##supply-module forms::).

     For example:

          $ cat foobar.sld
          (define-library (foobar)
            (import (scheme write))
            (begin (display "foobar library has executed\n")))
          $ gsi
          Gambit v4.9.4

          > (import (foobar))
          *** ERROR IN (stdin)@1.9 -- Cannot find library (foobar)
          > (module-search-order-add! ".")
          > (import (foobar))
          foobar library has executed
          > ,q
          $ gsi -:search=. foobar
          foobar library has executed
          $ gsi . foobar
          foobar library has executed

 -- procedure: module-whitelist-reset!
 -- procedure: module-whitelist-add! SOURCE

     When modules are installed it is done at the granularity of a
     “package”, which is defined as a ‘git’ repository possibly
     containing more than one module.  For example, if the hosted module
     ‘github.com/gambit/hello/demo’ needs to be installed it is all of
     the code at ‘github.com/gambit/hello’ that is installed (this
     includes the three modules ‘github.com/gambit/hello’,
     ‘github.com/gambit/hello/demo’, and
     ‘github.com/gambit/hello/test’).

     For convenience the runtime system will automatically install in
     the ‘~~userlib’ directory any hosted module that is from a trusted
     source.  The whitelist of trusted sources, which initially contains
     only ‘github.com/gambit’, can be modified using the procedures
     ‘module-whitelist-reset!’ and ‘module-whitelist-add!’ that
     respectively clear the list and extend the list with the source
     SOURCE which must be a string.  The list can also be extended with
     the ‘-:whitelist=’SOURCE runtime option.

     For example:

          $ gsi github.com/gambit/hello/demo  # auto-install of github.com/gambit/hello package
          People customarily greet each other when they meet.
          In English you can say: hello Bob, nice to see you!
          In French you can say: bonjour Bob, je suis enchant!
          Demo source code: /Users/feeley/.gambit_userlib/github.com/gambit/hello/@/demo.scm
          $ gsi github.com/feeley/roman/demo  # no auto-install because not on whitelist
          *** ERROR IN ##main -- No such file or directory
          (load "github.com/feeley/roman/demo")
          $ gsi -:whitelist=github.com/feeley github.com/feeley/roman/demo
          1 is I in roman numerals
          2 is II in roman numerals
          4 is IV in roman numerals
          8 is VIII in roman numerals
          16 is XVI in roman numerals
          32 is XXXII in roman numerals
          64 is LXIV in roman numerals
          $ gsi github.com/feeley/roman/demo   # OK because module is now installed
          1 is I in roman numerals
          2 is II in roman numerals
          4 is IV in roman numerals
          8 is VIII in roman numerals
          16 is XVI in roman numerals
          32 is XXXII in roman numerals
          64 is LXIV in roman numerals
          $ gsi github.com/feeley/roman/test   # the test module was also installed
          *** all tests passed out of a total of 19 tests

     The use of the runtime option ‘-:whitelist=’ (with no SOURCE) will
     disable the automatic installation of modules, even from
     ‘github.com/gambit’.  For example:

          $ gsi -:whitelist= github.com/gambit/hello/demo
          *** ERROR IN ##main -- No such file or directory
          (load "github.com/gambit/hello/demo")

     A manual management of packages is nevertheless possible with the
     ‘gsi’ package management operations.  These are invoked with the
     command line options ‘-install’, ‘-uninstall’, and ‘-update’ which
     respectively install, uninstall and update packages.  The package
     management operations accept a list of packages.  Packages are
     installed in ‘~~userlib’ which is mapped to ‘~/.gambit_userlib’ by
     default.  An optional ‘-dir DIR’ option can be used to install the
     package in some other directory.

     For example:

          $ gsi -install github.com/feeley/roman
          installing github.com/feeley/roman to /Users/feeley/.gambit_userlib/
          $ gsi github.com/feeley/roman/demo
          1 is I in roman numerals
          2 is II in roman numerals
          4 is IV in roman numerals
          8 is VIII in roman numerals
          16 is XVI in roman numerals
          32 is XXXII in roman numerals
          64 is LXIV in roman numerals
          $ gsi -uninstall github.com/feeley/roman
          uninstalling github.com/feeley/roman from /Users/feeley/.gambit_userlib/
          $ gsi -install -dir ~/mylibs github.com/feeley/roman
          installing github.com/feeley/roman to /Users/feeley/mylibs
          $ gsi ~/mylibs/ github.com/feeley/roman/demo
          1 is I in roman numerals
          2 is II in roman numerals
          4 is IV in roman numerals
          8 is VIII in roman numerals
          16 is XVI in roman numerals
          32 is XXXII in roman numerals
          64 is LXIV in roman numerals

     Local ‘git’ repositories can also be installed manually with the
     package management operations using a path to the local repository.
     This can be useful during the development phase before a library
     becomes hosted.

     For example:

          $ mkdir some
          $ mkdir some/dir
          $ mkdir some/dir/mylib
          $ cd some/dir/mylib
          $ cat > mylib.sld
          (define-library (mylib)
            (import (scheme write))
            (begin (display "mylib library has executed\n")))
          $ git init
          Initialized empty Git repository in /Users/feeley/doc/some/dir/mylib/.git/
          $ git add mylib.sld
          $ git commit -m "first commit"
          [master (root-commit) c3f6aff] first commit
           1 file changed, 3 insertions(+)
           create mode 100644 mylib.sld
          $ cd ../../..
          $ gsi some/dir/ mylib   # execution of mylib without installation
          mylib library has executed
          $ gsi -install some/dir/mylib
          installing some/dir/mylib to /Users/feeley/.gambit_userlib/
          $ gsi mylib   # execution of mylib after installation
          mylib library has executed

7.6 Compiling Modules
=====================

When ‘gsc’ finds a command line argument that is the name of a module
found on the list of module search order directories (after an automatic
installation if that is appropriate) that module’s main source code file
will be compiled.

   When a dynamic compilation is requested (which is the default
compilation mode and when the command line option ‘-dynamic’ is used)
the compiler will compile for the selected target the main source code
file to a target file and a dynamically loadable object file with a
‘.o1’ extension.  These files will be created in a directory next to the
module’s main source code file, with the same name stripped of it’s
extension and suffixed with the Gambit version and the target name.
This naming strategy aims to avoid loading the compiled file in an
inappropriate context.  The module loading algorithm knows it should
check this directory to find a compiled version of the module.

   For example:

     $ mkdir lib1 lib2
     $ cat > lib1/lib1.sld
     (define-library (lib1)
       (export fact)
       (import (scheme base) (scheme write))
       (begin
         (define (fact n) (if (<= n 1) 1 (* n (fact (- n 1)))))
         (display "lib1 loaded\n")))
     $ cat > lib2/lib2.sld
     (define-library (lib2)
       (import (lib1) (scheme base) (scheme write))
       (begin
         (display
          (cond-expand
            ((compilation-target C)   "lib2 compiled to C\n")
            ((compilation-target (_)) "lib2 interpreted\n")
            (else                     "lib2 compiled to other\n")))
         (display (fact 10))
         (newline)))
     $ gsi . lib2        # loads lib1.sld and lib2.sld
     lib1 loaded
     lib2 interpreted
     3628800
     lib1
     $ tree --charset=ascii --noreport lib1 lib2
     `-- lib1.sld
     lib2
     `-- lib2.sld
     $ gsc . lib1 lib2   # compile lib1.sld and lib2.sld using C target
     $ gsi . lib2        # loads generated lib1.o1 and lib2.o1
     lib1 loaded
     lib2 compiled to C
     3628800
     $ gsc -target js . lib1 lib2   # also compile them for js target
     $ tree --charset=ascii --noreport lib1 lib2
     lib1
     |-- lib1.sld
     |-- lib1@gambit409003@C
     |   |-- lib1.c
     |   `-- lib1.o1
     `-- lib1@gambit409003@js
         |-- lib1.js
         `-- lib1.o1
     lib2
     |-- lib2.sld
     |-- lib2@gambit409003@C
     |   |-- lib2.c
     |   `-- lib2.o1
     `-- lib2@gambit409003@js
         |-- lib2.js
         `-- lib2.o1

   To create an executable program from a set of non-legacy modules it
is important to use the ‘-nopreload’ linking option when linking so that
the modules will be initialized in an order that is consistent with the
module dependencies.  If the default ‘-preload’ linking option is used
some modules may be initialized out of order, leading to incorrect
execution.

   Here is an example that extends the previous example:

     $ gsc -exe -nopreload . lib1/lib1.sld lib2/lib2.sld
     $ lib2/lib2
     lib1 loaded
     lib2 compiled to C
     3628800
     $ gsc -target js -exe -nopreload . lib1/lib1.sld lib2/lib2.sld
     $ lib2/lib2
     lib1 loaded
     lib2 compiled to other
     3628800

8 Built-in data types
*********************

8.1 Numbers
===========

8.1.1 Extensions to numeric procedures
--------------------------------------

 -- procedure: = Z1...
 -- procedure: < X1...
 -- procedure: > X1...
 -- procedure: <= X1...
 -- procedure: >= X1...

     These procedures take any number of arguments including no
     argument.  This is useful to test if the elements of a list are
     sorted in a particular order.  For example, testing that the list
     of numbers ‘lst’ is sorted in nondecreasing order can be done with
     the call ‘(apply < lst)’.

8.1.2 IEEE floating point arithmetic
------------------------------------

To better conform to IEEE floating point arithmetic the standard numeric
tower is extended with these special inexact reals:

‘+inf.0’
     positive infinity
‘-inf.0’
     negative infinity
‘+nan.0’
     “not a number”
‘-0.’
     negative zero (‘0.’ is the positive zero)

   The infinities and “not a number” are reals (i.e.  ‘(real? +inf.0)’
is ‘#t’) but are not rational (i.e.  ‘(rational? +inf.0)’ is ‘#f’).

   Both zeros are numerically equal (i.e.  ‘(= -0. 0.)’ is ‘#t’) but are
not equivalent (i.e.  ‘(eqv? -0. 0.)’ and ‘(equal? -0. 0.)’ are ‘#f’).
All numerical comparisons with “not a number”, including ‘(= +nan.0
+nan.0)’, are ‘#f’.

8.1.3 Integer square root and nth root
--------------------------------------

 -- procedure: integer-sqrt N

     This procedure returns the integer part of the square root of the
     nonnegative exact integer N.

     For example:

          > (integer-sqrt 123)
          11

 -- procedure: integer-nth-root N1 N2

     This procedure returns the integer part of N1 raised to the power
     1/N2, where N1 is a nonnegative exact integer and N2 is a positive
     exact integer.

     For example:

          > (integer-nth-root 100 3)
          4

8.1.4 Bitwise-operations on exact integers
------------------------------------------

The procedures defined in this section are compatible with the withdrawn
“Integer Bitwise-operation Library SRFI” (SRFI 33).  Note that some of
the procedures specified in SRFI 33 are not provided.

   Most procedures in this section are specified in terms of the binary
representation of exact integers.  The two’s complement representation
is assumed where an integer is composed of an infinite number of bits.
The upper section of an integer (the most significant bits) are either
an infinite sequence of ones when the integer is negative, or they are
an infinite sequence of zeros when the integer is nonnegative.

 -- procedure: arithmetic-shift N1 N2

     This procedure returns N1 shifted to the left by N2 bits, that is
     ‘(floor (* N1 (expt 2 N2)))’.  Both N1 and N2 must be exact
     integers.

     For example:

          > (arithmetic-shift 1000 7)  ; n1=...0000001111101000
          128000
          > (arithmetic-shift 1000 -6) ; n1=...0000001111101000
          15
          > (arithmetic-shift -23 -3)  ; n1=...1111111111101001
          -3

 -- procedure: bitwise-merge N1 N2 N3

     This procedure returns an exact integer whose bits combine the bits
     from N2 and N3 depending on N1.  The bit at index I of the result
     depends only on the bits at index I in N1, N2 and N3: it is equal
     to the bit in N2 when the bit in N1 is 0 and it is equal to the bit
     in N3 when the bit in N1 is 1.  All arguments must be exact
     integers.

     For example:

          > (bitwise-merge -4 -11 10) ; ...11111100 ...11110101 ...00001010
          9
          > (bitwise-merge 12 -11 10) ; ...00001100 ...11110101 ...00001010
          -7

 -- procedure: bitwise-and N...

     This procedure returns the bitwise “and” of the exact integers
     N....  The value -1 is returned when there are no arguments.

     For example:

          > (bitwise-and 6 12)  ; ...00000110 ...00001100
          4
          > (bitwise-and 6 -4)  ; ...00000110 ...11111100
          4
          > (bitwise-and -6 -4) ; ...11111010 ...11111100
          -8
          > (bitwise-and)
          -1

 -- procedure: bitwise-andc1 N1 N2

     This procedure returns the bitwise “and” of the bitwise complement
     of the exact integer N1 and the exact integer N2.

     For example:

          > (bitwise-andc1 11 26)  ; ...00001011 ...00011010
          16
          > (bitwise-andc1 -12 26) ; ...11110100 ...00011010
          10

 -- procedure: bitwise-andc2 N1 N2

     This procedure returns the bitwise “and” of the exact integer N1
     and the bitwise complement of the exact integer N2.

     For example:

          > (bitwise-andc2 11 26)  ; ...00001011 ...00011010
          1
          > (bitwise-andc2 11 -27) ; ...00001011 ...11100101
          10

 -- procedure: bitwise-eqv N...

     This procedure returns the bitwise complement of the bitwise
     “exclusive-or” of the exact integers N....  The value -1 is
     returned when there are no arguments.

     For example:

          > (bitwise-eqv 6 12)  ; ...00000110 ...00001100
          -11
          > (bitwise-eqv 6 -4)  ; ...00000110 ...11111100
          5
          > (bitwise-eqv -6 -4) ; ...11111010 ...11111100
          -7
          > (bitwise-eqv)
          -1

 -- procedure: bitwise-ior N...

     This procedure returns the bitwise “inclusive-or” of the exact
     integers N....  The value 0 is returned when there are no
     arguments.

     For example:

          > (bitwise-ior 6 12)  ; ...00000110 ...00001100
          14
          > (bitwise-ior 6 -4)  ; ...00000110 ...11111100
          -2
          > (bitwise-ior -6 -4) ; ...11111010 ...11111100
          -2
          > (bitwise-ior)
          0

 -- procedure: bitwise-nand N1 N2

     This procedure returns the bitwise complement of the bitwise “and”
     of the exact integer N1 and the exact integer N2.

     For example:

          > (bitwise-nand 11 26)  ; ...00001011 ...00011010
          -11
          > (bitwise-nand 11 -27) ; ...00001011 ...11100101
          -2

 -- procedure: bitwise-nor N1 N2

     This procedure returns the bitwise complement of the bitwise
     “inclusive-or” of the exact integer N1 and the exact integer N2.

     For example:

          > (bitwise-nor 11 26)  ; ...00001011 ...00011010
          -28
          > (bitwise-nor 11 -27) ; ...00001011 ...11100101
          16

 -- procedure: bitwise-not N

     This procedure returns the bitwise complement of the exact integer
     N.

     For example:

          > (bitwise-not 3)  ; ...00000011
          -4
          > (bitwise-not -1) ; ...11111111
          0

 -- procedure: bitwise-orc1 N1 N2

     This procedure returns the bitwise “inclusive-or” of the bitwise
     complement of the exact integer N1 and the exact integer N2.

     For example:

          > (bitwise-orc1 11 26)  ; ...00001011 ...00011010
          -2
          > (bitwise-orc1 -12 26) ; ...11110100 ...00011010
          27

 -- procedure: bitwise-orc2 N1 N2

     This procedure returns the bitwise “inclusive-or” of the exact
     integer N1 and the bitwise complement of the exact integer N2.

     For example:

          > (bitwise-orc2 11 26)  ; ...00001011 ...00011010
          -17
          > (bitwise-orc2 11 -27) ; ...00001011 ...11100101
          27

 -- procedure: bitwise-xor N...

     This procedure returns the bitwise “exclusive-or” of the exact
     integers N....  The value 0 is returned when there are no
     arguments.

     For example:

          > (bitwise-xor 6 12)  ; ...00000110 ...00001100
          10
          > (bitwise-xor 6 -4)  ; ...00000110 ...11111100
          -6
          > (bitwise-xor -6 -4) ; ...11111010 ...11111100
          6
          > (bitwise-xor)
          0

 -- procedure: bit-count N

     This procedure returns the bit count of the exact integer N.  If N
     is nonnegative, the bit count is the number of 1 bits in the two’s
     complement representation of N.  If N is negative, the bit count is
     the number of 0 bits in the two’s complement representation of N.

     For example:

          > (bit-count 0)   ; ...00000000
          0
          > (bit-count 1)   ; ...00000001
          1
          > (bit-count 2)   ; ...00000010
          1
          > (bit-count 3)   ; ...00000011
          2
          > (bit-count 4)   ; ...00000100
          1
          > (bit-count -23) ; ...11101001
          3

 -- procedure: integer-length N

     This procedure returns the bit length of the exact integer N.  If N
     is a positive integer the bit length is one more than the index of
     the highest 1 bit (the least significant bit is at index 0).  If N
     is a negative integer the bit length is one more than the index of
     the highest 0 bit.  If N is zero, the bit length is 0.

     For example:

          > (integer-length 0)   ; ...00000000
          0
          > (integer-length 1)   ; ...00000001
          1
          > (integer-length 2)   ; ...00000010
          2
          > (integer-length 3)   ; ...00000011
          2
          > (integer-length 4)   ; ...00000100
          3
          > (integer-length -23) ; ...11101001
          5

 -- procedure: bit-set? N1 N2

     This procedure returns a boolean indicating if the bit at index N1
     of N2 is set (i.e.  equal to 1) or not.  Both N1 and N2 must be
     exact integers, and N1 must be nonnegative.

     For example:

          > (map (lambda (i) (bit-set? i -23)) ; ...11101001
                 '(7 6 5 4 3 2 1 0))
          (#t #t #t #f #t #f #f #t)

 -- procedure: any-bits-set? N1 N2

     This procedure returns a boolean indicating if the bitwise and of
     N1 and N2 is different from zero or not.  This procedure is
     implemented more efficiently than the naive definition:

          (define (any-bits-set? n1 n2) (not (zero? (bitwise-and n1 n2))))

     For example:

          > (any-bits-set? 5 10)   ; ...00000101 ...00001010
          #f
          > (any-bits-set? -23 32) ; ...11101001 ...00100000
          #t

 -- procedure: all-bits-set? N1 N2

     This procedure returns a boolean indicating if the bitwise and of
     N1 and N2 is equal to N1 or not.  This procedure is implemented
     more efficiently than the naive definition:

          (define (all-bits-set? n1 n2) (= n1 (bitwise-and n1 n2)))

     For example:

          > (all-bits-set? 1 3) ; ...00000001 ...00000011
          #t
          > (all-bits-set? 7 3) ; ...00000111 ...00000011
          #f

 -- procedure: first-set-bit N

     This procedure returns the bit index of the least significant bit
     of N equal to 1 (which is also the number of 0 bits that are below
     the least significant 1 bit).  This procedure returns ‘-1’ when N
     is zero.

     For example:

          > (first-set-bit 24) ; ...00011000
          3
          > (first-set-bit 0)  ; ...00000000
          -1

 -- procedure: extract-bit-field N1 N2 N3
 -- procedure: test-bit-field? N1 N2 N3
 -- procedure: clear-bit-field N1 N2 N3
 -- procedure: replace-bit-field N1 N2 N3 N4
 -- procedure: copy-bit-field N1 N2 N3 N4

     These procedures operate on a bit-field which is N1 bits wide
     starting at bit index N2.  All arguments must be exact integers and
     N1 and N2 must be nonnegative.

     The procedure ‘extract-bit-field’ returns the bit-field of N3
     shifted to the right so that the least significant bit of the
     bit-field is the least significant bit of the result.

     The procedure ‘test-bit-field?’ returns ‘#t’ if any bit in the
     bit-field of N3 is equal to 1, otherwise ‘#f’ is returned.

     The procedure ‘clear-bit-field’ returns N3 with all bits in the
     bit-field replaced with 0.

     The procedure ‘replace-bit-field’ returns N4 with the bit-field
     replaced with the least-significant N1 bits of N3.

     The procedure ‘copy-bit-field’ returns N4 with the bit-field
     replaced with the (same index and size) bit-field in N3.

     For example:

          > (extract-bit-field 5 2 -37)    ; ...11011011
          22
          > (test-bit-field? 5 2 -37)      ; ...11011011
          #t
          > (test-bit-field? 1 2 -37)      ; ...11011011
          #f
          > (clear-bit-field 5 2 -37)      ; ...11011011
          -125
          > (replace-bit-field 5 2 -6 -37) ; ...11111010 ...11011011
          -21
          > (copy-bit-field 5 2 -6 -37)    ; ...11111010 ...11011011
          -5

8.1.5 Fixnum specific operations
--------------------------------

 -- procedure: fixnum? OBJ

 -- procedure: fx* N1...

 -- procedure: fx+ N1...

 -- procedure: fx- N1 N2...

 -- procedure: fx< N1...

 -- procedure: fx<= N1...

 -- procedure: fx= N1...

 -- procedure: fx> N1...

 -- procedure: fx>= N1...

 -- procedure: fxabs N

 -- procedure: fxand N1...

 -- procedure: fxandc1 N1 N2

 -- procedure: fxandc2 N1 N2

 -- procedure: fxarithmetic-shift N1 N2

 -- procedure: fxarithmetic-shift-left N1 N2

 -- procedure: fxarithmetic-shift-right N1 N2

 -- procedure: fxbit-count N

 -- procedure: fxbit-set? N1 N2

 -- procedure: fxeqv N1...

 -- procedure: fxeven? N

 -- procedure: fxfirst-set-bit N

 -- procedure: fxif N1 N2 N3

 -- procedure: fxior N1...

 -- procedure: fxlength N

 -- procedure: fxmax N1 N2...

 -- procedure: fxmin N1 N2...

 -- procedure: fxmodulo N1 N2

 -- procedure: fxnegative? N

 -- procedure: fxnand N1 N2

 -- procedure: fxnor N1 N2

 -- procedure: fxnot N

 -- procedure: fxodd? N

 -- procedure: fxorc1 N1 N2

 -- procedure: fxorc2 N1 N2

 -- procedure: fxpositive? N

 -- procedure: fxquotient N1 N2

 -- procedure: fxremainder N1 N2

 -- procedure: fxwrap* N1...

 -- procedure: fxwrap+ N1...

 -- procedure: fxwrap- N1 N2...

 -- procedure: fxwrapabs N

 -- procedure: fxwraparithmetic-shift N1 N2

 -- procedure: fxwraparithmetic-shift-left N1 N2

 -- procedure: fxwraplogical-shift-right N1 N2

 -- procedure: fxwrapquotient N1 N2

 -- procedure: fxxor N1...

 -- procedure: fxzero? N

 -- procedure: fxsquare N

 -- procedure: fxwrapsquare N

 -- procedure: fixnum-overflow-exception? OBJ
 -- procedure: fixnum-overflow-exception-procedure EXC
 -- procedure: fixnum-overflow-exception-arguments EXC

     Fixnum-overflow-exception objects are raised by some of the fixnum
     specific procedures when the result is larger than can fit in a
     fixnum.  The parameter EXC must be a fixnum-overflow-exception
     object.

     The procedure ‘fixnum-overflow-exception?’ returns ‘#t’ when OBJ is
     a fixnum-overflow-exception object and ‘#f’ otherwise.

     The procedure ‘fixnum-overflow-exception-procedure’ returns the
     procedure that raised EXC.

     The procedure ‘fixnum-overflow-exception-arguments’ returns the
     list of arguments of the procedure that raised EXC.

     For example:

          > (define (handler exc)
              (if (fixnum-overflow-exception? exc)
                  (list (fixnum-overflow-exception-procedure exc)
                        (fixnum-overflow-exception-arguments exc))
                  'not-fixnum-overflow-exception))
          > (with-exception-catcher
              handler
              (lambda () (fx* 100000 100000)))
          (#<procedure #2 fx*> (100000 100000))

8.1.6 Flonum specific operations
--------------------------------

 -- procedure: flonum? OBJ

 -- procedure: fixnum->flonum N

 -- procedure: fl* X1...

 -- procedure: fl+ X1...

 -- procedure: fl- X1 X2...

 -- procedure: fl/ X1 X2

 -- procedure: fl< X1...

 -- procedure: fl<= X1...

 -- procedure: fl= X1...

 -- procedure: fl> X1...

 -- procedure: fl>= X1...

 -- procedure: flabs X

 -- procedure: flacos X

 -- procedure: flasin X

 -- procedure: flatan X
 -- procedure: flatan Y X

 -- procedure: flceiling X

 -- procedure: flcos X

 -- procedure: fldenominator X

 -- procedure: fleven? X

 -- procedure: flexp X

 -- procedure: flexpt X Y

 -- procedure: flhypot X Y

 -- procedure: flfinite? X

 -- procedure: flfloor X

 -- procedure: flinfinite? X

 -- procedure: flinteger? X

 -- procedure: fllog X

 -- procedure: flmax X1 X2...

 -- procedure: flmin X1 X2...

 -- procedure: flnan? X

 -- procedure: flnegative? X

 -- procedure: flnumerator X

 -- procedure: flodd? X

 -- procedure: flpositive? X

 -- procedure: flround X

 -- procedure: flsin X

 -- procedure: flsqrt X

 -- procedure: fltan X

 -- procedure: fltruncate X

 -- procedure: flzero? X

 -- procedure: fl+* X1 X2 X3

 -- procedure: flacosh X

 -- procedure: flasinh X

 -- procedure: flatanh X

 -- procedure: flcosh X

 -- procedure: flexpm1 X

 -- procedure: flilogb X

 -- procedure: fllog1p X

 -- procedure: flscalbn X

 -- procedure: flsinh X

 -- procedure: flsquare X

 -- procedure: fltanh X

8.1.7 Pseudo random numbers
---------------------------

The procedures and variables defined in this section are compatible with
the “Sources of Random Bits SRFI” (SRFI 27).  The implementation is
based on Pierre L’Ecuyer’s MRG32k3a pseudo random number generator.  At
the heart of SRFI 27’s interface is the random source type which
encapsulates the state of a pseudo random number generator.  The state
of a random source object changes every time a pseudo random number is
generated from this random source object.

 -- variable: default-random-source

     The global variable ‘default-random-source’ is bound to the random
     source object which is used by the ‘random-integer’, ‘random-real’,
     ‘random-u8vector’ and ‘random-f64vector’ procedures.

 -- procedure: random-integer N

     This procedure returns a pseudo random exact integer in the range 0
     to N-1.  The random source object in the global variable
     ‘default-random-source’ is used to generate this number.  The
     parameter N must be a positive exact integer.

     For example:

          > (random-integer 100)
          24
          > (random-integer 100)
          2
          > (random-integer 10000000000000000000000000000000000000000)
          6143360270902284438072426748425263488507

 -- procedure: random-real

     This procedure returns a pseudo random inexact real between, but
     not including, 0 and 1.  The random source object in the global
     variable ‘default-random-source’ is used to generate this number.

     For example:

          > (random-real)
          .24230672079133753
          > (random-real)
          .02317001922506932

 -- procedure: random-u8vector N

     This procedure returns a u8vector of length N containing pseudo
     random exact integers in the range 0 to 255.  The random source
     object in the global variable ‘default-random-source’ is used to
     generate these numbers.  The parameter N must be a nonnegative
     exact integer.

     For example:

          > (random-u8vector 10)
          #u8(200 53 29 202 3 85 208 187 73 219)

 -- procedure: random-f64vector N

     This procedure returns a f64vector of length N containing pseudo
     random inexact reals between, but not including, 0 and 1.  The
     random source object in the global variable ‘default-random-source’
     is used to generate these numbers.  The parameter N must be a
     nonnegative exact integer.

     For example:

          > (random-f64vector 3)
          #f64(.7145854494613069 .47089632669147946 .5400124875182746)

 -- procedure: make-random-source

     This procedure returns a new random source object initialized to a
     predetermined state (to initialize to a pseudo random state the
     procedure ‘random-source-randomize!’ should be called).

     For example:

          > (define rs (make-random-source))
          > ((random-source-make-integers rs) 10000000)
          8583952

 -- procedure: random-source? OBJ

     This procedure returns ‘#t’ when OBJ is a random source object and
     ‘#f’ otherwise.

     For example:

          > (random-source? default-random-source)
          #t
          > (random-source? 123)
          #f

 -- procedure: random-source-state-ref RANDOM-SOURCE
 -- procedure: random-source-state-set! RANDOM-SOURCE STATE

     The procedure ‘random-source-state-ref’ extracts the state of the
     random source object RANDOM-SOURCE and returns a vector containing
     the state.

     The procedure ‘random-source-state-set!’ restores the state of the
     random source object RANDOM-SOURCE to STATE which must be a vector
     returned from a call to the procedure ‘random-source-state-ref’.

     For example:

          > (define s (random-source-state-ref default-random-source))
          > (random-integer 10000000000000000000000000000000000000000)
          7583880188903074396261960585615270693321
          > (random-source-state-set! default-random-source s)
          > (random-integer 10000000000000000000000000000000000000000)
          7583880188903074396261960585615270693321

 -- procedure: random-source-randomize! RANDOM-SOURCE
 -- procedure: random-source-pseudo-randomize! RANDOM-SOURCE I J

     These procedures change the state of the random source object
     RANDOM-SOURCE.  The procedure ‘random-source-randomize!’ sets the
     random source object to a state that depends on the current time
     (which for typical uses can be considered to randomly initialize
     the state).  The procedure ‘random-source-pseudo-randomize!’ sets
     the random source object to a state that is determined only by the
     current state and the nonnegative exact integers I and J.  For both
     procedures the value returned is unspecified.

     For example:

          > (define s (random-source-state-ref default-random-source))
          > (random-source-pseudo-randomize! default-random-source 5 99)
          > (random-integer 10000000000000000000000000000000000000000)
          9816755163910623041601722050112674079767
          > (random-source-state-set! default-random-source s)
          > (random-source-pseudo-randomize! default-random-source 5 99)
          > (random-integer 10000000000000000000000000000000000000000)
          9816755163910623041601722050112674079767
          > (random-source-pseudo-randomize! default-random-source 5 99)
          > (random-integer 10000000000000000000000000000000000000000)
          9816755163910623041601722050112674079767
          > (random-source-state-set! default-random-source s)
          > (random-source-randomize! default-random-source)
          > (random-integer 10000000000000000000000000000000000000000)
          2271441220851914333384493143687768110622
          > (random-source-state-set! default-random-source s)
          > (random-source-randomize! default-random-source)
          > (random-integer 10000000000000000000000000000000000000000)
          6247966138948323029033944059178072366895

 -- procedure: random-source-make-integers RANDOM-SOURCE

     This procedure returns a procedure for generating pseudo random
     exact integers using the random source object RANDOM-SOURCE.  The
     returned procedure accepts a single parameter N, a positive exact
     integer, and returns a pseudo random exact integer in the range 0
     to N-1.

     For example:

          > (define rs (make-random-source))
          > (define ri (random-source-make-integers rs))
          > (ri 10000000)
          8583952
          > (ri 10000000)
          2879793

 -- procedure: random-source-make-reals RANDOM-SOURCE [PRECISION]

     This procedure returns a procedure for generating pseudo random
     inexact reals using the random source object RANDOM-SOURCE.  The
     returned procedure accepts no parameters and returns a pseudo
     random inexact real between, but not including, 0 and 1.  The
     optional parameter PRECISION specifies an upper bound on the
     minimum amount by which two generated pseudo-random numbers can be
     separated.

     For example:

          > (define rs (make-random-source))
          > (define rr (random-source-make-reals rs))
          > (rr)
          .857402537562821
          > (rr)
          .2876463473845367

 -- procedure: random-source-make-u8vectors RANDOM-SOURCE

     This procedure returns a procedure for generating pseudo random
     u8vectors using the random source object RANDOM-SOURCE.  The
     returned procedure accepts a single parameter N, a nonnegative
     exact integer, and returns a u8vector of length N containing pseudo
     random exact integers in the range 0 to 255.

     For example:

          > (define rs (make-random-source))
          > (define rv (random-source-make-u8vectors rs))
          > (rv 10)
          #u8(200 53 29 202 3 85 208 187 73 219)
          > (rv 10)
          #u8(113 8 182 120 138 103 53 192 40 176)

 -- procedure: random-source-make-f64vectors RANDOM-SOURCE [PRECISION]

     This procedure returns a procedure for generating pseudo random
     f64vectors using the random source object RANDOM-SOURCE.  The
     returned procedure accepts a single parameter N, a nonnegative
     exact integer, and returns an f64vector of length N containing
     pseudo random inexact reals between, but not including, 0 and 1.
     The optional parameter PRECISION specifies an upper bound on the
     minimum amount by which two generated pseudo-random numbers can be
     separated.

     For example:

          > (define rs (make-random-source))
          > (define rv (random-source-make-f64vectors rs))
          > (rv 3)
          #f64(.7342236104231586 .2876463473845367 .8574025375628211)
          > (rv 3)
          #f64(.013863292728449427 .33449296573515447 .8162050798467028)

8.2 Booleans
============

8.3 Pairs and lists
===================

8.4 Symbols and keywords
========================

8.5 Characters and strings
==========================

Gambit supports the Unicode character encoding standard.  Scheme
characters can be any of the characters whose Unicode encoding is in the
range 0 to #x10ffff (inclusive) but not in the range #xd800 to #xdfff.
Source code can also contain any Unicode character, however to read such
source code properly ‘gsi’ and ‘gsc’ must be told which character
encoding to use for reading the source code (i.e.  ISO-8859-1, UTF-8,
UTF-16, etc).  This can be done by specifying the runtime option
‘-:file-settings=...’ or ‘-:io-settings=...’ when ‘gsi’ and ‘gsc’ are
started.

8.6 Extensions to character procedures
======================================

 -- procedure: char->integer CHAR
 -- procedure: integer->char N

     The procedure ‘char->integer’ returns the Unicode encoding of the
     character CHAR.

     The procedure ‘integer->char’ returns the character whose Unicode
     encoding is the exact integer N.

     For example:

          > (char->integer #\!)
          33
          > (integer->char 65)
          #\A
          > (integer->char (char->integer #\u1234))
          #\u1234
          > (integer->char #xd800)
          *** ERROR IN (console)@4.1 -- (Argument 1) Out of range
          (integer->char 55296)

 -- procedure: char=? CHAR1...
 -- procedure: char<? CHAR1...
 -- procedure: char>? CHAR1...
 -- procedure: char<=? CHAR1...
 -- procedure: char>=? CHAR1...
 -- procedure: char-ci=? CHAR1...
 -- procedure: char-ci<? CHAR1...
 -- procedure: char-ci>? CHAR1...
 -- procedure: char-ci<=? CHAR1...
 -- procedure: char-ci>=? CHAR1...

     These procedures take any number of arguments including no
     argument.  This is useful to test if the elements of a list are
     sorted in a particular order.  For example, testing that the list
     of characters ‘lst’ is sorted in nondecreasing order can be done
     with the call ‘(apply char<? lst)’.

8.7 Extensions to string procedures
===================================

 -- procedure: string=? STRING1...
 -- procedure: string<? STRING1...
 -- procedure: string>? STRING1...
 -- procedure: string<=? STRING1...
 -- procedure: string>=? STRING1...
 -- procedure: string-ci=? STRING1...
 -- procedure: string-ci<? STRING1...
 -- procedure: string-ci>? STRING1...
 -- procedure: string-ci<=? STRING1...
 -- procedure: string-ci>=? STRING1...

     These procedures take any number of arguments including no
     argument.  This is useful to test if the elements of a list are
     sorted in a particular order.  For example, testing that the list
     of strings ‘lst’ is sorted in nondecreasing order can be done with
     the call ‘(apply string<? lst)’.

8.8 Vectors
===========

8.9 Homogeneous numeric vectors
===============================

Homogeneous vectors are vectors containing raw numbers of the same type
(signed or unsigned exact integers or inexact reals).  There are 10
types of homogeneous vectors: ‘s8vector’ (vector of exact integers in
the range -2^7 to 2^7-1), ‘u8vector’ (vector of exact integers in the
range 0 to 2^8-1), ‘s16vector’ (vector of exact integers in the range
-2^15 to 2^15-1), ‘u16vector’ (vector of exact integers in the range 0
to 2^16-1), ‘s32vector’ (vector of exact integers in the range -2^31 to
2^31-1), ‘u32vector’ (vector of exact integers in the range 0 to
2^32-1), ‘s64vector’ (vector of exact integers in the range -2^63 to
2^63-1), ‘u64vector’ (vector of exact integers in the range 0 to
2^64-1), ‘f32vector’ (vector of 32 bit floating point numbers), and
‘f64vector’ (vector of 64 bit floating point numbers).

   The lexical syntax of homogeneous vectors is specified in *note
Homogeneous vector syntax::.

   The procedures available for homogeneous vectors, listed below, are
the analog of the normal vector/string procedures for each of the
homogeneous vector types.

 -- procedure: s8vector? OBJ
 -- procedure: make-s8vector K [FILL]
 -- procedure: s8vector EXACT-INT8...
 -- procedure: s8vector-length S8VECTOR
 -- procedure: s8vector-ref S8VECTOR K
 -- procedure: s8vector-set S8VECTOR K EXACT-INT8
 -- procedure: s8vector-set! S8VECTOR K EXACT-INT8
 -- procedure: s8vector->list S8VECTOR
 -- procedure: list->s8vector LIST-OF-EXACT-INT8
 -- procedure: s8vector-fill! S8VECTOR FILL [START [END]]
 -- procedure: subs8vector-fill! VECTOR START END FILL
 -- procedure: s8vector-concatenate LST [SEPARATOR]
 -- procedure: s8vector-copy S8VECTOR [START [END]]
 -- procedure: s8vector-copy! DEST-S8VECTOR DEST-START S8VECTOR [START
          [END]]
 -- procedure: s8vector-append S8VECTOR...
 -- procedure: subs8vector S8VECTOR START END
 -- procedure: subs8vector-move! SRC-S8VECTOR SRC-START SRC-END
          DST-S8VECTOR DST-START
 -- procedure: s8vector-shrink! S8VECTOR K

 -- procedure: u8vector? OBJ
 -- procedure: make-u8vector K [FILL]
 -- procedure: u8vector EXACT-INT8...
 -- procedure: u8vector-length U8VECTOR
 -- procedure: u8vector-ref U8VECTOR K
 -- procedure: u8vector-set U8VECTOR K EXACT-INT8
 -- procedure: u8vector-set! U8VECTOR K EXACT-INT8
 -- procedure: u8vector->list U8VECTOR
 -- procedure: list->u8vector LIST-OF-EXACT-INT8
 -- procedure: u8vector-fill! U8VECTOR FILL [START [END]]
 -- procedure: subu8vector-fill! VECTOR START END FILL
 -- procedure: u8vector-concatenate LST [SEPARATOR]
 -- procedure: u8vector-copy U8VECTOR [START [END]]
 -- procedure: u8vector-copy! DEST-U8VECTOR DEST-START U8VECTOR [START
          [END]]
 -- procedure: u8vector-append U8VECTOR...
 -- procedure: subu8vector U8VECTOR START END
 -- procedure: subu8vector-move! SRC-U8VECTOR SRC-START SRC-END
          DST-U8VECTOR DST-START
 -- procedure: u8vector-shrink! U8VECTOR K

 -- procedure: s16vector? OBJ
 -- procedure: make-s16vector K [FILL]
 -- procedure: s16vector EXACT-INT16...
 -- procedure: s16vector-length S16VECTOR
 -- procedure: s16vector-ref S16VECTOR K
 -- procedure: s16vector-set S16VECTOR K EXACT-INT16
 -- procedure: s16vector-set! S16VECTOR K EXACT-INT16
 -- procedure: s16vector->list S16VECTOR
 -- procedure: list->s16vector LIST-OF-EXACT-INT16
 -- procedure: s16vector-fill! S16VECTOR FILL [START [END]]
 -- procedure: subs16vector-fill! VECTOR START END FILL
 -- procedure: s16vector-concatenate LST [SEPARATOR]
 -- procedure: s16vector-copy S16VECTOR [START [END]]
 -- procedure: s16vector-copy! DEST-S16VECTOR DEST-START S16VECTOR
          [START [END]]
 -- procedure: s16vector-append S16VECTOR...
 -- procedure: subs16vector S16VECTOR START END
 -- procedure: subs16vector-move! SRC-S16VECTOR SRC-START SRC-END
          DST-S16VECTOR DST-START
 -- procedure: s16vector-shrink! S16VECTOR K

 -- procedure: u16vector? OBJ
 -- procedure: make-u16vector K [FILL]
 -- procedure: u16vector EXACT-INT16...
 -- procedure: u16vector-length U16VECTOR
 -- procedure: u16vector-ref U16VECTOR K
 -- procedure: u16vector-set U16VECTOR K EXACT-INT16
 -- procedure: u16vector-set! U16VECTOR K EXACT-INT16
 -- procedure: u16vector->list U16VECTOR
 -- procedure: list->u16vector LIST-OF-EXACT-INT16
 -- procedure: u16vector-fill! U16VECTOR FILL [START [END]]
 -- procedure: subu16vector-fill! VECTOR START END FILL
 -- procedure: u16vector-concatenate LST [SEPARATOR]
 -- procedure: u16vector-copy U16VECTOR [START [END]]
 -- procedure: u16vector-copy! DEST-U16VECTOR DEST-START U16VECTOR
          [START [END]]
 -- procedure: u16vector-append U16VECTOR...
 -- procedure: subu16vector U16VECTOR START END
 -- procedure: subu16vector-move! SRC-U16VECTOR SRC-START SRC-END
          DST-U16VECTOR DST-START
 -- procedure: u16vector-shrink! U16VECTOR K

 -- procedure: s32vector? OBJ
 -- procedure: make-s32vector K [FILL]
 -- procedure: s32vector EXACT-INT32...
 -- procedure: s32vector-length S32VECTOR
 -- procedure: s32vector-ref S32VECTOR K
 -- procedure: s32vector-set S32VECTOR K EXACT-INT32
 -- procedure: s32vector-set! S32VECTOR K EXACT-INT32
 -- procedure: s32vector->list S32VECTOR
 -- procedure: list->s32vector LIST-OF-EXACT-INT32
 -- procedure: s32vector-fill! S32VECTOR FILL [START [END]]
 -- procedure: subs32vector-fill! VECTOR START END FILL
 -- procedure: s32vector-concatenate LST [SEPARATOR]
 -- procedure: s32vector-copy S32VECTOR [START [END]]
 -- procedure: s32vector-copy! DEST-S32VECTOR DEST-START S32VECTOR
          [START [END]]
 -- procedure: s32vector-append S32VECTOR...
 -- procedure: subs32vector S32VECTOR START END
 -- procedure: subs32vector-move! SRC-S32VECTOR SRC-START SRC-END
          DST-S32VECTOR DST-START
 -- procedure: s32vector-shrink! S32VECTOR K

 -- procedure: u32vector? OBJ
 -- procedure: make-u32vector K [FILL]
 -- procedure: u32vector EXACT-INT32...
 -- procedure: u32vector-length U32VECTOR
 -- procedure: u32vector-ref U32VECTOR K
 -- procedure: u32vector-set U32VECTOR K EXACT-INT32
 -- procedure: u32vector-set! U32VECTOR K EXACT-INT32
 -- procedure: u32vector->list U32VECTOR
 -- procedure: list->u32vector LIST-OF-EXACT-INT32
 -- procedure: u32vector-fill! U32VECTOR FILL [START [END]]
 -- procedure: subu32vector-fill! VECTOR START END FILL
 -- procedure: u32vector-concatenate LST [SEPARATOR]
 -- procedure: u32vector-copy U32VECTOR [START [END]]
 -- procedure: u32vector-copy! DEST-U32VECTOR DEST-START U32VECTOR
          [START [END]]
 -- procedure: u32vector-append U32VECTOR...
 -- procedure: subu32vector U32VECTOR START END
 -- procedure: subu32vector-move! SRC-U32VECTOR SRC-START SRC-END
          DST-U32VECTOR DST-START
 -- procedure: u32vector-shrink! U32VECTOR K

 -- procedure: s64vector? OBJ
 -- procedure: make-s64vector K [FILL]
 -- procedure: s64vector EXACT-INT64...
 -- procedure: s64vector-length S64VECTOR
 -- procedure: s64vector-ref S64VECTOR K
 -- procedure: s64vector-set S64VECTOR K EXACT-INT64
 -- procedure: s64vector-set! S64VECTOR K EXACT-INT64
 -- procedure: s64vector->list S64VECTOR
 -- procedure: list->s64vector LIST-OF-EXACT-INT64
 -- procedure: s64vector-fill! S64VECTOR FILL [START [END]]
 -- procedure: subs64vector-fill! VECTOR START END FILL
 -- procedure: s64vector-concatenate LST [SEPARATOR]
 -- procedure: s64vector-copy S64VECTOR [START [END]]
 -- procedure: s64vector-copy! DEST-S64VECTOR DEST-START S64VECTOR
          [START [END]]
 -- procedure: s64vector-append S64VECTOR...
 -- procedure: subs64vector S64VECTOR START END
 -- procedure: subs64vector-move! SRC-S64VECTOR SRC-START SRC-END
          DST-S64VECTOR DST-START
 -- procedure: s64vector-shrink! S64VECTOR K

 -- procedure: u64vector? OBJ
 -- procedure: make-u64vector K [FILL]
 -- procedure: u64vector EXACT-INT64...
 -- procedure: u64vector-length U64VECTOR
 -- procedure: u64vector-ref U64VECTOR K
 -- procedure: u64vector-set U64VECTOR K EXACT-INT64
 -- procedure: u64vector-set! U64VECTOR K EXACT-INT64
 -- procedure: u64vector->list U64VECTOR
 -- procedure: list->u64vector LIST-OF-EXACT-INT64
 -- procedure: u64vector-fill! U64VECTOR FILL [START [END]]
 -- procedure: subu64vector-fill! VECTOR START END FILL
 -- procedure: u64vector-concatenate LST [SEPARATOR]
 -- procedure: u64vector-copy U64VECTOR [START [END]]
 -- procedure: u64vector-copy! DEST-U64VECTOR DEST-START U64VECTOR
          [START [END]]
 -- procedure: u64vector-append U64VECTOR...
 -- procedure: subu64vector U64VECTOR START END
 -- procedure: subu64vector-move! SRC-U64VECTOR SRC-START SRC-END
          DST-U64VECTOR DST-START
 -- procedure: u64vector-shrink! U64VECTOR K

 -- procedure: f32vector? OBJ
 -- procedure: make-f32vector K [FILL]
 -- procedure: f32vector INEXACT-REAL...
 -- procedure: f32vector-length F32VECTOR
 -- procedure: f32vector-ref F32VECTOR K
 -- procedure: f32vector-set F32VECTOR K INEXACT-REAL
 -- procedure: f32vector-set! F32VECTOR K INEXACT-REAL
 -- procedure: f32vector->list F32VECTOR
 -- procedure: list->f32vector LIST-OF-INEXACT-REAL
 -- procedure: f32vector-fill! F32VECTOR FILL [START [END]]
 -- procedure: subf32vector-fill! VECTOR START END FILL
 -- procedure: f32vector-concatenate LST [SEPARATOR]
 -- procedure: f32vector-copy F32VECTOR [START [END]]
 -- procedure: f32vector-copy! DEST-F32VECTOR DEST-START F32VECTOR
          [START [END]]
 -- procedure: f32vector-append F32VECTOR...
 -- procedure: subf32vector F32VECTOR START END
 -- procedure: subf32vector-move! SRC-F32VECTOR SRC-START SRC-END
          DST-F32VECTOR DST-START
 -- procedure: f32vector-shrink! F32VECTOR K

 -- procedure: f64vector? OBJ
 -- procedure: make-f64vector K [FILL]
 -- procedure: f64vector INEXACT-REAL...
 -- procedure: f64vector-length F64VECTOR
 -- procedure: f64vector-ref F64VECTOR K
 -- procedure: f64vector-set F64VECTOR K INEXACT-REAL
 -- procedure: f64vector-set! F64VECTOR K INEXACT-REAL
 -- procedure: f64vector->list F64VECTOR
 -- procedure: list->f64vector LIST-OF-INEXACT-REAL
 -- procedure: f64vector-fill! F64VECTOR FILL [START [END]]
 -- procedure: subf64vector-fill! VECTOR START END FILL
 -- procedure: f64vector-concatenate LST [SEPARATOR]
 -- procedure: f64vector-copy F64VECTOR [START [END]]
 -- procedure: f64vector-copy! DEST-F64VECTOR DEST-START F64VECTOR
          [START [END]]
 -- procedure: f64vector-append F64VECTOR...
 -- procedure: subf64vector F64VECTOR START END
 -- procedure: subf64vector-move! SRC-F64VECTOR SRC-START SRC-END
          DST-F64VECTOR DST-START
 -- procedure: f64vector-shrink! F64VECTOR K

   For example:

     > (define v (u8vector 10 255 13))
     > (u8vector-set! v 2 99)
     > v
     #u8(10 255 99)
     > (u8vector-ref v 1)
     255
     > (u8vector->list v)
     (10 255 99)
     > (u8vector-shrink! v 2)
     > (v)
     #u8(10 255)

 -- procedure: object->u8vector OBJ [ENCODER]
 -- procedure: u8vector->object U8VECTOR [DECODER]

     The procedure ‘object->u8vector’ returns a u8vector that contains
     the sequence of bytes that encodes the object OBJ.  The procedure
     ‘u8vector->object’ decodes the sequence of bytes contained in the
     u8vector U8VECTOR, which was produced by the procedure
     ‘object->u8vector’, and reconstructs an object structurally equal
     to the original object.  In other words the procedures
     ‘object->u8vector’ and ‘u8vector->object’ respectively perform
     serialization and deserialization of Scheme objects.  Note that
     some objects are non-serializable (e.g.  threads, wills, some types
     of ports, and any object containing a non-serializable object).

     The optional ENCODER and DECODER parameters are single parameter
     procedures which default to the identity function.  The ENCODER
     procedure is called during serialization.  As the serializer walks
     through OBJ, it calls the ENCODER procedure on each sub-object X
     that is encountered.  The ENCODER transforms the object X into an
     object Y that will be serialized instead of X.  Similarly the
     DECODER procedure is called during deserialization.  When an object
     Y is encountered, the DECODER procedure is called to transform it
     into the object X that is the result of deserialization.

     The ENCODER and DECODER procedures are useful to customize the
     serialized representation of objects.  In particular, it can be
     used to define the semantics of serializing objects, such as
     threads and ports, that would otherwise not be serializable.  The
     DECODER procedure is typically the inverse of the ENCODER
     procedure, i.e.  ‘(DECODER (ENCODER X))’ = ‘X’.

     For example:

          > (define (make-adder x) (lambda (y) (+ x y)))
          > (define f (make-adder 10))
          > (define a (object->u8vector f))
          > (define b (u8vector->object a))
          > (u8vector-length a)
          1639
          > (f 5)
          15
          > (b 5)
          15
          > (pp b)
          (lambda (y) (+ x y))

8.10 Hashing and weak references
================================

8.10.1 Hashing
--------------

 -- procedure: object->serial-number OBJ
 -- procedure: serial-number->object N [DEFAULT]

     All Scheme objects are uniquely identified with a serial number
     which is a nonnegative exact integer.  The ‘object->serial-number’
     procedure returns the serial number of object OBJ.  This serial
     number is only allocated the first time the ‘object->serial-number’
     procedure is called on that object.  Objects which do not have an
     external textual representation that can be read by the ‘read’
     procedure, use an external textual representation that includes a
     serial number of the form ‘#N’.  Consequently, the procedures
     ‘write’, ‘pretty-print’, etc will call the ‘object->serial-number’
     procedure to get the serial number, and this may cause the serial
     number to be allocated.

     The ‘serial-number->object’ procedure takes an exact integer
     parameter N and returns the object whose serial number is N.  If no
     object currently exists with that serial number, DEFAULT is
     returned if it is specified, otherwise an
     unbound-serial-number-exception object is raised.  The reader
     defines the following abbreviation for calling
     ‘serial-number->object’: the syntax ‘#N’, where N is a sequence of
     decimal digits and it is not followed by ‘‘=’’ or ‘‘#’’, is
     equivalent to the list ‘(serial-number->object N)’.

     For example:

          > (define z (list (lambda (x) (* x x)) (lambda (y) (/ 1 y))))
          > z
          (#<procedure #2> #<procedure #3>)
          > (#3 10)
          1/10
          > '(#3 10)
          ((serial-number->object 3) 10)
          > car
          #<procedure #4 car>
          > (#4 z)
          #<procedure #2>

 -- procedure: unbound-serial-number-exception? OBJ
 -- procedure: unbound-serial-number-exception-procedure EXC
 -- procedure: unbound-serial-number-exception-arguments EXC

     Unbound-serial-number-exception objects are raised by the procedure
     ‘serial-number->object’ when no object currently exists with that
     serial number.  The parameter EXC must be an
     unbound-serial-number-exception object.

     The procedure ‘unbound-serial-number-exception?’ returns ‘#t’ when
     OBJ is a unbound-serial-number-exception object and ‘#f’ otherwise.

     The procedure ‘unbound-serial-number-exception-procedure’ returns
     the procedure that raised EXC.

     The procedure ‘unbound-serial-number-exception-arguments’ returns
     the list of arguments of the procedure that raised EXC.

     For example:

          > (define (handler exc)
              (if (unbound-serial-number-exception? exc)
                  (list (unbound-serial-number-exception-procedure exc)
                        (unbound-serial-number-exception-arguments exc))
                  'not-unbound-serial-number-exception))
          > (with-exception-catcher
              handler
              (lambda () (serial-number->object 1000)))
          (#<procedure #2 serial-number->object> (1000))

 -- procedure: symbol-hash SYMBOL

     The ‘symbol-hash’ procedure returns the hash number of the symbol
     SYMBOL.  The hash number is a small exact integer (fixnum).  When
     SYMBOL is an interned symbol the value returned is the same as
     ‘(string=?-hash (symbol->string SYMBOL))’.

     For example:

          > (symbol-hash 'car)
          444471047

 -- procedure: keyword-hash KEYWORD

     The ‘keyword-hash’ procedure returns the hash number of the keyword
     KEYWORD.  The hash number is a small exact integer (fixnum).  When
     KEYWORD is an interned keyword the value returned is the same as
     ‘(string=?-hash (keyword->string KEYWORD))’.

     For example:

          > (keyword-hash car:)
          444471047

 -- procedure: string=?-hash STRING

     The ‘string=?-hash’ procedure returns the hash number of the string
     STRING.  The hash number is a small exact integer (fixnum).  For
     any two strings S1 and S2, ‘(string=? S1 S2)’ implies ‘(=
     (string=?-hash S1) (string=?-hash S2))’.

     For example:

          > (string=?-hash "car")
          444471047

 -- procedure: string-ci=?-hash STRING

     The ‘string-ci=?-hash’ procedure returns the hash number of the
     string STRING.  The hash number is a small exact integer (fixnum).
     For any two strings S1 and S2, ‘(string-ci=? S1 S2)’ implies ‘(=
     (string-ci=?-hash S1) (string-ci=?-hash S2))’.

     For example:

          > (string-ci=?-hash "CaR")
          444471047

 -- procedure: eq?-hash OBJ

     The ‘eq?-hash’ procedure returns the hash number of the object OBJ.
     The hash number is a small exact integer (fixnum).  For any two
     objects O1 and O2, ‘(eq? O1 O2)’ implies ‘(= (eq?-hash O1)
     (eq?-hash O2))’.

     For example:

          > (eq?-hash #t)
          536870910

 -- procedure: eqv?-hash OBJ

     The ‘eqv?-hash’ procedure returns the hash number of the object
     OBJ.  The hash number is a small exact integer (fixnum).  For any
     two objects O1 and O2, ‘(eqv? O1 O2)’ implies ‘(= (eqv?-hash O1)
     (eqv?-hash O2))’.

     For example:

          > (eqv?-hash 1.5)
          496387656

 -- procedure: equal?-hash OBJ

     The ‘equal?-hash’ procedure returns the hash number of the object
     OBJ.  The hash number is a small exact integer (fixnum).  For any
     two objects O1 and O2, ‘(equal? O1 O2)’ implies ‘(= (equal?-hash
     O1) (equal?-hash O2))’.

     For example:

          > (equal?-hash (list 1 2 3))
          442438567

8.10.2 Weak references
----------------------

The garbage collector is responsible for reclaiming objects that are no
longer needed by the program.  This is done by analyzing the
reachability graph of all objects from the roots (i.e.  the global
variables, the runnable threads, permanently allocated objects such as
procedures defined in a compiled file, nonexecutable wills, etc).  If a
root or a reachable object X contains a reference to an object Y then Y
is reachable.  As a general rule, unreachable objects are reclaimed by
the garbage collector.

   There are two types of references: strong references and weak
references.  Most objects, including pairs, vectors, records and
closures, contain strong references.  An object X is “strongly
reachable” if there is a path from the roots to X that traverses only
strong references.  Weak references only occur in wills and tables.
There are two types of weak references: will-weak references and
table-weak references.  If all paths from the roots to an object Y
traverse at least one table-weak reference, then Y will be reclaimed by
the garbage collector.  The will-weak references are used for
finalization and are explained in the next section.

8.10.2.1 Wills
..............

The following procedures implement the “will” data type.  Will objects
provide support for finalization.  A will is an object that contains a
will-weak reference to a TESTATOR object (the object attached to the
will), and a strong reference to an ACTION procedure which is a one
parameter procedure which is called when the will is executed.

 -- procedure: make-will TESTATOR ACTION
 -- procedure: will? OBJ
 -- procedure: will-testator WILL
 -- procedure: will-execute! WILL

     The ‘make-will’ procedure creates a will object with the given
     TESTATOR object and ACTION procedure.  The ‘will?’ procedure tests
     if OBJ is a will object.  The ‘will-testator’ procedure gets the
     testator object attached to the WILL.  The ‘will-execute!’
     procedure executes WILL.

     A will becomes “executable” when its TESTATOR object is not
     strongly reachable (i.e.  the TESTATOR object is either unreachable
     or only reachable using paths from the roots that traverse at least
     one weak reference).  Some objects, including symbols, small exact
     integers (fixnums), booleans and characters, are considered to be
     always strongly reachable.

     When the runtime system detects that a will has become executable
     the current computation is interrupted, the will’s testator is set
     to ‘#f’ and the will’s action procedure is called with the will’s
     testator as the sole argument.  Currently only the garbage
     collector detects when wills become executable but this may change
     in future versions of Gambit (for example the compiler could
     perform an analysis to infer will executability at compile time).
     The garbage collector builds a list of all executable wills.
     Shortly after a garbage collection, the action procedures of these
     wills will be called.  The link from the will to the action
     procedure is severed when the action procedure is called.

     Note that the testator object will not be reclaimed during the
     garbage collection that determined executability of the will.  It
     is only when an object is not reachable from the roots that it is
     reclaimed by the garbage collector.

     A remarkable feature of wills is that an action procedure can
     “resurrect” an object.  An action procedure could for example
     assign the testator object to a global variable or create a new
     will with the same testator object.

     For example:

          > (define a (list 123))
          > (set-cdr! a a) ; create a circular list
          > (define b (vector a))
          > (define c #f)
          > (define w
              (let ((obj a))
                (make-will obj
                           (lambda (x) ; x will be eq? to obj
                             (display "executing action procedure")
                             (newline)
                             (set! c x)))))
          > (will? w)
          #t
          > (car (will-testator w))
          123
          > (##gc)
          > (set! a #f)
          > (##gc)
          > (set! b #f)
          > (##gc)
          executing action procedure
          > (will-testator w)
          #f
          > (car c)
          123

8.10.3 Tables
-------------

The following procedures implement the “table” data type.  Tables are
heterogenous structures whose elements are indexed by keys which are
arbitrary objects.  Tables are similar to association lists but are
abstract and the access time for large tables is typically smaller.
Each key contained in the table is bound to a value.  The length of the
table is the number of key/value bindings it contains.  New key/value
bindings can be added to a table, the value bound to a key can be
changed, and existing key/value bindings can be removed.

   The references to the keys can either be all strong or all table-weak
and the references to the values can either be all strong or all
table-weak.  The garbage collector removes key/value bindings from a
table when 1) the key is a table-weak reference and the key is
unreachable or only reachable using paths from the roots that traverse
at least one table-weak reference, or 2) the value is a table-weak
reference and the value is unreachable or only reachable using paths
from the roots that traverse at least one table-weak reference.
Key/value bindings that are removed by the garbage collector are
reclaimed immediately.

   Although there are several possible ways of implementing tables, the
current implementation uses hashing with open-addressing.  This is space
efficient and provides constant-time access.  Hash tables are
automatically resized to maintain the load within specified bounds.  The
load is the number of active entries (the length of the table) divided
by the total number of entries in the hash table.

   Tables are parameterized with a key comparison procedure.  By default
the ‘equal?’ procedure is used, but ‘eq?’, ‘eqv?’, ‘string=?’,
‘string-ci=?’, or a user defined procedure can also be used.  To support
arbitrary key comparison procedures, tables are also parameterized with
a hashing procedure accepting a key as its single parameter and
returning a fixnum result.  The hashing procedure HASH must be
consistent with the key comparison procedure TEST, that is, for any two
keys K1 and K2 in the table, ‘(TEST K1 K2)’ implies ‘(= (HASH K1) (HASH
K2))’.  A default hashing procedure consistent with the key comparison
procedure is provided by the system.  The default hashing procedure
generally gives good performance when the key comparison procedure is
‘eq?’, ‘eqv?’, ‘equal?’, ‘string=?’, and ‘string-ci=?’.  However, for
user defined key comparison procedures, the default hashing procedure
always returns 0.  This degrades the performance of the table to a
linear search.

   Tables can be compared for equality using the ‘equal?’ procedure.
Two tables ‘X’ and ‘Y’ are considered equal by ‘equal?’ when they have
the same weakness attributes, the same key comparison procedure, the
same hashing procedure, the same length, and for all the keys ‘K’ in
‘X’, ‘(equal? (table-ref X K) (table-ref Y K))’.

 -- procedure: make-table [size: SIZE] [init: INIT] [weak-keys:
          WEAK-KEYS] [weak-values: WEAK-VALUES] [test: TEST] [hash:
          HASH] [min-load: MIN-LOAD] [max-load: MAX-LOAD]

     The procedure ‘make-table’ returns a new table.  The optional
     keyword parameters specify various parameters of the table.

     The SIZE parameter is a nonnegative exact integer indicating the
     expected length of the table.  The system uses SIZE to choose an
     appropriate initial size of the hash table so that it does not need
     to be resized too often.

     The INIT parameter indicates a value that is associated to keys
     that are not in the table.  When INIT is not specified, no value is
     associated to keys that are not in the table.

     The WEAK-KEYS and WEAK-VALUES parameters are extended booleans
     indicating respectively whether the keys and values are table-weak
     references (true) or strong references (false).  By default the
     keys and values are strong references.

     The TEST parameter indicates the key comparison procedure.  The
     default key comparison procedure is ‘equal?’.  The key comparison
     procedures ‘eq?’, ‘eqv?’, ‘equal?’, ‘string=?’, and ‘string-ci=?’
     are special because the system will use a reasonably good hash
     procedure when none is specified.

     The HASH parameter indicates the hash procedure.  This procedure
     must accept a single key parameter, return a fixnum, and be
     consistent with the key comparison procedure.  When HASH is not
     specified, a default hash procedure is used.  The default hash
     procedure is reasonably good when the key comparison procedure is
     ‘eq?’, ‘eqv?’, ‘equal?’, ‘string=?’, or ‘string-ci=?’.

     The MIN-LOAD and MAX-LOAD parameters are real numbers that indicate
     the minimum and maximum load of the table respectively.  The table
     is resized when adding or deleting a key/value binding would bring
     the table’s load outside of this range.  The MIN-LOAD parameter
     must be no less than 0.05 and the MAX-LOAD parameter must be no
     greater than 0.95.  Moreover the difference between MIN-LOAD and
     MAX-LOAD must be at least 0.20.  When MIN-LOAD is not specified,
     the value 0.45 is used.  When MAX-LOAD is not specified, the value
     0.90 is used.

     For example:

          > (define t (make-table))
          > (table? t)
          #t
          > (table-length t)
          0
          > (table-set! t (list 1 2) 3)
          > (table-set! t (list 4 5) 6)
          > (table-ref t (list 1 2))
          3
          > (table-length t)
          2

 -- procedure: table? OBJ

     The procedure ‘table?’ returns ‘#t’ when OBJ is a table and ‘#f’
     otherwise.

     For example:

          > (table? (make-table))
          #t
          > (table? 123)
          #f

 -- procedure: table-length TABLE

     The procedure ‘table-length’ returns the number of key/value
     bindings contained in the table TABLE.

     For example:

          > (define t (make-table weak-keys: #t))
          > (define x (list 1 2))
          > (define y (list 3 4))
          > (table-set! t x 111)
          > (table-set! t y 222)
          > (table-length t)
          2
          > (table-set! t x)
          > (table-length t)
          1
          > (##gc)
          > (table-length t)
          1
          > (set! y #f)
          > (##gc)
          > (table-length t)
          0

 -- procedure: table-ref TABLE KEY [DEFAULT]

     The procedure ‘table-ref’ returns the value bound to the object KEY
     in the table TABLE.  When KEY is not bound and DEFAULT is
     specified, DEFAULT is returned.  When DEFAULT is not specified but
     an INIT parameter was specified when TABLE was created, INIT is
     returned.  Otherwise an unbound-key-exception object is raised.

     For example:

          > (define t1 (make-table init: 999))
          > (table-set! t1 (list 1 2) 3)
          > (table-ref t1 (list 1 2))
          3
          > (table-ref t1 (list 4 5))
          999
          > (table-ref t1 (list 4 5) #f)
          #f
          > (define t2 (make-table))
          > (table-ref t2 (list 4 5))
          *** ERROR IN (console)@7.1 -- Unbound key
          (table-ref '#<table #2> '(4 5))

 -- procedure: table-set! TABLE KEY [VALUE]

     The procedure ‘table-set!’ binds the object KEY to VALUE in the
     table TABLE.  When VALUE is not specified, if TABLE contains a
     binding for KEY then the binding is removed from TABLE.  The
     procedure ‘table-set!’ returns an unspecified value.

     For example:

          > (define t (make-table))
          > (table-set! t (list 1 2) 3)
          > (table-set! t (list 4 5) 6)
          > (table-set! t (list 4 5))
          > (table-set! t (list 7 8))
          > (table-ref t (list 1 2))
          3
          > (table-ref t (list 4 5))
          *** ERROR IN (console)@7.1 -- Unbound key
          (table-ref '#<table #2> '(4 5))

 -- procedure: table-search PROC TABLE

     The procedure ‘table-search’ searches the table TABLE for a
     key/value binding for which the two parameter procedure PROC
     returns a non false result.  For each key/value binding visited by
     ‘table-search’ the procedure PROC is called with the key as the
     first parameter and the value as the second parameter.  The
     procedure ‘table-search’ returns the first non false value returned
     by PROC, or ‘#f’ if PROC returned ‘#f’ for all key/value bindings
     in TABLE.

     The order in which the key/value bindings are visited is
     unspecified and may vary from one call of ‘table-search’ to the
     next.  While a call to ‘table-search’ is being performed on TABLE,
     it is an error to call any of the following procedures on TABLE:
     ‘table-ref’, ‘table-set!’, ‘table-search’, ‘table-for-each’,
     ‘table-copy’, ‘table-merge’, ‘table-merge!’, and ‘table->list’.  It
     is also an error to compare with ‘equal?’ (directly or indirectly
     with ‘member’, ‘assoc’, ‘table-ref’, etc.)  an object that contains
     TABLE.  All these procedures may cause TABLE to be reordered and
     resized.  This restriction allows a more efficient iteration over
     the key/value bindings.

     For example:

          > (define square (make-table))
          > (table-set! square 2 4)
          > (table-set! square 3 9)
          > (table-search (lambda (k v) (and (odd? k) v)) square)
          9

 -- procedure: table-for-each PROC TABLE

     The procedure ‘table-for-each’ calls the two parameter procedure
     PROC for each key/value binding in the table TABLE.  The procedure
     PROC is called with the key as the first parameter and the value as
     the second parameter.  The procedure ‘table-for-each’ returns an
     unspecified value.

     The order in which the key/value bindings are visited is
     unspecified and may vary from one call of ‘table-for-each’ to the
     next.  While a call to ‘table-for-each’ is being performed on
     TABLE, it is an error to call any of the following procedures on
     TABLE: ‘table-ref’, ‘table-set!’, ‘table-search’, ‘table-for-each’,
     and ‘table->list’.  It is also an error to compare with ‘equal?’
     (directly or indirectly with ‘member’, ‘assoc’, ‘table-ref’, etc.)
     an object that contains TABLE.  All these procedures may cause
     TABLE to be reordered and resized.  This restriction allows a more
     efficient iteration over the key/value bindings.

     For example:

          > (define square (make-table))
          > (table-set! square 2 4)
          > (table-set! square 3 9)
          > (table-for-each (lambda (k v) (write (list k v)) (newline)) square)
          (2 4)
          (3 9)

 -- procedure: table->list TABLE

     The procedure ‘table->list’ returns an association list containing
     the key/value bindings in the table TABLE.  Each key/value binding
     yields a pair whose car field is the key and whose cdr field is the
     value bound to that key.  The order of the bindings in the list is
     unspecified.

     For example:

          > (define square (make-table))
          > (table-set! square 2 4)
          > (table-set! square 3 9)
          > (table->list square)
          ((3 . 9) (2 . 4))

 -- procedure: list->table LIST [size: SIZE] [init: INIT] [weak-keys:
          WEAK-KEYS] [weak-values: WEAK-VALUES] [test: TEST] [hash:
          HASH] [min-load: MIN-LOAD] [max-load: MAX-LOAD]

     The procedure ‘list->table’ returns a new table containing the
     key/value bindings in the association list LIST.  The optional
     keyword parameters specify various parameters of the table and have
     the same meaning as for the ‘make-table’ procedure.

     Each element of LIST is a pair whose car field is a key and whose
     cdr field is the value bound to that key.  If a key appears more
     than once in LIST (tested using the table’s key comparison
     procedure) it is the first key/value binding in LIST that has
     precedence.

     For example:

          > (define t (list->table '((b . 2) (a . 1) (c . 3) (a . 4))))
          > (table->list t)
          ((a . 1) (b . 2) (c . 3))

 -- procedure: unbound-key-exception? OBJ
 -- procedure: unbound-key-exception-procedure EXC
 -- procedure: unbound-key-exception-arguments EXC

     Unbound-key-exception objects are raised by the procedure
     ‘table-ref’ when the key does not have a binding in the table.  The
     parameter EXC must be an unbound-key-exception object.

     The procedure ‘unbound-key-exception?’ returns ‘#t’ when OBJ is a
     unbound-key-exception object and ‘#f’ otherwise.

     The procedure ‘unbound-key-exception-procedure’ returns the
     procedure that raised EXC.

     The procedure ‘unbound-key-exception-arguments’ returns the list of
     arguments of the procedure that raised EXC.

     For example:

          > (define t (make-table))
          > (define (handler exc)
              (if (unbound-key-exception? exc)
                  (list (unbound-key-exception-procedure exc)
                        (unbound-key-exception-arguments exc))
                  'not-unbound-key-exception))
          > (with-exception-catcher
              handler
              (lambda () (table-ref t '(1 2))))
          (#<procedure #2 table-ref> (#<table #3> (1 2)))

 -- procedure: table-copy TABLE

     The procedure ‘table-copy’ returns a new table containing the same
     key/value bindings as TABLE and the same table parameters (i.e.
     hash procedure, key comparison procedure, key and value weakness,
     etc).

     For example:

          > (define t (list->table '((b . 2) (a . 1) (c . 3))))
          > (define x (table-copy t))
          > (table-set! t 'b 99)
          > (table->list t)
          ((a . 1) (b . 99) (c . 3))
          > (table->list x)
          ((a . 1) (b . 2) (c . 3))

 -- procedure: table-merge! TABLE1 TABLE2 [TABLE2-TAKES-PRECEDENCE?]

     The procedure ‘table-merge!’ returns TABLE1 after the key/value
     bindings contained in TABLE2 have been added to it.  When a key
     exists both in TABLE1 and TABLE2, then the parameter
     TABLE2-TAKES-PRECEDENCE? indicates which binding will be kept (the
     one in TABLE1 if TABLE2-TAKES-PRECEDENCE? is false, and the one in
     TABLE2 otherwise).  If TABLE2-TAKES-PRECEDENCE? is not specified
     the binding in TABLE1 is kept.

     For example:

          > (define t1 (list->table '((a . 1) (b . 2) (c . 3))))
          > (define t2 (list->table '((a . 4) (b . 5) (z . 6))))
          > (table->list (table-merge! t1 t2))
          ((a . 1) (b . 2) (c . 3) (z . 6))
          > (define t1 (list->table '((a . 1) (b . 2) (c . 3))))
          > (define t2 (list->table '((a . 4) (b . 5) (z . 6))))
          > (table->list (table-merge! t1 t2 #t))
          ((a . 4) (b . 5) (c . 3) (z . 6))

 -- procedure: table-merge TABLE1 TABLE2 [TABLE2-TAKES-PRECEDENCE?]

     The procedure ‘table-merge’ returns a copy of TABLE1 (created with
     ‘table-copy’) to which the key/value bindings contained in TABLE2
     have been added using ‘table-merge!’.  When a key exists both in
     TABLE1 and TABLE2, then the parameter TABLE2-TAKES-PRECEDENCE?
     indicates which binding will be kept (the one in TABLE1 if
     TABLE2-TAKES-PRECEDENCE? is false, and the one in TABLE2
     otherwise).  If TABLE2-TAKES-PRECEDENCE? is not specified the
     binding in TABLE1 is kept.

     For example:

          > (define t1 (list->table '((a . 1) (b . 2) (c . 3))))
          > (define t2 (list->table '((a . 4) (b . 5) (z . 6))))
          > (table->list (table-merge t1 t2))
          ((a . 1) (b . 2) (c . 3) (z . 6))
          > (table->list (table-merge t1 t2 #t))
          ((a . 4) (b . 5) (c . 3) (z . 6))

9 Records
*********

 -- special form: define-structure name field...

     Record data types similar to Pascal records and C ‘struct’ types
     can be defined using the ‘define-structure’ special form.  The
     identifier name specifies the name of the new data type.  The
     structure name is followed by K identifiers naming each field of
     the record.  The ‘define-structure’ expands into a set of
     definitions of the following procedures:

        • ‘make-name’ – A K argument procedure which constructs a new
          record from the value of its K fields.

        • ‘name?’ – A procedure which tests if its single argument is of
          the given record type.

        • ‘name-field’ – For each field, a procedure taking as its
          single argument a value of the given record type and returning
          the content of the corresponding field of the record.

        • ‘name-field-set!’ – For each field, a two argument procedure
          taking as its first argument a value of the given record type.
          The second argument gets assigned to the corresponding field
          of the record and the void object is returned.

     Record data types have a printed representation that includes the
     name of the type and the name and value of each field.  Record data
     types can not be read by the ‘read’ procedure.

     For example:

          > (define-structure point x y color)
          > (define p (make-point 3 5 'red))
          > p
          #<point #2 x: 3 y: 5 color: red>
          > (point-x p)
          3
          > (point-color p)
          red
          > (point-color-set! p 'black)
          > p
          #<point #2 x: 3 y: 5 color: black>

10 Threads
**********

Gambit supports the execution of multiple Scheme threads.  These threads
are managed entirely by Gambit’s runtime and are not related to the host
operating system’s threads.  Gambit’s runtime does not currently take
advantage of multiprocessors (i.e.  at most one thread is running).

10.1 Introduction
=================

Multithreading is a paradigm that is well suited for building complex
systems such as: servers, GUIs, and high-level operating systems.
Gambit’s thread system offers mechanisms for creating threads of
execution and for synchronizing them.  The thread system also supports
features which are useful in a real-time context, such as priorities,
priority inheritance and timeouts.

   The thread system provides the following data types:

   • Thread (a virtual processor which shares object space with all
     other threads)

   • Mutex (a mutual exclusion device, also known as a lock and binary
     semaphore)

   • Condition variable (a set of blocked threads)

10.2 Thread objects
===================

A “running thread” is a thread that is currently executing.  A “runnable
thread” is a thread that is ready to execute or running.  A thread is
“blocked” if it is waiting for a mutex to become unlocked, an I/O
operation to become possible, the end of a “sleep” period, etc.  A “new
thread” is a thread that has been allocated but has not yet been
initialized.  An “initialized thread” is a thread that can be made
runnable.  A new thread becomes runnable when it is started by calling
‘thread-start!’.  A “terminated thread” is a thread that can no longer
become runnable (but “deadlocked threads” are not considered
terminated).  The only valid transitions between the thread states are
from new to initialized, from initialized to runnable, between runnable
and blocked, and from any state except new to terminated as indicated in
the following diagram:

                                                 unblock
                               start            <-------
     NEW -------> INITIALIZED -------> RUNNABLE -------> BLOCKED
                            \             |      block  /
                             \            v            /
                              +-----> TERMINATED <----+

   Each thread has a “base priority”, which is a real number (where a
higher numerical value means a higher priority), a “priority boost”,
which is a nonnegative real number representing the priority increase
applied to a thread when it blocks, and a “quantum”, which is a
nonnegative real number representing a duration in seconds.

   Each thread has a “specific field” which can be used in an
application specific way to associate data with the thread (some thread
systems call this “thread local storage”).

   Each thread has a “mailbox” which is used for inter-thread
communication.

10.3 Mutex objects
==================

A mutex can be in one of four states: “locked” (either “owned” or “not
owned”) and “unlocked” (either “abandoned” or “not abandoned”).

   An attempt to lock a mutex only succeeds if the mutex is in an
unlocked state, otherwise the current thread will wait.  A mutex in the
locked/owned state has an associated “owner thread”, which by convention
is the thread that is responsible for unlocking the mutex (this case is
typical of critical sections implemented as “lock mutex, perform
operation, unlock mutex”).  A mutex in the locked/not-owned state is not
linked to a particular thread.

   A mutex becomes locked when a thread locks it using the ‘mutex-lock!’
primitive.  A mutex becomes unlocked/abandoned when the owner of a
locked/owned mutex terminates.  A mutex becomes unlocked/not-abandoned
when a thread unlocks it using the ‘mutex-unlock!’ primitive.

   The mutex primitives do not implement “recursive mutex semantics”.
An attempt to lock a mutex that is locked implies that the current
thread waits even if the mutex is owned by the current thread (this can
lead to a deadlock if no other thread unlocks the mutex).

   Each mutex has a “specific field” which can be used in an application
specific way to associate data with the mutex.

10.4 Condition variable objects
===============================

A condition variable represents a set of blocked threads.  These blocked
threads are waiting for a certain condition to become true.  When a
thread modifies some program state that might make the condition true,
the thread unblocks some number of threads (one or all depending on the
primitive used) so they can check if the condition is now true.  This
allows complex forms of interthread synchronization to be expressed more
conveniently than with mutexes alone.

   Each condition variable has a “specific field” which can be used in
an application specific way to associate data with the condition
variable.

10.5 Fairness
=============

In various situations the scheduler must select one thread from a set of
threads (e.g.  which thread to run when a running thread blocks or
expires its quantum, which thread to unblock when a mutex becomes
unlocked or a condition variable is signaled).  The constraints on the
selection process determine the scheduler’s “fairness”.  The selection
depends on the order in which threads become runnable or blocked and on
the “priority” attached to the threads.

   The definition of fairness requires the notion of time ordering, i.e.
“event A occured before event B”.  For the purpose of establishing time
ordering, the scheduler uses a clock with a discrete, usually variable,
resolution (a “tick”).  Events occuring in a given tick can be
considered to be simultaneous (i.e.  if event A occured before event B
in real time, then the scheduler will claim that event A occured before
event B unless both events fall within the same tick, in which case the
scheduler arbitrarily chooses a time ordering).

   Each thread T has three priorities which affect fairness; the “base
priority”, the “boosted priority”, and the “effective priority”.

   • The “base priority” is the value contained in T’s “base priority”
     field (which is set with the ‘thread-base-priority-set!’
     primitive).

   • T’s “boosted flag” field contains a boolean that affects T’s
     “boosted priority”.  When the boosted flag field is false, the
     boosted priority is equal to the base priority, otherwise the
     boosted priority is equal to the base priority plus the value
     contained in T’s “priority boost” field (which is set with the
     ‘thread-priority-boost-set!’ primitive).  The boosted flag field is
     set to false when a thread is created, when its quantum expires,
     and when “thread-yield!”  is called.  The boosted flag field is set
     to true when a thread blocks.  By carefully choosing the base
     priority and priority boost, relatively to the other threads, it is
     possible to set up an interactive thread so that it has good I/O
     response time without being a CPU hog when it performs long
     computations.

   • The “effective priority” is equal to the maximum of T’s boosted
     priority and the effective priority of all the threads that are
     blocked on a mutex owned by T.  This “priority inheritance” avoids
     priority inversion problems that would prevent a high priority
     thread blocked at the entry of a critical section to progress
     because a low priority thread inside the critical section is
     preempted for an arbitrary long time by a medium priority thread.

   Let P(T) be the effective priority of thread T and let R(T) be the
most recent time when one of the following events occurred for thread T,
thus making it runnable: T was started by calling ‘thread-start!’, T
called ‘thread-yield!’, T expired its quantum, or T became unblocked.
Let the relation NL(T1,T2), “T1 no later than T2”, be true if
P(T1)<P(T2) or P(T1)=P(T2) and R(T1)>R(T2), and false otherwise.  The
scheduler will schedule the execution of threads in such a way that
whenever there is at least one runnable thread, 1) within a finite time
at least one thread will be running, and 2) there is never a pair of
runnable threads T1 and T2 for which NL(T1,T2) is true and T1 is not
running and T2 is running.

   A thread T expires its quantum when an amount of time equal to T’s
quantum has elapsed since T entered the running state and T did not
block, terminate or call ‘thread-yield!’.  At that point T exits the
running state to allow other threads to run.  A thread’s quantum is thus
an indication of the rate of progress of the thread relative to the
other threads of the same priority.  Moreover, the resolution of the
timer measuring the running time may cause a certain deviation from the
quantum, so a thread’s quantum should only be viewed as an approximation
of the time it can run before yielding to another thread.

   Threads blocked on a given mutex or condition variable will unblock
in an order which is consistent with decreasing priority and increasing
blocking time (i.e.  the highest priority thread unblocks first, and
among equal priority threads the one that blocked first unblocks first).

10.6 Memory coherency
=====================

Read and write operations on the store (such as reading and writing a
variable, an element of a vector or a string) are not atomic.  It is an
error for a thread to write a location in the store while some other
thread reads or writes that same location.  It is the responsibility of
the application to avoid write/read and write/write races through
appropriate uses of the synchronization primitives.

   Concurrent reads and writes to ports are allowed.  It is the
responsibility of the implementation to serialize accesses to a given
port using the appropriate synchronization primitives.

10.7 Timeouts
=============

All synchronization primitives which take a timeout parameter accept
three types of values as a timeout, with the following meaning:

   • a time object represents an absolute point in time

   • an exact or inexact real number represents a relative time in
     seconds from the moment the primitive was called

   • ‘#f’ means that there is no timeout

   When a timeout denotes the current time or a time in the past, the
synchronization primitive claims that the timeout has been reached only
after the other synchronization conditions have been checked.  Moreover
the thread remains running (it does not enter the blocked state).  For
example, ‘(mutex-lock! m 0)’ will lock mutex ‘m’ and return ‘#t’ if ‘m’
is currently unlocked, otherwise ‘#f’ is returned because the timeout is
reached.

10.8 Primordial thread
======================

The execution of a program is initially under the control of a single
thread known as the “primordial thread”.  The primordial thread has an
unspecified base priority, priority boost, boosted flag, quantum, name,
specific field, dynamic environment, ‘dynamic-wind’ stack, and
exception-handler.  All threads are terminated when the primordial
thread terminates (normally or not).

10.9 Procedures
===============

 -- procedure: current-thread

     This procedure returns the current thread.  For example:

          > (current-thread)
          #<thread #1 primordial>
          > (eq? (current-thread) (current-thread))
          #t

 -- procedure: thread? OBJ

     This procedure returns ‘#t’ when OBJ is a thread object and ‘#f’
     otherwise.

     For example:

          > (thread? (current-thread))
          #t
          > (thread? 'foo)
          #f

 -- procedure: make-thread THUNK [NAME [THREAD-GROUP]]
 -- procedure: make-root-thread THUNK [NAME [THREAD-GROUP [INPUT-PORT
          [OUTPUT-PORT]]]]

     The ‘make-thread’ procedure creates and returns an initialized
     thread.  This thread is not automatically made runnable (the
     procedure ‘thread-start!’ must be used for this).  A thread has the
     following fields: base priority, priority boost, boosted flag,
     quantum, name, specific, end-result, end-exception, and a list of
     locked/owned mutexes it owns.  The thread’s execution consists of a
     call to THUNK with the “initial continuation”.  This continuation
     causes the (then) current thread to store the result in its
     end-result field, abandon all mutexes it owns, and finally
     terminate.  The ‘dynamic-wind’ stack of the initial continuation is
     empty.  The optional NAME is an arbitrary Scheme object which
     identifies the thread (useful for debugging); it defaults to an
     unspecified value.  The specific field is set to an unspecified
     value.  The optional THREAD-GROUP indicates which thread group this
     thread belongs to; it defaults to the thread group of the current
     thread.  The base priority, priority boost, and quantum of the
     thread are set to the same value as the current thread and the
     boosted flag is set to false.  The thread’s mailbox is initially
     empty.  The thread inherits the dynamic environment from the
     current thread.  Moreover, in this dynamic environment the
     exception-handler is bound to the “initial exception-handler” which
     is a unary procedure which causes the (then) current thread to
     store in its end-exception field an uncaught-exception object whose
     “reason” is the argument of the handler, abandon all mutexes it
     owns, and finally terminate.

     The ‘make-root-thread’ procedure behaves like the ‘make-thread’
     procedure except the created thread does not inherit the dynamic
     environment from the current thread and the base priority is set to
     0, the priority boost is set to 1.0e-6, and the quantum is set to
     0.02.  The dynamic environment of the thread has the global
     bindings of the parameter objects, except ‘current-input-port’
     which is bound to INPUT-PORT, ‘current-output-port’ which is bound
     to OUTPUT-PORT, and ‘current-directory’ which is bound to the
     initial current working directory of the current process.  If
     INPUT-PORT is not specified it defaults to a port corresponding to
     the standard input (‘stdin’).  If OUTPUT-PORT is not specified it
     defaults to a port corresponding to the standard output (‘stdout’).

     For example:

          > (make-thread (lambda () (write 'hello)))
          #<thread #2>
          > (make-root-thread (lambda () (write 'world)) 'a-name)
          #<thread #3 a-name>

 -- procedure: thread THUNK

     The ‘thread’ procedure creates, starts and returns a new thread.
     The call ‘(thread THUNK)’ is equivalent to ‘(thread-start!
     (make-thread THUNK))’.

     For example:

          > (define a (thread (lambda () (expt 2 1005))))
          > (define b (thread (lambda () (expt 2 1000))))
          > (/ (thread-join! a) (thread-join! b))
          32

 -- procedure: thread-name THREAD

     This procedure returns the name of the THREAD.  For example:

          > (thread-name (make-thread (lambda () #f) 'foo))
          foo

 -- procedure: thread-specific THREAD
 -- procedure: thread-specific-set! THREAD OBJ

     The ‘thread-specific’ procedure returns the content of the THREAD’s
     specific field.

     The ‘thread-specific-set!’ procedure stores OBJ into the THREAD’s
     specific field and returns an unspecified value.

     For example:

          > (thread-specific-set! (current-thread) "hello")
          > (thread-specific (current-thread))
          "hello"

 -- procedure: thread-base-priority THREAD
 -- procedure: thread-base-priority-set! THREAD PRIORITY

     The procedure ‘thread-base-priority’ returns a real number which
     corresponds to the base priority of the THREAD.

     The procedure ‘thread-base-priority-set!’ changes the base priority
     of the THREAD to PRIORITY and returns an unspecified value.  The
     PRIORITY must be a real number.

     For example:

          > (thread-base-priority-set! (current-thread) 12.3)
          > (thread-base-priority (current-thread))
          12.3

 -- procedure: thread-priority-boost THREAD
 -- procedure: thread-priority-boost-set! THREAD PRIORITY-BOOST

     The procedure ‘thread-priority-boost’ returns a real number which
     corresponds to the priority boost of the THREAD.

     The procedure ‘thread-priority-boost-set!’ changes the priority
     boost of the THREAD to PRIORITY-BOOST and returns an unspecified
     value.  The PRIORITY-BOOST must be a nonnegative real.

     For example:

          > (thread-priority-boost-set! (current-thread) 2.5)
          > (thread-priority-boost (current-thread))
          2.5

 -- procedure: thread-quantum THREAD
 -- procedure: thread-quantum-set! THREAD QUANTUM

     The procedure ‘thread-quantum’ returns a real number which
     corresponds to the quantum of the THREAD.

     The procedure ‘thread-quantum-set!’ changes the quantum of the
     THREAD to QUANTUM and returns an unspecified value.  The QUANTUM
     must be a nonnegative real.  A value of zero selects the smallest
     quantum supported by the implementation.

     For example:

          > (thread-quantum-set! (current-thread) 1.5)
          > (thread-quantum (current-thread))
          1.5
          > (thread-quantum-set! (current-thread) 0)
          > (thread-quantum (current-thread))
          0.

 -- procedure: thread-start! THREAD

     This procedure makes THREAD runnable and returns the THREAD.  The
     THREAD must be an initialized thread.

     For example:

          > (let ((t (thread-start! (make-thread (lambda () (write 'a))))))
              (write 'b)
              (thread-join! t))
          ab> or ba>

     NOTE: It is useful to separate thread creation and thread
     activation to avoid the race condition that would occur if the
     created thread tries to examine a table in which the current thread
     stores the created thread.  See the last example of the
     ‘thread-terminate!’ procedure which contains mutually recursive
     threads.

 -- procedure: thread-yield!

     This procedure causes the current thread to exit the running state
     as if its quantum had expired and returns an unspecified value.

     For example:

          ; a busy loop that avoids being too wasteful of the CPU

          (let loop ()
            (if (mutex-lock! m 0) ; try to lock m but don’t block
                (begin
                  (display "locked mutex m")
                  (mutex-unlock! m))
                (begin
                  (do-something-else)
                  (thread-yield!) ; relinquish rest of quantum
                  (loop))))

 -- procedure: thread-sleep! TIMEOUT

     This procedure causes the current thread to wait until the timeout
     is reached and returns an unspecified value.  This blocks the
     thread only if TIMEOUT represents a point in the future.  It is an
     error for TIMEOUT to be ‘#f’.

     For example:

          ; a clock with a gradual drift:

          (let loop ((x 1))
            (thread-sleep! 1)
            (write x)
            (loop (+ x 1)))

          ; a clock with no drift:

          (let ((start (time->seconds (current-time)))
            (let loop ((x 1))
              (thread-sleep! (seconds->time (+ x start)))
              (write x)
              (loop (+ x 1))))

 -- procedure: thread-terminate! THREAD

     This procedure causes an abnormal termination of the THREAD.  If
     the THREAD is not already terminated, all mutexes owned by the
     THREAD become unlocked/abandoned and a terminated-thread-exception
     object is stored in the THREAD’s end-exception field.  If THREAD is
     the current thread, ‘thread-terminate!’ does not return.  Otherwise
     ‘thread-terminate!’ returns an unspecified value; the termination
     of the THREAD will occur at some point between the calling of
     ‘thread-terminate!’ and a finite time in the future (an explicit
     thread synchronization is needed to detect termination, see
     ‘thread-join!’).

     For example:

          (define (amb thunk1 thunk2)
            (let ((result #f)
                  (result-mutex (make-mutex))
                  (done-mutex (make-mutex)))
              (letrec ((child1
                        (make-thread
                          (lambda ()
                            (let ((x (thunk1)))
                              (mutex-lock! result-mutex #f #f)
                              (set! result x)
                              (thread-terminate! child2)
                              (mutex-unlock! done-mutex)))))
                       (child2
                        (make-thread
                          (lambda ()
                            (let ((x (thunk2)))
                              (mutex-lock! result-mutex #f #f)
                              (set! result x)
                              (thread-terminate! child1)
                              (mutex-unlock! done-mutex))))))
                (mutex-lock! done-mutex #f #f)
                (thread-start! child1)
                (thread-start! child2)
                (mutex-lock! done-mutex #f #f)
                result)))

     NOTE: This operation must be used carefully because it terminates a
     thread abruptly and it is impossible for that thread to perform any
     kind of cleanup.  This may be a problem if the thread is in the
     middle of a critical section where some structure has been put in
     an inconsistent state.  However, another thread attempting to enter
     this critical section will raise an abandoned-mutex-exception
     object because the mutex is unlocked/abandoned.  This helps avoid
     observing an inconsistent state.  Clean termination can be obtained
     by polling, as shown in the example below.

     For example:

          (define (spawn thunk)
            (let ((t (make-thread thunk)))
              (thread-specific-set! t #t)
              (thread-start! t)
              t))

          (define (stop! thread)
            (thread-specific-set! thread #f)
            (thread-join! thread))

          (define (keep-going?)
            (thread-specific (current-thread)))

          (define count!
            (let ((m (make-mutex))
                  (i 0))
              (lambda ()
                (mutex-lock! m)
                (let ((x (+ i 1)))
                  (set! i x)
                  (mutex-unlock! m)
                  x))))

          (define (increment-forever!)
            (let loop () (count!) (if (keep-going?) (loop))))

          (let ((t1 (spawn increment-forever!))
                (t2 (spawn increment-forever!)))
            (thread-sleep! 1)
            (stop! t1)
            (stop! t2)
            (count!))  ==>  377290

 -- procedure: thread-join! thread [TIMEOUT [TIMEOUT-VAL]]

     This procedure causes the current thread to wait until the THREAD
     terminates (normally or not) or until the timeout is reached if
     TIMEOUT is supplied.  If the timeout is reached, THREAD-JOIN!
     returns TIMEOUT-VAL if it is supplied, otherwise a
     join-timeout-exception object is raised.  If the THREAD terminated
     normally, the content of the end-result field is returned,
     otherwise the content of the end-exception field is raised.

     For example:

          (let ((t (thread-start! (make-thread (lambda () (expt 2 100))))))
            (do-something-else)
            (thread-join! t))  ==>  1267650600228229401496703205376

          (let ((t (thread-start! (make-thread (lambda () (raise 123))))))
            (do-something-else)
            (with-exception-handler
              (lambda (exc)
                (if (uncaught-exception? exc)
                    (* 10 (uncaught-exception-reason exc))
                    99999))
              (lambda ()
                (+ 1 (thread-join! t)))))  ==>  1231

          (define thread-alive?
            (let ((unique (list 'unique)))
              (lambda (thread)
                ; Note: this procedure raises an exception if
                ; the thread terminated abnormally.
                (eq? (thread-join! thread 0 unique) unique))))

          (define (wait-for-termination! thread)
            (let ((eh (current-exception-handler)))
              (with-exception-handler
                (lambda (exc)
                  (if (not (or (terminated-thread-exception? exc)
                               (uncaught-exception? exc)))
                      (eh exc))) ; unexpected exceptions are handled by eh
                (lambda ()
                  ; The following call to thread-join! will wait until the
                  ; thread terminates.  If the thread terminated normally
                  ; thread-join! will return normally.  If the thread
                  ; terminated abnormally then one of these two exception
                  ; objects is raised by thread-join!:
                  ;   - terminated-thread-exception object
                  ;   - uncaught-exception object
                  (thread-join! thread)
                  #f)))) ; ignore result of thread-join!

 -- procedure: thread-send THREAD MSG

     Each thread has a mailbox which stores messages delivered to the
     thread in the order delivered.

     The procedure ‘thread-send’ adds the message MSG at the end of the
     mailbox of thread THREAD and returns an unspecified value.

     For example:

          > (thread-send (current-thread) 111)
          > (thread-send (current-thread) 222)
          > (thread-receive)
          111
          > (thread-receive)
          222

 -- procedure: thread-receive [TIMEOUT [DEFAULT]]
 -- procedure: thread-mailbox-next [TIMEOUT [DEFAULT]]
 -- procedure: thread-mailbox-rewind
 -- procedure: thread-mailbox-extract-and-rewind

     To allow a thread to examine the messages in its mailbox without
     removing them from the mailbox, each thread has a “mailbox cursor”
     which normally points to the last message accessed in the mailbox.
     When a mailbox cursor is rewound using the procedure
     ‘thread-mailbox-rewind’ or ‘thread-mailbox-extract-and-rewind’ or
     ‘thread-receive’, the cursor does not point to a message, but the
     next call to ‘thread-receive’ and ‘thread-mailbox-next’ will set
     the cursor to the oldest message in the mailbox.

     The procedure ‘thread-receive’ advances the mailbox cursor of the
     current thread to the next message, removes that message from the
     mailbox, rewinds the mailbox cursor, and returns the message.  When
     TIMEOUT is not specified, the current thread will wait until a
     message is available in the mailbox.  When TIMEOUT is specified and
     DEFAULT is not specified, a mailbox-receive-timeout-exception
     object is raised if the timeout is reached before a message is
     available.  When TIMEOUT is specified and DEFAULT is specified,
     DEFAULT is returned if the timeout is reached before a message is
     available.

     The procedure ‘thread-mailbox-next’ behaves like ‘thread-receive’
     except that the message remains in the mailbox and the mailbox
     cursor is not rewound.

     The procedures ‘thread-mailbox-rewind’ or
     ‘thread-mailbox-extract-and-rewind’ rewind the mailbox cursor of
     the current thread so that the next call to ‘thread-mailbox-next’
     and ‘thread-receive’ will access the oldest message in the mailbox.
     Additionally the procedure ‘thread-mailbox-extract-and-rewind’ will
     remove from the mailbox the message most recently accessed by a
     call to ‘thread-mailbox-next’.  When ‘thread-mailbox-next’ has not
     been called since the last call to ‘thread-receive’ or
     ‘thread-mailbox-rewind’ or ‘thread-mailbox-extract-and-rewind’, a
     call to ‘thread-mailbox-extract-and-rewind’ only resets the mailbox
     cursor (no message is removed).

     For example:

          > (thread-send (current-thread) 111)
          > (thread-receive 1 999)
          111
          > (thread-send (current-thread) 222)
          > (thread-send (current-thread) 333)
          > (thread-mailbox-next 1 999)
          222
          > (thread-mailbox-next 1 999)
          333
          > (thread-mailbox-next 1 999)
          999
          > (thread-mailbox-extract-and-rewind)
          > (thread-receive 1 999)
          222
          > (thread-receive 1 999)
          999

 -- procedure: mailbox-receive-timeout-exception? OBJ
 -- procedure: mailbox-receive-timeout-exception-procedure EXC
 -- procedure: mailbox-receive-timeout-exception-arguments EXC

     Mailbox-receive-timeout-exception objects are raised by the
     procedures ‘thread-receive’ and ‘thread-mailbox-next’ when a
     timeout expires before a message is available and no default value
     is specified.  The parameter EXC must be a
     mailbox-receive-timeout-exception object.

     The procedure ‘mailbox-receive-timeout-exception?’ returns ‘#t’
     when OBJ is a mailbox-receive-timeout-exception object and ‘#f’
     otherwise.

     The procedure ‘mailbox-receive-timeout-exception-procedure’ returns
     the procedure that raised EXC.

     The procedure ‘mailbox-receive-timeout-exception-arguments’ returns
     the list of arguments of the procedure that raised EXC.

     For example:

          > (define (handler exc)
              (if (mailbox-receive-timeout-exception? exc)
                  (list (mailbox-receive-timeout-exception-procedure exc)
                        (mailbox-receive-timeout-exception-arguments exc))
                  'not-mailbox-receive-timeout-exception))
          > (with-exception-catcher
              handler
              (lambda () (thread-receive 1)))
          (#<procedure #2 thread-receive> (1))

 -- procedure: mutex? OBJ

     This procedure returns ‘#t’ when OBJ is a mutex object and ‘#f’
     otherwise.

     For example:

          > (mutex? (make-mutex))
          #t
          > (mutex? 'foo)
          #f

 -- procedure: make-mutex [NAME]

     This procedure returns a new mutex in the unlocked/not-abandoned
     state.  The optional NAME is an arbitrary Scheme object which
     identifies the mutex (useful for debugging); it defaults to an
     unspecified value.  The mutex’s specific field is set to an
     unspecified value.

     For example:

          > (make-mutex)
          #<mutex #2>
          > (make-mutex 'foo)
          #<mutex #3 foo>

 -- procedure: mutex-name MUTEX

     Returns the name of the MUTEX.  For example:

          > (mutex-name (make-mutex 'foo))
          foo

 -- procedure: mutex-specific MUTEX
 -- procedure: mutex-specific-set! MUTEX OBJ

     The ‘mutex-specific’ procedure returns the content of the MUTEX’s
     specific field.

     The ‘mutex-specific-set!’ procedure stores OBJ into the MUTEX’s
     specific field and returns an unspecified value.

     For example:

          > (define m (make-mutex))
          > (mutex-specific-set! m "hello")
          > (mutex-specific m)
          "hello"
          > (define (mutex-lock-recursively! mutex)
              (if (eq? (mutex-state mutex) (current-thread))
                  (let ((n (mutex-specific mutex)))
                    (mutex-specific-set! mutex (+ n 1)))
                  (begin
                    (mutex-lock! mutex)
                    (mutex-specific-set! mutex 0))))
          > (define (mutex-unlock-recursively! mutex)
              (let ((n (mutex-specific mutex)))
                (if (= n 0)
                    (mutex-unlock! mutex)
                    (mutex-specific-set! mutex (- n 1)))))
          > (mutex-lock-recursively! m)
          > (mutex-lock-recursively! m)
          > (mutex-lock-recursively! m)
          > (mutex-specific m)
          2

 -- procedure: mutex-state MUTEX

     Thos procedure returns information about the state of the MUTEX.
     The possible results are:

        • thread T: the MUTEX is in the locked/owned state and thread T
          is the owner of the MUTEX

        • symbol ‘not-owned’: the MUTEX is in the locked/not-owned state

        • symbol ‘abandoned’: the MUTEX is in the unlocked/abandoned
          state

        • symbol ‘not-abandoned’: the MUTEX is in the
          unlocked/not-abandoned state

     For example:

          (mutex-state (make-mutex))  ==>  not-abandoned

          (define (thread-alive? thread)
            (let ((mutex (make-mutex)))
              (mutex-lock! mutex #f thread)
              (let ((state (mutex-state mutex)))
                (mutex-unlock! mutex) ; avoid space leak
                (eq? state thread))))

 -- procedure: mutex-lock! MUTEX [TIMEOUT [THREAD]]

     This procedure locks MUTEX.  If the MUTEX is currently locked, the
     current thread waits until the MUTEX is unlocked, or until the
     timeout is reached if TIMEOUT is supplied.  If the timeout is
     reached, ‘mutex-lock!’ returns ‘#f’.  Otherwise, the state of the
     MUTEX is changed as follows:

        • if THREAD is ‘#f’ the MUTEX becomes locked/not-owned,

        • otherwise, let T be THREAD (or the current thread if THREAD is
          not supplied),

             • if T is terminated the MUTEX becomes unlocked/abandoned,

             • otherwise MUTEX becomes locked/owned with T as the owner.

     After changing the state of the MUTEX, an abandoned-mutex-exception
     object is raised if the MUTEX was unlocked/abandoned before the
     state change, otherwise ‘mutex-lock!’ returns ‘#t’.  It is not an
     error if the MUTEX is owned by the current thread (but the current
     thread will have to wait).

     For example:

          ; an implementation of a mailbox object of depth one; this
          ; implementation does not behave well in the presence of forced
          ; thread terminations using thread-terminate! (deadlock can occur
          ; if a thread is terminated in the middle of a put! or get! operation)

          (define (make-empty-mailbox)
            (let ((put-mutex (make-mutex)) ; allow put! operation
                  (get-mutex (make-mutex))
                  (cell #f))

              (define (put! obj)
                (mutex-lock! put-mutex #f #f) ; prevent put! operation
                (set! cell obj)
                (mutex-unlock! get-mutex)) ; allow get! operation

              (define (get!)
                (mutex-lock! get-mutex #f #f) ; wait until object in mailbox
                (let ((result cell))
                  (set! cell #f) ; prevent space leaks
                  (mutex-unlock! put-mutex) ; allow put! operation
                  result))

              (mutex-lock! get-mutex #f #f) ; prevent get! operation

              (lambda (msg)
                (case msg
                  ((put!) put!)
                  ((get!) get!)
                  (else (error "unknown message"))))))

          (define (mailbox-put! m obj) ((m 'put!) obj))
          (define (mailbox-get! m) ((m 'get!)))

          ; an alternate implementation of thread-sleep!

          (define (sleep! timeout)
            (let ((m (make-mutex)))
              (mutex-lock! m #f #f)
              (mutex-lock! m timeout #f)))

          ; a procedure that waits for one of two mutexes to unlock

          (define (lock-one-of! mutex1 mutex2)
            ; this procedure assumes that neither mutex1 or mutex2
            ; are owned by the current thread
            (let ((ct (current-thread))
                  (done-mutex (make-mutex)))
              (mutex-lock! done-mutex #f #f)
              (let ((t1 (thread-start!
                         (make-thread
                          (lambda ()
                            (mutex-lock! mutex1 #f ct)
                            (mutex-unlock! done-mutex)))))
                    (t2 (thread-start!
                         (make-thread
                          (lambda ()
                            (mutex-lock! mutex2 #f ct)
                            (mutex-unlock! done-mutex))))))
                (mutex-lock! done-mutex #f #f)
                (thread-terminate! t1)
                (thread-terminate! t2)
                (if (eq? (mutex-state mutex1) ct)
                    (begin
                      (if (eq? (mutex-state mutex2) ct)
                          (mutex-unlock! mutex2)) ; don't lock both
                      mutex1)
                    mutex2))))

 -- procedure: mutex-unlock! MUTEX [CONDITION-VARIABLE [TIMEOUT]]

     This procedure unlocks the MUTEX by making it
     unlocked/not-abandoned.  It is not an error to unlock an unlocked
     mutex and a mutex that is owned by any thread.  If
     CONDITION-VARIABLE is supplied, the current thread is blocked and
     added to the CONDITION-VARIABLE before unlocking MUTEX; the thread
     can unblock at any time but no later than when an appropriate call
     to ‘condition-variable-signal!’ or ‘condition-variable-broadcast!’
     is performed (see below), and no later than the timeout (if TIMEOUT
     is supplied).  If there are threads waiting to lock this MUTEX, the
     scheduler selects a thread, the mutex becomes locked/owned or
     locked/not-owned, and the thread is unblocked.  ‘mutex-unlock!’
     returns ‘#f’ when the timeout is reached, otherwise it returns
     ‘#t’.

     NOTE: The reason the thread can unblock at any time (when
     CONDITION-VARIABLE is supplied) is that the scheduler, when it
     detects a serious problem such as a deadlock, must interrupt one of
     the blocked threads (such as the primordial thread) so that it can
     perform some appropriate action.  After a thread blocked on a
     condition-variable has handled such an interrupt it would be wrong
     for the scheduler to return the thread to the blocked state,
     because any calls to ‘condition-variable-broadcast!’ during the
     interrupt will have gone unnoticed.  It is necessary for the thread
     to remain runnable and return from the call to ‘mutex-unlock!’ with
     a result of ‘#t’.

     NOTE: ‘mutex-unlock!’ is related to the “wait” operation on
     condition variables available in other thread systems.  The main
     difference is that “wait” automatically locks MUTEX just after the
     thread is unblocked.  This operation is not performed by
     ‘mutex-unlock!’ and so must be done by an explicit call to
     ‘mutex-lock!’.  This has the advantages that a different timeout
     and exception-handler can be specified on the ‘mutex-lock!’ and
     ‘mutex-unlock!’ and the location of all the mutex operations is
     clearly apparent.

     For example:

          (let loop ()
            (mutex-lock! m)
            (if (condition-is-true?)
                (begin
                  (do-something-when-condition-is-true)
                  (mutex-unlock! m))
                (begin
                  (mutex-unlock! m cv)
                  (loop))))

 -- procedure: condition-variable? OBJ

     This procedure returns ‘#t’ when OBJ is a condition-variable object
     and ‘#f’ otherwise.

     For example:

          > (condition-variable? (make-condition-variable))
          #t
          > (condition-variable? 'foo)
          #f

 -- procedure: make-condition-variable [NAME]

     This procedure returns a new empty condition variable.  The
     optional NAME is an arbitrary Scheme object which identifies the
     condition variable (useful for debugging); it defaults to an
     unspecified value.  The condition variable’s specific field is set
     to an unspecified value.

     For example:

          > (make-condition-variable)
          #<condition-variable #2>

 -- procedure: condition-variable-name CONDITION-VARIABLE

     This procedure returns the name of the CONDITION-VARIABLE.  For
     example:

          > (condition-variable-name (make-condition-variable 'foo))
          foo

 -- procedure: condition-variable-specific CONDITION-VARIABLE
 -- procedure: condition-variable-specific-set! CONDITION-VARIABLE OBJ

     The ‘condition-variable-specific’ procedure returns the content of
     the CONDITION-VARIABLE’s specific field.

     The ‘condition-variable-specific-set!’ procedure stores OBJ into
     the CONDITION-VARIABLE’s specific field and returns an unspecified
     value.

     For example:

          > (define cv (make-condition-variable))
          > (condition-variable-specific-set! cv "hello")
          > (condition-variable-specific cv)
          "hello"

 -- procedure: condition-variable-signal! CONDITION-VARIABLE

     This procedure unblocks a thread blocked on the CONDITION-VARIABLE
     (if there is at least one) and returns an unspecified value.

     For example:

          ; an implementation of a mailbox object of depth one; this
          ; implementation behaves gracefully when threads are forcibly
          ; terminated using thread-terminate! (an abandoned-mutex-exception
          ; object will be raised when a put! or get! operation is attempted
          ; after a thread is terminated in the middle of a put! or get!
          ; operation)

          (define (make-empty-mailbox)
            (let ((mutex (make-mutex))
                  (put-condvar (make-condition-variable))
                  (get-condvar (make-condition-variable))
                  (full? #f)
                  (cell #f))

              (define (put! obj)
                (mutex-lock! mutex)
                (if full?
                    (begin
                      (mutex-unlock! mutex put-condvar)
                      (put! obj))
                    (begin
                      (set! cell obj)
                      (set! full? #t)
                      (condition-variable-signal! get-condvar)
                      (mutex-unlock! mutex))))

              (define (get!)
                (mutex-lock! mutex)
                (if (not full?)
                    (begin
                      (mutex-unlock! mutex get-condvar)
                      (get!))
                    (let ((result cell))
                      (set! cell #f) ; avoid space leaks
                      (set! full? #f)
                      (condition-variable-signal! put-condvar)
                      (mutex-unlock! mutex)
                      result)))

              (lambda (msg)
                (case msg
                  ((put!) put!)
                  ((get!) get!)
                  (else (error "unknown message"))))))

          (define (mailbox-put! m obj) ((m 'put!) obj))
          (define (mailbox-get! m) ((m 'get!)))

 -- procedure: condition-variable-broadcast! CONDITION-VARIABLE

     This procedure unblocks all the thread blocked on the
     CONDITION-VARIABLE and returns an unspecified value.

     For example:

          (define (make-semaphore n)
            (vector n (make-mutex) (make-condition-variable)))

          (define (semaphore-wait! sema)
            (mutex-lock! (vector-ref sema 1))
            (let ((n (vector-ref sema 0)))
              (if (> n 0)
                  (begin
                    (vector-set! sema 0 (- n 1))
                    (mutex-unlock! (vector-ref sema 1)))
                  (begin
                    (mutex-unlock! (vector-ref sema 1) (vector-ref sema 2))
                    (semaphore-wait! sema))))

          (define (semaphore-signal-by! sema increment)
            (mutex-lock! (vector-ref sema 1))
            (let ((n (+ (vector-ref sema 0) increment)))
              (vector-set! sema 0 n)
              (if (> n 0)
                  (condition-variable-broadcast! (vector-ref sema 2)))
              (mutex-unlock! (vector-ref sema 1))))

11 Dynamic environment
**********************

The “dynamic environment” is the structure which allows the system to
find the value returned by the standard procedures ‘current-input-port’
and ‘current-output-port’.  The standard procedures
‘with-input-from-file’ and ‘with-output-to-file’ extend the dynamic
environment to produce a new dynamic environment which is in effect for
the dynamic extent of the call to the thunk passed as their last
argument.  These procedures are essentially special purpose dynamic
binding operations on hidden dynamic variables (one for
‘current-input-port’ and one for ‘current-output-port’).  Gambit
generalizes this dynamic binding mechanism to allow the user to
introduce new dynamic variables, called “parameter objects”, and
dynamically bind them.  The parameter objects implemented by Gambit are
compatible with the specification of the “Parameter objects SRFI” (SRFI
39).

   One important issue is the relationship between the dynamic
environments of the parent and child threads when a thread is created.
Each thread has its own dynamic environment that is accessed when
looking up the value bound to a parameter object by that thread.  When a
thread’s dynamic environment is extended it does not affect the dynamic
environment of other threads.  When a thread is created it is given a
dynamic environment whose bindings are inherited from the parent thread.
In this inherited dynamic environment the parameter objects are bound to
the same cells as the parent’s dynamic environment (in other words an
assignment of a new value to a parameter object is visible in the other
thread).

   Another important issue is the interaction between the ‘dynamic-wind’
procedure and dynamic environments.  When a thread creates a
continuation, the thread’s dynamic environment and the ‘dynamic-wind’
stack are saved within the continuation (an alternate but equivalent
point of view is that the ‘dynamic-wind’ stack is part of the dynamic
environment).  When this continuation is invoked the required
‘dynamic-wind’ before and after thunks are called and the saved dynamic
environment is reinstated as the dynamic environment of the current
thread.  During the call to each required ‘dynamic-wind’ before and
after thunk, the dynamic environment and the ‘dynamic-wind’ stack in
effect when the corresponding ‘dynamic-wind’ was executed are
reinstated.  Note that this specification precisely defines the
semantics of calling ‘call-with-current-continuation’ or invoking a
continuation within a before or after thunk.  The semantics are well
defined even when a continuation created by another thread is invoked.
Below is an example exercising the subtleties of this semantics.

     (with-output-to-file
      "foo"
      (lambda ()
        (let ((k (call-with-current-continuation
                  (lambda (exit)
                    (with-output-to-file
                     "bar"
                     (lambda ()
                       (dynamic-wind
                        (lambda ()
                          (write '(b1))
                          (force-output))
                        (lambda ()
                          (let ((x (call-with-current-continuation
                                    (lambda (cont) (exit cont)))))
                            (write '(t1))
                            (force-output)
                            x))
                        (lambda ()
                          (write '(a1))
                          (force-output)))))))))
          (if k
              (dynamic-wind
               (lambda ()
                 (write '(b2))
                 (force-output))
               (lambda ()
                 (with-output-to-file
                  "baz"
                  (lambda ()
                    (write '(t2))
                    (force-output)
                    ; go back inside (with-output-to-file "bar" ...)
                    (k #f))))
               (lambda ()
                 (write '(a2))
                 (force-output)))))))

   The following actions will occur when this code is executed:
‘(b1)(a1)’ is written to “bar”, ‘(b2)’ is then written to “foo”, ‘(t2)’
is then written to “baz”, ‘(a2)’ is then written to “foo”, and finally
‘(b1)(t1)(a1)’ is written to “bar”.

 -- procedure: make-parameter OBJ [SET-FILTER [GET-FILTER]]

     The dynamic environment is composed of two parts: the “local
     dynamic environment” and the “global dynamic environment”.  There
     is a single global dynamic environment, and it is used to lookup
     parameter objects that can’t be found in the local dynamic
     environment.

     The ‘make-parameter’ procedure returns a new “parameter object”.
     The SET-FILTER argument is a one argument “set” conversion
     procedure.  The GET-FILTER argument is a one argument “get”
     conversion procedure.  If they are not specified the conversion
     procedures default to the identity function.

     The global dynamic environment is updated to associate the
     parameter object to a new cell.  The initial content of the cell is
     the result of applying the “set” conversion procedure to OBJ.

     A parameter object is a procedure which accepts zero or one
     argument.  The cell bound to a particular parameter object in the
     dynamic environment is accessed by calling the parameter object.
     When no argument is passed, the value returned is the result of
     applying the “get” conversion procedure to the content of the cell.
     When one argument is passed the content of the cell is updated with
     the result of applying the parameter object’s “set” conversion
     procedure to the argument.  Note that the conversion procedures can
     be used for guaranteeing the type of the parameter object’s binding
     and/or to perform some conversion of the value.

     For example:

          > (define radix (make-parameter 10))
          > (radix)
          10
          > (radix 2)
          > (radix)
          2
          > (define prompt
              (make-parameter
                123
                (lambda (x)
                  (if (string? x)
                      x
                      (object->string x)))))
          > (prompt)
          "123"
          > (prompt "$")
          > (prompt)
          "$"
          > (define p
              (make-parameter
                100
                (lambda (val) ;; set filter
                  (pp (list val: val))
                  (list 0 val))
                (lambda (state) ;; get filter
                  (pp (list state: state))
                  (set-car! state (+ 1 (car state)))
                  (+ (car state) (cadr state)))))
          (val: 100)
          > (p)
          (state: (0 100))
          101
          > (p)
          (state: (1 100))
          102
          > (p)
          (state: (2 100))
          103
          > (p 555)
          (val: 555)
          > (p)
          (state: (0 555))
          556
          > (p)
          (state: (1 555))
          557
          > (define write-shared
              (make-parameter
                #f
                (lambda (x)
                  (if (boolean? x)
                      x
                      (error "only booleans are accepted by write-shared")))))
          > (write-shared 123)
          *** ERROR IN ##make-parameter -- only booleans are accepted by write-shared

 -- special form: parameterize ((procedure value)...) body

     The ‘parameterize’ form, evaluates all procedure and value
     expressions in an unspecified order.  All the procedure expressions
     must evaluate to procedures, either parameter objects or procedures
     accepting zero and one argument.  Then, for each procedure p and in
     an unspecified order:

        • If p is a parameter object a new cell is created, initialized,
          and bound to the parameter object in the local dynamic
          environment.  The value contained in the cell is the result of
          applying the parameter object’s “set” conversion procedure to
          value.  The resulting dynamic environment is then used for
          processing the remaining bindings (or the evaluation of body
          if there are no other bindings).

        • Otherwise p will be used according to the following protocol:
          we say that the call ‘(p)’ “gets p’s value” and that the call
          ‘(p x)’ “sets p’s value to x”.  First, the ‘parameterize’ form
          gets p’s value and saves it in a local variable.  It then sets
          p’s value to value.  It then processes the remaining bindings
          (or evaluates body if there are no other bindings).  Then it
          sets p’s value to the saved value.  These steps are performed
          in a ‘dynamic-wind’ so that it is possible to use
          continuations to jump into and out of the body (i.e.  the
          ‘dynamic-wind’’s before thunk sets p’s value to value and the
          after thunk sets p’s value to the saved value).

     The result(s) of the ‘parameterize’ form are the result(s) of the
     body.

     Note that using procedures instead of parameter objects may lead to
     unexpected results in multithreaded programs because the before and
     after thunks of the ‘dynamic-wind’ are not called when control
     switches between threads.

     For example:

          > (define radix (make-parameter 2))
          > (define prompt
              (make-parameter
                123
                (lambda (x)
                  (if (string? x)
                      x
                      (object->string x)))))
          > (radix)
          2
          > (parameterize ((radix 16)) (radix))
          16
          > (radix)
          2
          > (define (f n) (number->string n (radix)))
          > (f 10)
          "1010"
          > (parameterize ((radix 8)) (f 10))
          "12"
          > (parameterize ((radix 8) (prompt (f 10))) (prompt))
          "1010"
          > (define p
              (let ((x 1))
                (lambda args
                  (if (null? args) x (set! x (car args))))))
          > (let* ((a (p))
                   (b (parameterize ((p 2)) (list (p))))
                   (c (p)))
              (list a b c))
          (1 (2) 1)

12 Exceptions
*************

12.1 Exception-handling
=======================

Gambit’s exception-handling model is inspired from the withdrawn
“Exception Handling SRFI” (SRFI 12), the “Multithreading support SRFI”
(SRFI 18), and the “Exception Handling for Programs SRFI” (SRFI 34).
The two fundamental operations are the dynamic binding of an exception
handler (i.e.  the procedure ‘with-exception-handler’) and the
invocation of the exception handler (i.e.  the procedure ‘raise’).

   All predefined procedures which check for errors (including type
errors, memory allocation errors, host operating-system errors, etc)
report these errors using the exception-handling system (i.e.  they
“raise” an exception that can be handled in a user-defined exception
handler).  When an exception is raised and the exception is not handled
by a user-defined exception handler, the predefined exception handler
will display an error message (if the primordial thread raised the
exception) or the thread will silently terminate with no error message
(if it is not the primordial thread that raised the exception).  This
default behavior can be changed through the ‘-:debug=...’ runtime option
(*note Runtime options::).

   Predefined procedures normally raise exceptions by performing a
tail-call to the exception handler (the exceptions are “complex”
procedures such as ‘eval’, ‘compile-file’, ‘read’, ‘write’, etc).  This
means that the continuation of the exception handler and of the REPL
that may be started due to this is normally the continuation of the
predefined procedure that raised the exception.  By exiting the REPL
with the ‘,(c EXPRESSION)’ command it is thus possible to resume the
program as though the call to the predefined procedure returned the
value of EXPRESSION.  For example:

     > (define (f x) (+ (car x) 1))
     > (f 2) ; typo... we meant to say (f ’(2))
     *** ERROR IN f, (console)@1.18 -- (Argument 1) PAIR expected
     (car 2)
     1> ,(c 2)
     3

 -- procedure: current-exception-handler [NEW-EXCEPTION-HANDLER]

     The parameter object ‘current-exception-handler’ is bound to the
     current exception-handler.  Calling this procedure with no argument
     returns the current exception-handler and calling this procedure
     with one argument sets the current exception-handler to
     NEW-EXCEPTION-HANDLER.

     For example:

          > (current-exception-handler)
          #<procedure #2 primordial-exception-handler>
          > (current-exception-handler (lambda (exc) (pp exc) 999))
          > (/ 1 0)
          #<divide-by-zero-exception #3>
          999

 -- procedure: with-exception-handler HANDLER THUNK

     Returns the result(s) of calling THUNK with no arguments.  The
     HANDLER, which must be a procedure, is installed as the current
     exception-handler in the dynamic environment in effect during the
     call to THUNK.  Note that the dynamic environment in effect during
     the call to HANDLER has HANDLER as the exception-handler.
     Consequently, an exception raised during the call to HANDLER may
     lead to an infinite loop.

     For example:

          > (with-exception-handler
              (lambda (e) (write e) 5)
              (lambda () (+ 1 (* 2 3) 4)))
          11
          > (with-exception-handler
              (lambda (e) (write e) 5)
              (lambda () (+ 1 (* 'foo 3) 4)))
          #<type-exception #2>10
          > (with-exception-handler
              (lambda (e) (write e 9))
              (lambda () (+ 1 (* 'foo 3) 4)))
          INFINITE LOOP

 -- procedure: with-exception-catcher HANDLER THUNK

     Returns the result(s) of calling THUNK with no arguments.  A new
     exception-handler is installed as the current exception-handler in
     the dynamic environment in effect during the call to THUNK.  This
     new exception-handler will call the HANDLER, which must be a
     procedure, with the exception object as an argument and with the
     same continuation as the call to ‘with-exception-catcher’.  This
     implies that the dynamic environment in effect during the call to
     HANDLER is the same as the one in effect at the call to
     ‘with-exception-catcher’.  Consequently, an exception raised during
     the call to HANDLER will not lead to an infinite loop.

     For example:

          > (with-exception-catcher
              (lambda (e) (write e) 5)
              (lambda () (+ 1 (* 2 3) 4)))
          11
          > (with-exception-catcher
              (lambda (e) (write e) 5)
              (lambda () (+ 1 (* 'foo 3) 4)))
          #<type-exception #2>5
          > (with-exception-catcher
              (lambda (e) (write e 9))
              (lambda () (+ 1 (* 'foo 3) 4)))
          *** ERROR IN (console)@7.1 -- (Argument 2) OUTPUT PORT expected
          (write '#<type-exception #3> 9)

 -- procedure: raise OBJ

     This procedure tail-calls the current exception-handler with OBJ as
     the sole argument.  If the exception-handler returns, the
     continuation of the call to ‘raise’ is invoked.

     For example:

          > (with-exception-handler
              (lambda (exc)
                (pp exc)
                100)
              (lambda ()
                (+ 1 (raise "hello"))))
          "hello"
          101

 -- procedure: abort OBJ
 -- procedure: noncontinuable-exception? OBJ
 -- procedure: noncontinuable-exception-reason EXC

     The procedure ‘abort’ calls the current exception-handler with OBJ
     as the sole argument.  If the exception-handler returns, the
     procedure ‘abort’ will be tail-called with a
     noncontinuable-exception object, whose reason field is OBJ, as sole
     argument.

     Noncontinuable-exception objects are raised by the ‘abort’
     procedure when the exception-handler returns.  The parameter EXC
     must be a noncontinuable-exception object.

     The procedure ‘noncontinuable-exception?’ returns ‘#t’ when OBJ is
     a noncontinuable-exception object and ‘#f’ otherwise.

     The procedure ‘noncontinuable-exception-reason’ returns the
     argument of the call to ‘abort’ that raised EXC.

     For example:

          > (call-with-current-continuation
              (lambda (k)
                (with-exception-handler
                  (lambda (exc)
                    (pp exc)
                    (if (noncontinuable-exception? exc)
                        (k (list (noncontinuable-exception-reason exc)))
                        100))
                  (lambda ()
                    (+ 1 (abort "hello"))))))
          "hello"
          #<noncontinuable-exception #2>
          ("hello")

12.2 Exception objects related to memory management
===================================================

 -- procedure: heap-overflow-exception? OBJ

     Heap-overflow-exception objects are raised when the allocation of
     an object would cause the heap to use more memory space than is
     available.

     The procedure ‘heap-overflow-exception?’ returns ‘#t’ when OBJ is a
     heap-overflow-exception object and ‘#f’ otherwise.

     For example:

          > (define (handler exc)
              (if (heap-overflow-exception? exc)
                  exc
                  'not-heap-overflow-exception))
          > (with-exception-catcher
              handler
              (lambda ()
                (define (f x) (f (cons 1 x)))
                (f '())))
          #<heap-overflow-exception #2>

 -- procedure: stack-overflow-exception? OBJ

     Stack-overflow-exception objects are raised when the allocation of
     a continuation frame would cause the heap to use more memory space
     than is available.

     The procedure ‘stack-overflow-exception?’ returns ‘#t’ when OBJ is
     a stack-overflow-exception object and ‘#f’ otherwise.

     For example:

          > (define (handler exc)
              (if (stack-overflow-exception? exc)
                  exc
                  'not-stack-overflow-exception))
          > (with-exception-catcher
              handler
              (lambda ()
                (define (f) (+ 1 (f)))
                (f)))
          #<stack-overflow-exception #2>

12.3 Exception objects related to the host environment
======================================================

 -- procedure: os-exception? OBJ
 -- procedure: os-exception-procedure EXC
 -- procedure: os-exception-arguments EXC
 -- procedure: os-exception-code EXC
 -- procedure: os-exception-message EXC

     Os-exception objects are raised by procedures which access the host
     operating-system’s services when the requested operation fails.
     The parameter EXC must be a os-exception object.

     The procedure ‘os-exception?’ returns ‘#t’ when OBJ is a
     os-exception object and ‘#f’ otherwise.

     The procedure ‘os-exception-procedure’ returns the procedure that
     raised EXC.

     The procedure ‘os-exception-arguments’ returns the list of
     arguments of the procedure that raised EXC.

     The procedure ‘os-exception-code’ returns an exact integer error
     code that can be converted to a string by the ‘err-code->string’
     procedure.  Note that the error code is operating-system dependent.

     The procedure ‘os-exception-message’ returns ‘#f’ or a string
     giving details of the exception in a human-readable form.

     For example:

          > (define (handler exc)
              (if (os-exception? exc)
                  (list (os-exception-procedure exc)
                        (os-exception-arguments exc)
                        (err-code->string (os-exception-code exc))
                        (os-exception-message exc))
                  'not-os-exception))
          > (with-exception-catcher
              handler
              (lambda () (host-info "x.y.z")))
          (#<procedure #2 host-info> ("x.y.z") "Unknown host" #f)

 -- procedure: no-such-file-or-directory-exception? OBJ
 -- procedure: no-such-file-or-directory-exception-procedure EXC
 -- procedure: no-such-file-or-directory-exception-arguments EXC

     No-such-file-or-directory-exception objects are raised by
     procedures which access the filesystem (such as ‘open-input-file’
     and ‘directory-files’) when the path specified can’t be found on
     the filesystem.  The parameter EXC must be a
     no-such-file-or-directory-exception object.

     The procedure ‘no-such-file-or-directory-exception?’ returns ‘#t’
     when OBJ is a no-such-file-or-directory-exception object and ‘#f’
     otherwise.

     The procedure ‘no-such-file-or-directory-exception-procedure’
     returns the procedure that raised EXC.

     The procedure ‘no-such-file-or-directory-exception-arguments’
     returns the list of arguments of the procedure that raised EXC.

     For example:

          > (define (handler exc)
              (if (no-such-file-or-directory-exception? exc)
                  (list (no-such-file-or-directory-exception-procedure exc)
                        (no-such-file-or-directory-exception-arguments exc))
                  'not-no-such-file-or-directory-exception))
          > (with-exception-catcher
              handler
              (lambda () (with-input-from-file "nofile" read)))
          (#<procedure #2 with-input-from-file> ("nofile" #<procedure #3 read>))

 -- procedure: file-exists-exception? OBJ
 -- procedure: file-exists-exception-procedure EXC
 -- procedure: file-exists-exception-arguments EXC

     File-exists-exception objects are raised by procedures which access
     the filesystem (such as ‘open-output-file’ and ‘create-directory’)
     when the path specified is an existing file on the filesystem.  The
     parameter EXC must be a file-exists-exception object.

     The procedure ‘file-exists-exception?’ returns ‘#t’ when OBJ is a
     file-exists-exception object and ‘#f’ otherwise.

     The procedure ‘file-exists-exception-procedure’ returns the
     procedure that raised EXC.

     The procedure ‘file-exists-exception-arguments’ returns the list of
     arguments of the procedure that raised EXC.

     For example:

          > (define (handler exc)
              (if (file-exists-exception? exc)
                  (list (file-exists-exception-procedure exc)
                        (file-exists-exception-arguments exc))
                  'not-file-exists-exception))
          > (with-exception-catcher
              handler
              (lambda () (with-output-to-file '(path: "foo" create: #t) newline)))
          > (with-exception-catcher
              handler
              (lambda () (with-output-to-file '(path: "foo" create: #t) newline)))
          (#<procedure #2 with-output-to-file>
           ((path: "foo" create: #t) #<procedure #3 newline>))

 -- procedure: permission-denied-exception? OBJ
 -- procedure: permission-denied-exception-procedure EXC
 -- procedure: permission-denied-exception-arguments EXC

     Permission-denied-exception objects are raised by procedures which
     access the filesystem (such as ‘open-file’ and ‘open-directory’)
     when the access to the specified path is not allowed, or search
     permission is denied for a directory in the path prefix, or write
     access to the parent directory isn’t allowed for a file that
     doesn’t exist yet on the filesystem.  The parameter EXC must be a
     permission-denied-exception object.

     The procedure ‘permission-denied-exception?’ returns ‘#t’ when OBJ
     is a permission-denied-exception object and ‘#f’ otherwise.

     The procedure ‘permission-denied-exception-procedure’ returns the
     procedure that raised EXC.

     The procedure ‘permission-denied-exception-arguments’ returns the
     list of arguments of the procedure that raised EXC.

     For example:

          > (define (handler exc)
              (if (permission-denied-exception? exc)
                  (list (permission-denied-exception-procedure exc)
                        (permission-denied-exception-arguments exc))
                  'not-permission-denied-exception))
          > (with-exception-catcher
              handler
              (lambda () (with-input-from-file "empty" read)))
          #!eof
          > (with-exception-catcher
              handler
              (lambda () (with-input-from-file "noperm" read)))
          (#<procedure #2 with-input-from-file> ("noperm" #<procedure #3 read>))

 -- procedure: unbound-os-environment-variable-exception? OBJ
 -- procedure: unbound-os-environment-variable-exception-procedure EXC
 -- procedure: unbound-os-environment-variable-exception-arguments EXC

     Unbound-os-environment-variable-exception objects are raised when
     an unbound operating-system environment variable is accessed by the
     procedures ‘getenv’ and ‘setenv’.  The parameter EXC must be an
     unbound-os-environment-variable-exception object.

     The procedure ‘unbound-os-environment-variable-exception?’ returns
     ‘#t’ when OBJ is an unbound-os-environment-variable-exception
     object and ‘#f’ otherwise.

     The procedure ‘unbound-os-environment-variable-exception-procedure’
     returns the procedure that raised EXC.

     The procedure ‘unbound-os-environment-variable-exception-arguments’
     returns the list of arguments of the procedure that raised EXC.

     For example:

          > (define (handler exc)
              (if (unbound-os-environment-variable-exception? exc)
                  (list (unbound-os-environment-variable-exception-procedure exc)
                        (unbound-os-environment-variable-exception-arguments exc))
                  'not-unbound-os-environment-variable-exception))
          > (with-exception-catcher
              handler
              (lambda () (getenv "DOES_NOT_EXIST")))
          (#<procedure #2 getenv> ("DOES_NOT_EXIST"))

12.4 Exception objects related to threads
=========================================

 -- procedure: scheduler-exception? OBJ
 -- procedure: scheduler-exception-reason EXC

     Scheduler-exception objects are raised by the scheduler when some
     operation requested from the host operating system failed (e.g.
     checking the status of the devices in order to wake up threads
     waiting to perform I/O on these devices).  The parameter EXC must
     be a scheduler-exception object.

     The procedure ‘scheduler-exception?’ returns ‘#t’ when OBJ is a
     scheduler-exception object and ‘#f’ otherwise.

     The procedure ‘scheduler-exception-reason’ returns the os-exception
     object that describes the failure detected by the scheduler.

 -- procedure: deadlock-exception? OBJ

     Deadlock-exception objects are raised when the scheduler discovers
     that all threads are blocked and can make no further progress.  In
     that case the scheduler unblocks the primordial-thread and forces
     it to raise a deadlock-exception object.

     The procedure ‘deadlock-exception?’ returns ‘#t’ when OBJ is a
     deadlock-exception object and ‘#f’ otherwise.

     For example:

          > (define (handler exc)
              (if (deadlock-exception? exc)
                  exc
                  'not-deadlock-exception))
          > (with-exception-catcher
              handler
              (lambda () (read (open-vector))))
          #<deadlock-exception #2>

 -- procedure: abandoned-mutex-exception? OBJ

     Abandoned-mutex-exception objects are raised when the current
     thread locks a mutex that was owned by a thread which terminated
     (see ‘mutex-lock!’).

     The procedure ‘abandoned-mutex-exception?’ returns ‘#t’ when OBJ is
     a abandoned-mutex-exception object and ‘#f’ otherwise.

     For example:

          > (define (handler exc)
              (if (abandoned-mutex-exception? exc)
                  exc
                  'not-abandoned-mutex-exception))
          > (with-exception-catcher
              handler
              (lambda ()
                (let ((m (make-mutex)))
                  (thread-join!
                    (thread-start!
                      (make-thread
                        (lambda () (mutex-lock! m)))))
                  (mutex-lock! m))))
          #<abandoned-mutex-exception #2>

 -- procedure: join-timeout-exception? OBJ
 -- procedure: join-timeout-exception-procedure EXC
 -- procedure: join-timeout-exception-arguments EXC

     Join-timeout-exception objects are raised when a call to the
     ‘thread-join!’ procedure reaches its timeout before the target
     thread terminates and a timeout-value parameter is not specified.
     The parameter EXC must be a join-timeout-exception object.

     The procedure ‘join-timeout-exception?’ returns ‘#t’ when OBJ is a
     join-timeout-exception object and ‘#f’ otherwise.

     The procedure ‘join-timeout-exception-procedure’ returns the
     procedure that raised EXC.

     The procedure ‘join-timeout-exception-arguments’ returns the list
     of arguments of the procedure that raised EXC.

     For example:

          > (define (handler exc)
              (if (join-timeout-exception? exc)
                  (list (join-timeout-exception-procedure exc)
                        (join-timeout-exception-arguments exc))
                  'not-join-timeout-exception))
          > (with-exception-catcher
              handler
              (lambda ()
                (thread-join!
                  (thread-start!
                    (make-thread
                      (lambda () (thread-sleep! 10))))
                  5)))
          (#<procedure #2 thread-join!> (#<thread #3> 5))

 -- procedure: started-thread-exception? OBJ
 -- procedure: started-thread-exception-procedure EXC
 -- procedure: started-thread-exception-arguments EXC

     Started-thread-exception objects are raised when the target thread
     of a call to the procedure ‘thread-start!’ is already started.  The
     parameter EXC must be a started-thread-exception object.

     The procedure ‘started-thread-exception?’ returns ‘#t’ when OBJ is
     a started-thread-exception object and ‘#f’ otherwise.

     The procedure ‘started-thread-exception-procedure’ returns the
     procedure that raised EXC.

     The procedure ‘started-thread-exception-arguments’ returns the list
     of arguments of the procedure that raised EXC.

     For example:

          > (define (handler exc)
              (if (started-thread-exception? exc)
                  (list (started-thread-exception-procedure exc)
                        (started-thread-exception-arguments exc))
                  'not-started-thread-exception))
          > (with-exception-catcher
              handler
              (lambda ()
                (let ((t (make-thread (lambda () (expt 2 1000)))))
                  (thread-start! t)
                  (thread-start! t))))
          (#<procedure #2 thread-start!> (#<thread #3>))

 -- procedure: terminated-thread-exception? OBJ
 -- procedure: terminated-thread-exception-procedure EXC
 -- procedure: terminated-thread-exception-arguments EXC

     Terminated-thread-exception objects are raised when the
     ‘thread-join!’ procedure is called and the target thread has
     terminated as a result of a call to the ‘thread-terminate!’
     procedure.  The parameter EXC must be a terminated-thread-exception
     object.

     The procedure ‘terminated-thread-exception?’ returns ‘#t’ when OBJ
     is a terminated-thread-exception object and ‘#f’ otherwise.

     The procedure ‘terminated-thread-exception-procedure’ returns the
     procedure that raised EXC.

     The procedure ‘terminated-thread-exception-arguments’ returns the
     list of arguments of the procedure that raised EXC.

     For example:

          > (define (handler exc)
              (if (terminated-thread-exception? exc)
                  (list (terminated-thread-exception-procedure exc)
                        (terminated-thread-exception-arguments exc))
                  'not-terminated-thread-exception))
          > (with-exception-catcher
              handler
              (lambda ()
                (thread-join!
                  (thread-start!
                    (make-thread
                      (lambda () (thread-terminate! (current-thread))))))))
          (#<procedure #2 thread-join!> (#<thread #3>))

 -- procedure: uncaught-exception? OBJ
 -- procedure: uncaught-exception-procedure EXC
 -- procedure: uncaught-exception-arguments EXC
 -- procedure: uncaught-exception-reason EXC

     Uncaught-exception objects are raised when an object is raised in a
     thread and that thread does not handle it (i.e.  the thread
     terminated because it did not catch an exception it raised).  The
     parameter EXC must be an uncaught-exception object.

     The procedure ‘uncaught-exception?’ returns ‘#t’ when OBJ is an
     uncaught-exception object and ‘#f’ otherwise.

     The procedure ‘uncaught-exception-procedure’ returns the procedure
     that raised EXC.

     The procedure ‘uncaught-exception-arguments’ returns the list of
     arguments of the procedure that raised EXC.

     The procedure ‘uncaught-exception-reason’ returns the object that
     was raised by the thread and not handled by that thread.

     For example:

          > (define (handler exc)
              (if (uncaught-exception? exc)
                  (list (uncaught-exception-procedure exc)
                        (uncaught-exception-arguments exc)
                        (uncaught-exception-reason exc))
                  'not-uncaught-exception))
          > (with-exception-catcher
              handler
              (lambda ()
                (thread-join!
                  (thread-start!
                    (make-thread
                      (lambda () (open-input-file "data" 99)))))))
          (#<procedure #2 thread-join!>
           (#<thread #3>)
           #<wrong-number-of-arguments-exception #4>)

12.5 Exception objects related to C-interface
=============================================

 -- procedure: cfun-conversion-exception? OBJ
 -- procedure: cfun-conversion-exception-procedure EXC
 -- procedure: cfun-conversion-exception-arguments EXC
 -- procedure: cfun-conversion-exception-code EXC
 -- procedure: cfun-conversion-exception-message EXC

     Cfun-conversion-exception objects are raised by the C-interface
     when converting between the Scheme representation and the C
     representation of a value during a call from Scheme to C. The
     parameter EXC must be a cfun-conversion-exception object.

     The procedure ‘cfun-conversion-exception?’ returns ‘#t’ when OBJ is
     a cfun-conversion-exception object and ‘#f’ otherwise.

     The procedure ‘cfun-conversion-exception-procedure’ returns the
     procedure that raised EXC.

     The procedure ‘cfun-conversion-exception-arguments’ returns the
     list of arguments of the procedure that raised EXC.

     The procedure ‘cfun-conversion-exception-code’ returns an exact
     integer error code that can be converted to a string by the
     ‘err-code->string’ procedure.

     The procedure ‘cfun-conversion-exception-message’ returns ‘#f’ or a
     string giving details of the exception in a human-readable form.

     For example:

          $ cat test1.scm
          (define weird
            (c-lambda (char-string) nonnull-char-string
              "___return(___arg1);"))
          $ gsc test1.scm
          $ gsi
          Gambit v4.9.4

          > (load "test1")
          "/Users/feeley/gambit/doc/test1.o1"
          > (weird "hello")
          "hello"
          > (define (handler exc)
              (if (cfun-conversion-exception? exc)
                  (list (cfun-conversion-exception-procedure exc)
                        (cfun-conversion-exception-arguments exc)
                        (err-code->string (cfun-conversion-exception-code exc))
                        (cfun-conversion-exception-message exc))
                  'not-cfun-conversion-exception))
          > (with-exception-catcher
              handler
              (lambda () (weird 'not-a-string)))
          (#<procedure #2 weird>
           (not-a-string)
           "(Argument 1) Can't convert to C char-string"
           #f)
          > (with-exception-catcher
              handler
              (lambda () (weird #f)))
          (#<procedure #2 weird>
           (#f)
           "Can't convert result from C nonnull-char-string"
           #f)

 -- procedure: sfun-conversion-exception? OBJ
 -- procedure: sfun-conversion-exception-procedure EXC
 -- procedure: sfun-conversion-exception-arguments EXC
 -- procedure: sfun-conversion-exception-code EXC
 -- procedure: sfun-conversion-exception-message EXC

     Sfun-conversion-exception objects are raised by the C-interface
     when converting between the Scheme representation and the C
     representation of a value during a call from C to Scheme.  The
     parameter EXC must be a sfun-conversion-exception object.

     The procedure ‘sfun-conversion-exception?’ returns ‘#t’ when OBJ is
     a sfun-conversion-exception object and ‘#f’ otherwise.

     The procedure ‘sfun-conversion-exception-procedure’ returns the
     procedure that raised EXC.

     The procedure ‘sfun-conversion-exception-arguments’ returns the
     list of arguments of the procedure that raised EXC.

     The procedure ‘sfun-conversion-exception-code’ returns an exact
     integer error code that can be converted to a string by the
     ‘err-code->string’ procedure.

     The procedure ‘sfun-conversion-exception-message’ returns ‘#f’ or a
     string giving details of the exception in a human-readable form.

     For example:

          $ cat test2.scm
          (c-define (f str) (nonnull-char-string) int "f" ""
            (string->number str))
          (define t1 (c-lambda () int "___return(f (\"123\"));"))
          (define t2 (c-lambda () int "___return(f (0));"))
          (define t3 (c-lambda () int "___return(f (\"1.5\"));"))
          $ gsc test2.scm
          $ gsi
          Gambit v4.9.4

          > (load "test2")
          "/u/feeley/test2.o1"
          > (t1)
          123
          > (define (handler exc)
              (if (sfun-conversion-exception? exc)
                  (list (sfun-conversion-exception-procedure exc)
                        (sfun-conversion-exception-arguments exc)
                        (err-code->string (sfun-conversion-exception-code exc))
                        (sfun-conversion-exception-message exc))
                  'not-sfun-conversion-exception))
          > (with-exception-catcher handler t2)
          (#<procedure #2 f>
           ()
           "(Argument 1) Can't convert from C nonnull-char-string"
           #f)
          > (with-exception-catcher handler t3)
          (#<procedure #2 f> () "Can't convert result to C int" #f)

 -- procedure: multiple-c-return-exception? OBJ

     Multiple-c-return-exception objects are raised by the C-interface
     when a C to Scheme procedure call returns and that call’s stack
     frame is no longer on the C stack because the call has already
     returned, or has been removed from the C stack by a ‘longjump’.

     The procedure ‘multiple-c-return-exception?’ returns ‘#t’ when OBJ
     is a multiple-c-return-exception object and ‘#f’ otherwise.

     For example:

          $ cat test3.scm
          (c-define (f str) (char-string) scheme-object "f" ""
            (pp (list 'entry 'str= str))
            (let ((k (call-with-current-continuation (lambda (k) k))))
              (pp (list 'exit 'k= k))
              k))
          (define scheme-to-c-to-scheme-and-back
            (c-lambda (char-string) scheme-object
              "___return(f (___arg1));"))
          $ gsc test3.scm
          $ gsi
          Gambit v4.9.4

          > (load "test3")
          "/Users/feeley/gambit/doc/test3.o1"
          > (define (handler exc)
              (if (multiple-c-return-exception? exc)
                  exc
                  'not-multiple-c-return-exception))
          > (with-exception-catcher
              handler
              (lambda ()
                (let ((c (scheme-to-c-to-scheme-and-back "hello")))
                  (pp c)
                  (c 999))))
          (entry str= "hello")
          (exit k= #<procedure #2>)
          #<procedure #2>
          (exit k= 999)
          #<multiple-c-return-exception #3>

 -- procedure: wrong-processor-c-return-exception? OBJ

     Wrong-processor-c-return-exception objects are raised by the
     runtime system when a C to Scheme procedure call returns and that
     call’s stack frame was created by another processor.

     The procedure ‘wrong-processor-c-return-exception?’ returns ‘#t’
     when OBJ is a wrong-processor-c-return-exception object and ‘#f’
     otherwise.

12.6 Exception objects related to the reader
============================================

 -- procedure: datum-parsing-exception? OBJ
 -- procedure: datum-parsing-exception-kind EXC
 -- procedure: datum-parsing-exception-parameters EXC
 -- procedure: datum-parsing-exception-readenv EXC

     Datum-parsing-exception objects are raised by the reader (i.e.  the
     ‘read’ procedure) when the input does not conform to the grammar
     for datum.  The parameter EXC must be a datum-parsing-exception
     object.

     The procedure ‘datum-parsing-exception?’ returns ‘#t’ when OBJ is a
     datum-parsing-exception object and ‘#f’ otherwise.

     The procedure ‘datum-parsing-exception-kind’ returns a symbol
     denoting the kind of parsing error that was encountered by the
     reader when it raised EXC.  Here is a table of the possible return
     values:

     ‘datum-or-eof-expected’              Datum or EOF expected
     ‘datum-expected’                     Datum expected
     ‘improperly-placed-dot’              Improperly placed dot
     ‘incomplete-form-eof-reached’        Incomplete form, EOF reached
     ‘incomplete-form’                    Incomplete form
     ‘character-out-of-range’             Character out of range
     ‘invalid-character-name’             Invalid ’#\’ name
     ‘illegal-character’                  Illegal character
     ‘s8-expected’                        Signed 8 bit exact integer
                                          expected
     ‘u8-expected’                        Unsigned 8 bit exact integer
                                          expected
     ‘s16-expected’                       Signed 16 bit exact integer
                                          expected
     ‘u16-expected’                       Unsigned 16 bit exact integer
                                          expected
     ‘s32-expected’                       Signed 32 bit exact integer
                                          expected
     ‘u32-expected’                       Unsigned 32 bit exact integer
                                          expected
     ‘s64-expected’                       Signed 64 bit exact integer
                                          expected
     ‘u64-expected’                       Unsigned 64 bit exact integer
                                          expected
     ‘inexact-real-expected’              Inexact real expected
     ‘invalid-hex-escape’                 Invalid hexadecimal escape
     ‘invalid-escaped-character’          Invalid escaped character
     ‘open-paren-expected’                ’(’ expected
     ‘invalid-token’                      Invalid token
     ‘invalid-sharp-bang-name’            Invalid ’#!’ name
     ‘duplicate-label-definition’         Duplicate definition for label
     ‘missing-label-definition’           Missing definition for label
     ‘illegal-label-definition’           Illegal definition of label
     ‘invalid-infix-syntax-character’     Invalid infix syntax character
     ‘invalid-infix-syntax-number’        Invalid infix syntax number
     ‘invalid-infix-syntax’               Invalid infix syntax
                                          

     The procedure ‘datum-parsing-exception-parameters’ returns a list
     of the parameters associated with the parsing error that was
     encountered by the reader when it raised EXC.

     For example:

          > (define (handler exc)
              (if (datum-parsing-exception? exc)
                  (list (datum-parsing-exception-kind exc)
                        (datum-parsing-exception-parameters exc))
                  'not-datum-parsing-exception))
          > (with-exception-catcher
              handler
              (lambda ()
                (with-input-from-string "(s #\\pace)" read)))
          (invalid-character-name ("pace"))

12.7 Exception objects related to evaluation and compilation
============================================================

 -- procedure: expression-parsing-exception? OBJ
 -- procedure: expression-parsing-exception-kind EXC
 -- procedure: expression-parsing-exception-parameters EXC
 -- procedure: expression-parsing-exception-source EXC

     Expression-parsing-exception objects are raised by the evaluator
     and compiler (i.e.  the procedures ‘eval’, ‘compile-file’, etc)
     when the input does not conform to the grammar for expression.  The
     parameter EXC must be a expression-parsing-exception object.

     The procedure ‘expression-parsing-exception?’ returns ‘#t’ when OBJ
     is a expression-parsing-exception object and ‘#f’ otherwise.

     The procedure ‘expression-parsing-exception-kind’ returns a symbol
     denoting the kind of parsing error that was encountered by the
     evaluator or compiler when it raised EXC.  Here is a table of the
     possible return values:

     ‘id-expected’                        Identifier expected
     ‘ill-formed-namespace’               Ill-formed namespace
     ‘ill-formed-namespace-prefix’        Ill-formed namespace prefix
     ‘namespace-prefix-must-be-string’    Namespace prefix must be a string
     ‘macro-used-as-variable’             Macro name can’t be used as a
                                          variable
     ‘variable-is-immutable’              Variable is immutable
     ‘ill-formed-macro-transformer’       Macro transformer must be a lambda
                                          expression
     ‘reserved-used-as-variable’          Reserved identifier can’t be used
                                          as a variable
     ‘ill-formed-special-form’            Ill-formed special form
     ‘cannot-open-file’                   Can’t open file
     ‘filename-expected’                  Filename expected
     ‘ill-placed-define’                  Ill-placed ’define’
     ‘ill-placed-**include’               Ill-placed ’##include’
     ‘ill-placed-**define-macro’          Ill-placed ’##define-macro’
     ‘ill-placed-**declare’               Ill-placed ’##declare’
     ‘ill-placed-**namespace’             Ill-placed ’##namespace’
     ‘ill-formed-expression’              Ill-formed expression
     ‘unsupported-special-form’           Interpreter does not support
     ‘ill-placed-unquote’                 Ill-placed ’unquote’
     ‘ill-placed-unquote-splicing’        Ill-placed ’unquote-splicing’
     ‘parameter-must-be-id’               Parameter must be an identifier
     ‘parameter-must-be-id-or-default’    Parameter must be an identifier or
                                          default binding
     ‘duplicate-parameter’                Duplicate parameter in parameter
                                          list
     ‘ill-placed-dotted-rest-parameter’   Ill-placed dotted rest parameter
     ‘parameter-expected-after-rest’      #!rest must be followed by a
                                          parameter
     ‘ill-formed-default’                 Ill-formed default binding
     ‘ill-placed-optional’                Ill-placed #!optional
     ‘ill-placed-rest’                    Ill-placed #!rest
     ‘ill-placed-key’                     Ill-placed #!key
     ‘key-expected-after-rest’            #!key expected after rest
                                          parameter
     ‘ill-placed-default’                 Ill-placed default binding
     ‘duplicate-variable-definition’      Duplicate definition of a variable
     ‘empty-body’                         Body must contain at least one
                                          expression
     ‘variable-must-be-id’                Defined variable must be an
                                          identifier
     ‘else-clause-not-last’               Else clause must be last
     ‘ill-formed-selector-list’           Ill-formed selector list
     ‘duplicate-variable-binding’         Duplicate variable in bindings
     ‘ill-formed-binding-list’            Ill-formed binding list
     ‘ill-formed-call’                    Ill-formed procedure call
     ‘ill-formed-cond-expand’             Ill-formed ’cond-expand’
     ‘unfulfilled-cond-expand’            Unfulfilled ’cond-expand’
                                          

     The procedure ‘expression-parsing-exception-parameters’ returns a
     list of the parameters associated with the parsing error that was
     encountered by the evaluator or compiler when it raised EXC.

     For example:

          > (define (handler exc)
              (if (expression-parsing-exception? exc)
                  (list (expression-parsing-exception-kind exc)
                        (expression-parsing-exception-parameters exc))
                  'not-expression-parsing-exception))
          > (with-exception-catcher
              handler
              (lambda ()
                (eval '(+ do 1))))
          (reserved-used-as-variable (do))

 -- procedure: unbound-global-exception? OBJ
 -- procedure: unbound-global-exception-variable EXC
 -- procedure: unbound-global-exception-code EXC
 -- procedure: unbound-global-exception-rte EXC

     Unbound-global-exception objects are raised when an unbound global
     variable is accessed.  The parameter EXC must be an
     unbound-global-exception object.

     The procedure ‘unbound-global-exception?’ returns ‘#t’ when OBJ is
     an unbound-global-exception object and ‘#f’ otherwise.

     The procedure ‘unbound-global-exception-variable’ returns a symbol
     identifying the unbound global variable.

     For example:

          > (define (handler exc)
              (if (unbound-global-exception? exc)
                  (list 'variable= (unbound-global-exception-variable exc))
                  'not-unbound-global-exception))
          > (with-exception-catcher
              handler
              (lambda () foo))
          (variable= foo)

 -- procedure: not-in-compilation-context-exception? OBJ
 -- procedure: not-in-compilation-context-exception-procedure EXC
 -- procedure: not-in-compilation-context-exception-arguments EXC

     Not-in-compilation-context-exception objects are raised by the
     procedure ‘compilation-target’ when it is executed outside of a
     compilation context.  The parameter EXC must be a
     not-in-compilation-context-exception object.

     The procedure ‘not-in-compilation-context-exception?’ returns ‘#t’
     when OBJ is a not-in-compilation-context-exception object and ‘#f’
     otherwise.

     The procedure ‘not-in-compilation-context-exception-procedure’
     returns the procedure that raised EXC.

     The procedure ‘not-in-compilation-context-exception-arguments’
     returns the list of arguments of the procedure that raised EXC.

     For example:

          > (define (handler exc)
              (if (not-in-compilation-context-exception? exc)
                  (list (not-in-compilation-context-exception-procedure exc)
                        (not-in-compilation-context-exception-arguments exc))
                  'not-not-in-compilation-context-exception))
          > (with-exception-catcher
              handler
              (lambda () (compilation-target)))
          (#<procedure #2 compilation-target> ())

12.8 Exception objects related to type checking
===============================================

 -- procedure: type-exception? OBJ
 -- procedure: type-exception-procedure EXC
 -- procedure: type-exception-arguments EXC
 -- procedure: type-exception-arg-id EXC
 -- procedure: type-exception-type-id EXC

     Type-exception objects are raised when a primitive procedure is
     called with an argument of incorrect type (i.e.  when a run time
     type-check fails).  The parameter EXC must be a type-exception
     object.

     The procedure ‘type-exception?’ returns ‘#t’ when OBJ is a
     type-exception object and ‘#f’ otherwise.

     The procedure ‘type-exception-procedure’ returns the procedure that
     raised EXC.

     The procedure ‘type-exception-arguments’ returns the list of
     arguments of the procedure that raised EXC.

     The procedure ‘type-exception-arg-id’ returns the identity of the
     argument whose type is incorrect, which can be an exact integer
     position (1 for the first argument) or a pair whose ‘car’ is the
     position and the ‘cdr’ is the parameter name as a symbol.

     The procedure ‘type-exception-type-id’ returns an identifier of the
     type expected.  The type-id can be a symbol, such as ‘number’ and
     ‘string-or-nonnegative-fixnum’, or a record type descriptor.

     For example:

          > (define (handler exc)
              (if (type-exception? exc)
                  (list (type-exception-procedure exc)
                        (type-exception-arguments exc)
                        (type-exception-arg-id exc)
                        (type-exception-type-id exc))
                  'not-type-exception))
          > (with-exception-catcher
              handler
              (lambda () (vector-ref '#(a b c) 'foo)))
          (#<procedure #2 vector-ref> (#(a b c) foo) 2 exact-integer)
          > (with-exception-catcher
              handler
              (lambda () (time->seconds 'foo)))
          (#<procedure #3 time->seconds> (foo) 1 #<type #4 time>)

 -- procedure: range-exception? OBJ
 -- procedure: range-exception-procedure EXC
 -- procedure: range-exception-arguments EXC
 -- procedure: range-exception-arg-id EXC

     Range-exception objects are raised when a numeric parameter is not
     in the allowed range.  The parameter EXC must be a range-exception
     object.

     The procedure ‘range-exception?’ returns ‘#t’ when OBJ is a
     range-exception object and ‘#f’ otherwise.

     The procedure ‘range-exception-procedure’ returns the procedure
     that raised EXC.

     The procedure ‘range-exception-arguments’ returns the list of
     arguments of the procedure that raised EXC.

     The procedure ‘range-exception-arg-id’ returns the identity of the
     argument which is not in the allowed range, which can be an exact
     integer position (1 for the first argument) or a pair whose ‘car’
     is the position and the ‘cdr’ is the parameter name as a symbol.

     For example:

          > (define (handler exc)
              (if (range-exception? exc)
                  (list (range-exception-procedure exc)
                        (range-exception-arguments exc)
                        (range-exception-arg-id exc))
                  'not-range-exception))
          > (with-exception-catcher
              handler
              (lambda () (string-ref "abcde" 10)))
          (#<procedure #2 string-ref> ("abcde" 10) 2)

 -- procedure: divide-by-zero-exception? OBJ
 -- procedure: divide-by-zero-exception-procedure EXC
 -- procedure: divide-by-zero-exception-arguments EXC

     Divide-by-zero-exception objects are raised when a division by zero
     is attempted.  The parameter EXC must be a divide-by-zero-exception
     object.

     The procedure ‘divide-by-zero-exception?’ returns ‘#t’ when OBJ is
     a divide-by-zero-exception object and ‘#f’ otherwise.

     The procedure ‘divide-by-zero-exception-procedure’ returns the
     procedure that raised EXC.

     The procedure ‘divide-by-zero-exception-arguments’ returns the list
     of arguments of the procedure that raised EXC.

     For example:

          > (define (handler exc)
              (if (divide-by-zero-exception? exc)
                  (list (divide-by-zero-exception-procedure exc)
                        (divide-by-zero-exception-arguments exc))
                  'not-divide-by-zero-exception))
          > (with-exception-catcher
              handler
              (lambda () (/ 5 0 7)))
          (#<procedure #2 /> (5 0 7))

 -- procedure: length-mismatch-exception? OBJ
 -- procedure: length-mismatch-exception-procedure EXC
 -- procedure: length-mismatch-exception-arguments EXC
 -- procedure: length-mismatch-exception-arg-id EXC

     Length-mismatch-exception objects are raised by some procedures
     when they are called with two or more list arguments and the lists
     are not of the same length.  The parameter EXC must be a
     length-mismatch-exception object.

     The procedure ‘length-mismatch-exception?’ returns ‘#t’ when OBJ is
     an length-mismatch-exception object and ‘#f’ otherwise.

     The procedure ‘length-mismatch-exception-procedure’ returns the
     procedure that raised EXC.

     The procedure ‘length-mismatch-exception-arguments’ returns the
     list of arguments of the procedure that raised EXC.

     The procedure ‘length-mismatch-exception-arg-id’ returns the
     identity of the argument whose length is the shortest, which can be
     an exact integer position (1 for the first argument) or a pair
     whose ‘car’ is the position and the ‘cdr’ is the parameter name as
     a symbol.

12.9 Exception objects related to procedure call
================================================

 -- procedure: wrong-number-of-arguments-exception? OBJ
 -- procedure: wrong-number-of-arguments-exception-procedure EXC
 -- procedure: wrong-number-of-arguments-exception-arguments EXC

     Wrong-number-of-arguments-exception objects are raised when a
     procedure is called with the wrong number of arguments.  The
     parameter EXC must be a wrong-number-of-arguments-exception object.

     The procedure ‘wrong-number-of-arguments-exception?’ returns ‘#t’
     when OBJ is a wrong-number-of-arguments-exception object and ‘#f’
     otherwise.

     The procedure ‘wrong-number-of-arguments-exception-procedure’
     returns the procedure that raised EXC.

     The procedure ‘wrong-number-of-arguments-exception-arguments’
     returns the list of arguments of the procedure that raised EXC.

     For example:

          > (define (handler exc)
              (if (wrong-number-of-arguments-exception? exc)
                  (list (wrong-number-of-arguments-exception-procedure exc)
                        (wrong-number-of-arguments-exception-arguments exc))
                  'not-wrong-number-of-arguments-exception))
          > (with-exception-catcher
              handler
              (lambda () (open-input-file "data" 99)))
          (#<procedure #2 open-input-file> ("data" 99))

 -- procedure: number-of-arguments-limit-exception? OBJ
 -- procedure: number-of-arguments-limit-exception-procedure EXC
 -- procedure: number-of-arguments-limit-exception-arguments EXC

     Number-of-arguments-limit-exception objects are raised by the
     ‘apply’ procedure when the procedure being called is passed more
     than 8192 arguments.  The parameter EXC must be a
     number-of-arguments-limit-exception object.

     The procedure ‘number-of-arguments-limit-exception?’ returns ‘#t’
     when OBJ is a number-of-arguments-limit-exception object and ‘#f’
     otherwise.

     The procedure ‘number-of-arguments-limit-exception-procedure’
     returns the target procedure of the call to ‘apply’ that raised
     EXC.

     The procedure ‘number-of-arguments-limit-exception-arguments’
     returns the list of arguments of the target procedure of the call
     to ‘apply’ that raised EXC.

     For example:

          > (define (iota n) (if (= n 0) '() (cons n (iota (- n 1)))))
          > (define (handler exc)
              (if (number-of-arguments-limit-exception? exc)
                  (list (number-of-arguments-limit-exception-procedure exc)
                        (length (number-of-arguments-limit-exception-arguments exc)))
                  'not-number-of-arguments-limit-exception))
          > (with-exception-catcher
              handler
              (lambda () (apply + 1 2 3 (iota 8190))))
          (#<procedure #2 +> 8193)

 -- procedure: nonprocedure-operator-exception? OBJ
 -- procedure: nonprocedure-operator-exception-operator EXC
 -- procedure: nonprocedure-operator-exception-arguments EXC
 -- procedure: nonprocedure-operator-exception-code EXC
 -- procedure: nonprocedure-operator-exception-rte EXC

     Nonprocedure-operator-exception objects are raised when a procedure
     call is executed and the operator position is not a procedure.  The
     parameter EXC must be an nonprocedure-operator-exception object.

     The procedure ‘nonprocedure-operator-exception?’ returns ‘#t’ when
     OBJ is an nonprocedure-operator-exception object and ‘#f’
     otherwise.

     The procedure ‘nonprocedure-operator-exception-operator’ returns
     the value in operator position of the procedure call that raised
     EXC.

     The procedure ‘nonprocedure-operator-exception-arguments’ returns
     the list of arguments of the procedure call that raised EXC.

     For example:

          > (define (handler exc)
              (if (nonprocedure-operator-exception? exc)
                  (list (nonprocedure-operator-exception-operator exc)
                        (nonprocedure-operator-exception-arguments exc))
                  'not-nonprocedure-operator-exception))
          > (with-exception-catcher
              handler
              (lambda () (11 22 33)))
          (11 (22 33))

 -- procedure: wrong-number-of-values-exception? OBJ
 -- procedure: wrong-number-of-values-exception-vals EXC
 -- procedure: wrong-number-of-values-exception-code EXC
 -- procedure: wrong-number-of-values-exception-rte EXC

     Wrong-number-of-values-exception objects are raised by the
     ‘let-values’ and ‘define-values’ forms when the number of values
     does not conform to the number of variables to be bound.  The
     parameter EXC must be an wrong-number-of-values-exception object.

     The procedure ‘wrong-number-of-values-exception?’ returns ‘#t’ when
     OBJ is an wrong-number-of-values-exception object and ‘#f’
     otherwise.

     The procedure ‘wrong-number-of-values-exception-vals’ returns the
     values that were to be bound.

     For example:

          > (define (handler exc)
              (if (wrong-number-of-values-exception? exc)
                  (call-with-values
                   (lambda () (wrong-number-of-values-exception-vals exc))
                   list)
                  'not-wrong-number-of-values-exception))
          > (with-exception-catcher
              handler
              (lambda () (let-values (((a b) (values 11 22 33))) (* a b))))
          (11 22 33)

 -- procedure: unknown-keyword-argument-exception? OBJ
 -- procedure: unknown-keyword-argument-exception-procedure EXC
 -- procedure: unknown-keyword-argument-exception-arguments EXC

     Unknown-keyword-argument-exception objects are raised when a
     procedure accepting keyword arguments is called and one of the
     keywords supplied is not among those that are expected.  The
     parameter EXC must be an unknown-keyword-argument-exception object.

     The procedure ‘unknown-keyword-argument-exception?’ returns ‘#t’
     when OBJ is an unknown-keyword-argument-exception object and ‘#f’
     otherwise.

     The procedure ‘unknown-keyword-argument-exception-procedure’
     returns the procedure that raised EXC.

     The procedure ‘unknown-keyword-argument-exception-arguments’
     returns the list of arguments of the procedure that raised EXC.

     For example:

          > (define (handler exc)
              (if (unknown-keyword-argument-exception? exc)
                  (list (unknown-keyword-argument-exception-procedure exc)
                        (unknown-keyword-argument-exception-arguments exc))
                  'not-unknown-keyword-argument-exception))
          > (with-exception-catcher
              handler
              (lambda () ((lambda (#!key (foo 5)) foo) bar: 11)))
          (#<procedure #2> (bar: 11))

 -- procedure: keyword-expected-exception? OBJ
 -- procedure: keyword-expected-exception-procedure EXC
 -- procedure: keyword-expected-exception-arguments EXC

     Keyword-expected-exception objects are raised when a procedure
     accepting keyword arguments is called and a nonkeyword was supplied
     where a keyword was expected.  The parameter EXC must be an
     keyword-expected-exception object.

     The procedure ‘keyword-expected-exception?’ returns ‘#t’ when OBJ
     is an keyword-expected-exception object and ‘#f’ otherwise.

     The procedure ‘keyword-expected-exception-procedure’ returns the
     procedure that raised EXC.

     The procedure ‘keyword-expected-exception-arguments’ returns the
     list of arguments of the procedure that raised EXC.

     For example:

          > (define (handler exc)
              (if (keyword-expected-exception? exc)
                  (list (keyword-expected-exception-procedure exc)
                        (keyword-expected-exception-arguments exc))
                  'not-keyword-expected-exception))
          > (with-exception-catcher
              handler
              (lambda () ((lambda (#!key (foo 5)) foo) 11 22)))
          (#<procedure #2> (11 22))

12.10 Other exception objects
=============================

 -- procedure: error-exception? OBJ
 -- procedure: error-exception-message EXC
 -- procedure: error-exception-parameters EXC
 -- procedure: error MESSAGE OBJ...

     Error-exception objects are raised when the ‘error’ procedure is
     called.  The parameter EXC must be an error-exception object.

     The procedure ‘error-exception?’ returns ‘#t’ when OBJ is an
     error-exception object and ‘#f’ otherwise.

     The procedure ‘error-exception-message’ returns the first argument
     of the call to ‘error’ that raised EXC.

     The procedure ‘error-exception-parameters’ returns the list of
     arguments, starting with the second argument, of the call to
     ‘error’ that raised EXC.

     The ‘error’ procedure raises an error-exception object whose
     message field is MESSAGE and parameters field is the list of values
     OBJ....

     For example:

          > (define (handler exc)
              (if (error-exception? exc)
                  (list (error-exception-message exc)
                        (error-exception-parameters exc))
                  'not-error-exception))
          > (with-exception-catcher
              handler
              (lambda () (error "unexpected object:" 123)))
          ("unexpected object:" (123))

13 Host environment
*******************

The host environment is the set of resources, such as the filesystem,
network and processes, that are managed by the operating system within
which the Scheme program is executing.  This chapter specifies how the
host environment can be accessed from within the Scheme program.

   In this chapter we say that the Scheme program being executed is a
process, even though the concept of process does not exist in some
operating systems supported by Gambit (e.g.  MSDOS).

13.1 Handling of file names
===========================

Gambit uses a naming convention for files that is compatible with the
one used by the host environment but extended to allow referring to the
“home directory” of the current user or some specific user and the
“installation directories”.

   A “path” is a string that denotes a file, for example
‘"src/readme.txt"’.  Each component of a path is separated by a ‘/’
under UNIX and macOS and by a ‘/’ or ‘\’ under MSDOS and Microsoft
Windows.  A leading separator indicates an absolute path under UNIX,
macOS, MSDOS and Microsoft Windows.  A path which does not contain a
path separator is relative to the “current working directory” on all
operating systems.  A volume specifier such as ‘C:’ may prefix a file
name under MSDOS and Microsoft Windows.

   A path which starts with the characters ‘~~’ denotes a file in an
installation directory.  If nothing follows the ‘~~’ then the directory
denoted is the central installation directory.  Otherwise what follows
the ‘~~’ is the name of the installation directory, for example ‘~~lib’
denotes the ‘lib’ installation directory.  Note that the location of the
installation directories may be overridden by using the
‘-:~~NAME=DIRECTORY’ runtime option or by defining the ‘GAMBOPT’
environment variable.  Unless explicitly overridden, ‘~~execdir’ denotes
the directory containing the current executable program.

   A path which starts with the character ‘~’ not followed by ‘~’
denotes a file in the user’s home directory.  The user’s home directory
is contained in the ‘HOME’ environment variable under UNIX, macOS, MSDOS
and Microsoft Windows.  Under MSDOS and Microsoft Windows, if the ‘HOME’
environment variable is not defined, the environment variables
‘HOMEDRIVE’ and ‘HOMEPATH’ are concatenated if they are defined.  If
this fails to yield a home directory, the central installation directory
is used instead.

   A path which starts with the characters ‘~USERNAME’ denotes a file in
the home directory of the given user.  Under UNIX and macOS this is
found using the password file.  There is no equivalent under MSDOS and
Microsoft Windows.

 -- procedure: initial-current-directory
 -- procedure: current-directory [NEW-CURRENT-DIRECTORY]

     The procedure ‘initial-current-directory’ returns the absolute
     “normalized path” of the current working directory of the current
     process when it was started.

     The parameter object ‘current-directory’ is bound to the current
     working directory.  Calling this procedure with no argument returns
     the absolute “normalized path” of the directory and calling this
     procedure with one argument sets the directory to
     NEW-CURRENT-DIRECTORY.  The initial binding of this parameter
     object is the path returned by ‘initial-current-directory’.  The
     path returned by ‘current-directory’ always contains a trailing
     directory separator.  Modifications of the parameter object do not
     change the current working directory of the current process (i.e.
     that is accessible with the UNIX ‘getcwd()’ function and the
     Microsoft Windows ‘GetCurrentDirectory’ function).  It is an error
     to mutate the string returned by ‘current-directory’.

     For example under UNIX:

          > (current-directory)
          "/Users/feeley/gambit/doc/"
          > (current-directory "..")
          > (current-directory)
          "/Users/feeley/gambit/"
          > (initial-current-directory)
          "/Users/feeley/gambit/doc/"
          > (path-expand "foo" "~~")
          "/usr/local/Gambit/foo"
          > (parameterize ((current-directory "~~")) (path-expand "foo"))
          "/usr/local/Gambit/foo"

 -- procedure: path-expand PATH [ORIGIN-DIRECTORY]

     The procedure ‘path-expand’ takes the path of a file or directory
     and returns an expanded path, which is an absolute path when PATH
     or ORIGIN-DIRECTORY are absolute paths.  The optional
     ORIGIN-DIRECTORY parameter, which defaults to the current working
     directory, is the directory used to resolve relative paths.
     Components of the paths PATH and ORIGIN-DIRECTORY need not exist.

     For example under UNIX:

          > (path-expand "foo")
          "/Users/feeley/gambit/doc/foo"
          > (path-expand "~/foo")
          "/Users/feeley/foo"
          > (path-expand "~~lib/foo")
          "/usr/local/Gambit/lib/foo"
          > (path-expand "../foo")
          "/Users/feeley/gambit/doc/../foo"
          > (path-expand "foo" "")
          "foo"
          > (path-expand "foo" "/tmp")
          "/tmp/foo"
          > (path-expand "this/file/does/not/exist")
          "/Users/feeley/gambit/doc/this/file/does/not/exist"
          > (path-expand "")
          "/Users/feeley/gambit/doc/"

 -- procedure: path-normalize PATH [ALLOW-RELATIVE? [ORIGIN-DIRECTORY]]

     The procedure ‘path-normalize’ takes a path of a file or directory
     and returns its normalized path.  The optional ORIGIN-DIRECTORY
     parameter, which defaults to the current working directory, is the
     directory used to resolve relative paths.  All components of the
     paths PATH and ORIGIN-DIRECTORY must exist, except possibly the
     last component of PATH.  A normalized path is a path containing no
     redundant parts and which is consistent with the current structure
     of the filesystem.  A normalized path of a directory will always
     end with a path separator (i.e.  ‘/’, ‘\’, or ‘:’ depending on the
     operating system).  The optional ALLOW-RELATIVE? parameter, which
     defaults to ‘#f’, indicates if the path returned can be expressed
     relatively to ORIGIN-DIRECTORY: a ‘#f’ requests an absolute path,
     the symbol ‘shortest’ requests the shortest of the absolute and
     relative paths, and any other value requests the relative path.
     The shortest path is useful for interaction with the user because
     short relative paths are typically easier to read than long
     absolute paths.

     For example under UNIX:

          > (path-expand "../foo")
          "/Users/feeley/gambit/doc/../foo"
          > (path-normalize "../foo")
          "/Users/feeley/gambit/foo"
          > (path-normalize "this/file/does/not/exist")
          *** ERROR IN (console)@3.1 -- No such file or directory
          (path-normalize "this/file/does/not/exist")

 -- procedure: path-extension PATH
 -- procedure: path-strip-extension PATH
 -- procedure: path-directory PATH
 -- procedure: path-strip-directory PATH
 -- procedure: path-strip-trailing-directory-separator PATH
 -- procedure: path-volume PATH
 -- procedure: path-strip-volume PATH

     These procedures extract various parts of a path, which need not be
     a normalized path.  The procedure ‘path-extension’ returns the file
     extension (including the period) or the empty string if there is no
     extension.  The procedure ‘path-strip-extension’ returns the path
     with the extension stripped off.  The procedure ‘path-directory’
     returns the file’s directory (including the last path separator) or
     the empty string if no directory is specified in the path.  The
     procedure ‘path-strip-directory’ returns the path with the
     directory stripped off.  The procedure
     ‘path-strip-trailing-directory-separator’ returns the path with the
     directory separator stripped off if one is at the end of the path.
     The procedure ‘path-volume’ returns the file’s volume (including
     the last path separator) or the empty string if no volume is
     specified in the path.  The procedure ‘path-strip-volume’ returns
     the path with the volume stripped off.

     For example under UNIX:

          > (path-extension "/tmp/foo")
          ""
          > (path-extension "/tmp/foo.txt")
          ".txt"
          > (path-strip-extension "/tmp/foo.txt")
          "/tmp/foo"
          > (path-directory "/tmp/foo.txt")
          "/tmp/"
          > (path-strip-directory "/tmp/foo.txt")
          "foo.txt"
          > (path-strip-trailing-directory-separator "/usr/local/bin/")
          "/usr/local/bin"
          > (path-strip-trailing-directory-separator "/usr/local/bin")
          "/usr/local/bin"
          > (path-volume "/tmp/foo.txt")
          ""
          > (path-volume "C:/tmp/foo.txt")
          "" ; result is "C:" under Microsoft Windows
          > (path-strip-volume "C:/tmp/foo.txt")
          "C:/tmp/foo.txt" ; result is "/tmp/foo.txt" under Microsoft Windows

13.2 Filesystem operations
==========================

 -- procedure: create-directory PATH-OR-SETTINGS
 -- procedure: create-temporary-directory PATH-OR-SETTINGS

     These procedures create directories.  The argument PATH-OR-SETTINGS
     is either a string denoting a filesystem path or a list of port
     settings which must contain a ‘path:’ setting.  The procedure
     ‘create-directory’ returns an unspecified value.  In the case of
     ‘create-temporary-directory’ the path is used as a prefix to
     generate new directory paths until the path of a directory not
     currently existing is generated and that path is returned.  Here
     are the settings allowed:

        • ‘path:’ STRING

          This setting indicates the location of the directory to create
          in the filesystem.  There is no default value for this
          setting.

        • ‘permissions:’ 12-BIT-EXACT-INTEGER

          This setting controls the UNIX permissions that will be
          attached to the file if it is created.  The default value of
          this setting is ‘#o777’.

     For example:

          > (create-directory "newdir")
          > (create-temporary-directory "/tmp/foo.")
          "/tmp/foo.85812"
          > (create-directory "newdir")
          *** ERROR IN (console)@2.1 -- File exists
          (create-directory "newdir")

 -- procedure: create-fifo PATH-OR-SETTINGS

     This procedure creates a FIFO. The argument PATH-OR-SETTINGS is
     either a string denoting a filesystem path or a list of port
     settings which must contain a ‘path:’ setting.  Here are the
     settings allowed:

        • ‘path:’ STRING

          This setting indicates the location of the FIFO to create in
          the filesystem.  There is no default value for this setting.

        • ‘permissions:’ 12-BIT-EXACT-INTEGER

          This setting controls the UNIX permissions that will be
          attached to the file if it is created.  The default value of
          this setting is ‘#o666’.

     For example:

          > (create-fifo "fifo")
          > (define a (open-input-file "fifo"))
          > (define b (open-output-file "fifo"))
          > (display "1 22 333" b)
          > (force-output b)
          > (read a)
          1
          > (read a)
          22

 -- procedure: create-link SOURCE-PATH DESTINATION-PATH

     This procedure creates a hard link between SOURCE-PATH and
     DESTINATION-PATH.  The argument SOURCE-PATH must be a string
     denoting the path of an existing file.  The argument
     DESTINATION-PATH must be a string denoting the path of the link to
     create.

 -- procedure: create-symbolic-link SOURCE-PATH DESTINATION-PATH

     This procedure creates a symbolic link between SOURCE-PATH and
     DESTINATION-PATH.  The argument SOURCE-PATH must be a string
     denoting the path of an existing file.  The argument
     DESTINATION-PATH must be a string denoting the path of the symbolic
     link to create.

 -- procedure: rename-file SOURCE-PATH DESTINATION-PATH [REPLACE?]

     This procedure renames the file SOURCE-PATH to DESTINATION-PATH.
     The argument SOURCE-PATH must be a string denoting the path of an
     existing file.  The argument DESTINATION-PATH must be a string
     denoting the new path of the file.  If REPLACE? is absent or true,
     an existing DESTINATION-PATH will be replaced by SOURCE-PATH.
     Otherwise, the rename operation will fail if DESTINATION-PATH
     exists.  Not all filesystems support atomic renaming and existence
     testing.

 -- procedure: copy-file SOURCE-PATH DESTINATION-PATH

     This procedure copies the file SOURCE-PATH to DESTINATION-PATH.
     The argument SOURCE-PATH must be a string denoting the path of an
     existing file.  The argument DESTINATION-PATH must be a string
     denoting the path of the file to create.

 -- procedure: delete-file PATH

     This procedure deletes the file PATH.  The argument PATH must be a
     string denoting the path of an existing file.

 -- procedure: delete-directory PATH

     This procedure deletes the directory PATH.  The argument PATH must
     be a string denoting the path of an existing empty directory.

 -- procedure: delete-file-or-directory PATH [RECURSIVE?]

     This procedure deletes the file or directory PATH.  The argument
     PATH must be a string denoting the path of an existing file or
     directory.  If RECURSIVE? is specified and is true, directories are
     recursively deleted.  Otherwise only empty directories can be
     deleted.

 -- procedure: directory-files [PATH-OR-SETTINGS]

     This procedure returns the list of the files in a directory.  The
     argument PATH-OR-SETTINGS is either a string denoting a filesystem
     path to a directory or a list of settings which must contain a
     ‘path:’ setting.  If it is not specified, PATH-OR-SETTINGS defaults
     to the current directory (the value bound to the
     ‘current-directory’ parameter object).  Here are the settings
     allowed:

        • ‘path:’ STRING

          This setting indicates the location of the directory in the
          filesystem.  There is no default value for this setting.

        • ‘ignore-hidden:’ ( ‘#f’ | ‘#t’ | ‘dot-and-dot-dot’ )

          This setting controls whether hidden-files will be returned.
          Under UNIX and macOS hidden-files are those that start with a
          period (such as ‘.’, ‘..’, and ‘.profile’).  Under Microsoft
          Windows hidden files are the ‘.’ and ‘..’ entries and the
          files whose “hidden file” attribute is set.  A setting of ‘#f’
          will enumerate all the files.  A setting of ‘#t’ will only
          enumerate the files that are not hidden.  A setting of
          ‘dot-and-dot-dot’ will enumerate all the files except for the
          ‘.’ and ‘..’ hidden files.  The default value of this setting
          is ‘#t’.

     For example:

          > (directory-files)
          ("complex" "README" "simple")
          > (directory-files "../include")
          ("config.h" "config.h.in" "gambit.h" "makefile" "makefile.in")
          > (directory-files (list path: "../include" ignore-hidden: #f))
          ("." ".." "config.h" "config.h.in" "gambit.h" "makefile" "makefile.in")

13.3 Shell command execution
============================

 -- procedure: shell-command COMMAND [CAPTURE?]

     The procedure ‘shell-command’ calls up the shell to execute COMMAND
     which must be a string.  The argument CAPTURE?, which defaults to
     ‘#f’, indicates if the output of the command is captured as a
     string.  If CAPTURE? is ‘#f’, this procedure returns the exit
     status of the shell in the form that the C library’s ‘system’
     routine returns.  If CAPTURE? is not ‘#f’, this procedure returns a
     pair consisting of the exit status of the shell in the ‘car’ field,
     and the captured output in the ‘cdr’ field.  Be advised that the
     shell that is used, and consequently the syntax of COMMAND, depends
     on the operating system.  On Unix, the shell ‘/bin/sh’ is usually
     invoked.  On Windows, the shell ‘cmd.exe’ is usually invoked.

     For example under UNIX:

          > (shell-command "ls -sk f*.scm")
          4 fact.scm   4 fib.scm
          0
          > (shell-command "ls -sk f*.scm" #t)
          (0 . "4 fact.scm   4 fib.scm\n")
          > (shell-command "echo x\\\\\\\\y $HOME" #t)
          (0 . "x\\y /Users/feeley\n")

     For example under Windows:

          > (shell-command "echo x\\\\\\\\y %HOME%" #t)
          (0 . "x\\\\\\\\y C:\\Users\\feeley\r\n")

13.4 Process termination
========================

 -- procedure: exit [STATUS]

     The procedure ‘exit’ causes the process to terminate with the
     status STATUS which must be an exact integer in the range 0 to 255
     or ‘#f’.  If it is not specified, STATUS defaults to 0.  When
     STATUS is ‘#f’ the process terminates with an error status.

     For example under UNIX:

          $ gsi
          Gambit v4.9.4

          > (exit #f)
          $ echo $?
          70

13.5 Command line arguments
===========================

 -- procedure: command-line

     This procedure returns a list of strings corresponding to the
     command line arguments, including the program file name as the
     first element of the list.  When the interpreter executes a Scheme
     script, the list returned by ‘command-line’ contains the script’s
     absolute path followed by the remaining command line arguments.

     For example under UNIX:

          $ gsi -:debug -e "(pretty-print (command-line))"
          ("gsi" "-e" "(pretty-print (command-line))")
          $ cat foo
          #!/usr/local/Gambit/bin/gsi-script
          (pretty-print (command-line))
          $ ./foo 1 2 "3 4"
          ("/u/feeley/./foo" "1" "2" "3 4")

13.6 Environment variables
==========================

 -- procedure: getenv NAME [DEFAULT]
 -- procedure: setenv NAME [NEW-VALUE]

     The procedure ‘getenv’ returns the value of the environment
     variable NAME of the current process.  Variable names are denoted
     with strings.  A string is returned if the environment variable is
     bound, otherwise DEFAULT is returned if it is specified, otherwise
     an exception is raised.

     The procedure ‘setenv’ changes the binding of the environment
     variable NAME to NEW-VALUE which must be a string.  If NEW-VALUE is
     not specified the binding is removed.

     For example under UNIX:

          > (getenv "HOME")
          "/Users/feeley"
          > (getenv "DOES_NOT_EXIST" #f)
          #f
          > (setenv "DOES_NOT_EXIST" "it does now")
          > (getenv "DOES_NOT_EXIST" #f)
          "it does now"
          > (setenv "DOES_NOT_EXIST")
          > (getenv "DOES_NOT_EXIST" #f)
          #f
          > (getenv "DOES_NOT_EXIST")
          *** ERROR IN (console)@7.1 -- Unbound OS environment variable
          (getenv "DOES_NOT_EXIST")

13.7 Measuring time
===================

Procedures are available for measuring real time (aka “wall” time) and
cpu time (the amount of time the cpu has been executing the process).
The resolution of the real time and cpu time clock is operating system
dependent.  Typically the resolution of the cpu time clock is rather
coarse (measured in “ticks” of 1/60th or 1/100th of a second).  Real
time is internally computed relative to some arbitrary point in time
using floating point numbers, which means that there is a gradual loss
of resolution as time elapses.  Moreover, some operating systems report
time in number of ticks using a 32 bit integer so the value returned by
the time related procedures may wraparound much before any significant
loss of resolution occurs (for example 2.7 years if ticks are 1/50th of
a second).

 -- procedure: current-time
 -- procedure: time? OBJ
 -- procedure: time->seconds TIME
 -- procedure: seconds->time X

     The procedure ‘current-time’ returns a “time object” representing
     the current point in real time.

     The procedure ‘time?’ returns ‘#t’ when OBJ is a time object and
     ‘#f’ otherwise.

     The procedure ‘time->seconds’ converts the time object TIME into an
     inexact real number representing the number of seconds elapsed
     since the “epoch” (which is 00:00:00 Coordinated Universal Time
     01-01-1970).

     The procedure ‘seconds->time’ converts the real number X
     representing the number of seconds elapsed since the “epoch” into a
     time object.

     For example:

          > (current-time)
          #<time #2>
          > (time? (current-time))
          #t
          > (time? 123)
          #f
          > (time->seconds (current-time))
          1083118758.63973
          > (time->seconds (current-time))
          1083118759.909163
          > (seconds->time (+ 10 (time->seconds (current-time))
          #<time #3>  ; a time object representing 10 seconds in the future

 -- procedure: process-times
 -- procedure: cpu-time
 -- procedure: real-time

     The procedure ‘process-times’ returns a three element f64vector
     containing the cpu time that has been used by the program and the
     real time that has elapsed since it was started.  The first element
     corresponds to “user” time in seconds, the second element
     corresponds to “system” time in seconds and the third element is
     the elapsed real time in seconds.  On operating systems that can’t
     differentiate user and system time, the system time is zero.  On
     operating systems that can’t measure cpu time, the user time is
     equal to the elapsed real time and the system time is zero.

     The procedure ‘cpu-time’ returns the cpu time in seconds that has
     been used by the program (user time plus system time).

     The procedure ‘real-time’ returns the real time that has elapsed
     since the program was started.

     For example:

          > (process-times)
          #f64(.02794 .021754 .159926176071167)
          > (cpu-time)
          .051223
          > (real-time)
          .40660619735717773

 -- special form: time expr [PORT]

     The ‘time’ special form evaluates expr and returns the result.  As
     a side effect it displays a message on the port PORT which
     indicates various statistics about the evaluation of expr including
     how long the evaluation took (in real time and cpu time), how much
     time was spent in the garbage collector, how much memory was
     allocated during the evaluation and how many minor and major page
     faults occured (0 is reported if not running under UNIX). If it is
     not specified, PORT defaults to the interaction channel (i.e.  the
     output will appear at the REPL).

     For example:

          > (define (f x)
              (let loop ((x x) (lst '()))
                (if (= x 0)
                    lst
                    (loop (- x 1) (cons x lst)))))
          > (length (time (f 100000)))
          (time (f 100000))
              683 ms real time
              558 ms cpu time (535 user, 23 system)
              8 collections accounting for 102 ms real time (70 user, 5 system)
              6400160 bytes allocated
              no minor faults
              no major faults
          100000

13.8 File information
=====================

 -- procedure: file-exists? PATH [CHASE?]

     The PATH argument must be a string.  This procedure returns ‘#t’
     when a file by that name exists, and returns ‘#f’ otherwise.

     When CHASE? is present and ‘#f’, symbolic links will not be chased,
     in other words if PATH refers to a symbolic link, ‘file-exists?’
     will return ‘#t’ whether or not it points to an existing file.

     For example:

          > (file-exists? "nofile")
          #f

 -- procedure: file-info PATH [CHASE?]

     This procedure accesses the filesystem to get information about the
     file whose location is given by the string PATH.  A
     file-information record is returned that contains the file’s type,
     the device number, the inode number, the mode (permission bits),
     the number of links, the file’s user id, the file’s group id, the
     file’s size in bytes, the times of last-access, last-modification
     and last-change, the attributes, and the creation time.

     When CHASE? is present and ‘#f’, symbolic links will not be chased,
     in other words if PATH refers to a symbolic link the ‘file-info’
     procedure will return information about the link rather than the
     file it links to.

     For example:

          > (file-info "/dev/tty")
          #<file-info #2
             type: character-special
             device: 19513156
             inode: 20728196
             mode: 438
             number-of-links: 1
             owner: 0
             group: 0
             size: 0
             last-access-time: #<time #3>
             last-modification-time: #<time #4>
             last-change-time: #<time #5>
             attributes: 128
             creation-time: #<time #6>>

 -- procedure: file-info? OBJ

     This procedure returns ‘#t’ when OBJ is a file-information record
     and ‘#f’ otherwise.

     For example:

          > (file-info? (file-info "/dev/tty"))
          #t
          > (file-info? 123)
          #f

 -- procedure: file-info-type FILE-INFO

     Returns the type field of the file-information record FILE-INFO.
     The type is denoted by a symbol.  The following types are possible:

     ‘regular’
          Regular file
     ‘directory’
          Directory
     ‘character-special’
          Character special device
     ‘block-special’
          Block special device
     ‘fifo’
          FIFO
     ‘symbolic-link’
          Symbolic link
     ‘socket’
          Socket
     ‘unknown’
          File is of an unknown type

     For example:

          > (file-info-type (file-info "/dev/tty"))
          character-special
          > (file-info-type (file-info "/dev"))
          directory

 -- procedure: file-info-device FILE-INFO

     Returns the device field of the file-information record FILE-INFO.

     For example:

          > (file-info-device (file-info "/dev/tty"))
          19513156

 -- procedure: file-info-inode FILE-INFO

     Returns the inode field of the file-information record FILE-INFO.

     For example:

          > (file-info-inode (file-info "/dev/tty"))
          20728196

 -- procedure: file-info-mode FILE-INFO

     Returns the mode field of the file-information record FILE-INFO.

     For example:

          > (file-info-mode (file-info "/dev/tty"))
          438

 -- procedure: file-info-number-of-links FILE-INFO

     Returns the number-of-links field of the file-information record
     FILE-INFO.

     For example:

          > (file-info-number-of-links (file-info "/dev/tty"))
          1

 -- procedure: file-info-owner FILE-INFO

     Returns the owner field of the file-information record FILE-INFO.

     For example:

          > (file-info-owner (file-info "/dev/tty"))
          0

 -- procedure: file-info-group FILE-INFO

     Returns the group field of the file-information record FILE-INFO.

     For example:

          > (file-info-group (file-info "/dev/tty"))
          0

 -- procedure: file-info-size FILE-INFO

     Returns the size field of the file-information record FILE-INFO.

     For example:

          > (file-info-size (file-info "/dev/tty"))
          0

 -- procedure: file-info-last-access-time FILE-INFO

     Returns the last-access-time field of the file-information record
     FILE-INFO.

     For example:

          > (file-info-last-access-time (file-info "/dev/tty"))
          #<time #2>

 -- procedure: file-info-last-modification-time FILE-INFO

     Returns the last-modification-time field of the file-information
     record FILE-INFO.

     For example:

          > (file-info-last-modification-time (file-info "/dev/tty"))
          #<time #2>

 -- procedure: file-info-last-change-time FILE-INFO

     Returns the last-change-time field of the file-information record
     FILE-INFO.

     For example:

          > (file-info-last-change-time (file-info "/dev/tty"))
          #<time #2>

 -- procedure: file-info-attributes FILE-INFO

     Returns the attributes field of the file-information record
     FILE-INFO.

     For example:

          > (file-info-attributes (file-info "/dev/tty"))
          128

 -- procedure: file-info-creation-time FILE-INFO

     Returns the creation-time field of the file-information record
     FILE-INFO.

     For example:

          > (file-info-creation-time (file-info "/dev/tty"))
          #<time #2>

 -- procedure: file-type PATH
 -- procedure: file-device PATH
 -- procedure: file-inode PATH
 -- procedure: file-mode PATH
 -- procedure: file-number-of-links PATH
 -- procedure: file-owner PATH
 -- procedure: file-group PATH
 -- procedure: file-size PATH
 -- procedure: file-last-access-time PATH
 -- procedure: file-last-modification-time PATH
 -- procedure: file-last-change-time PATH
 -- procedure: file-attributes PATH
 -- procedure: file-creation-time PATH

     These procedures combine a call to the ‘file-info’ procedure and a
     call to a file-information record field accessor.  For instance
     ‘(file-type PATH)’ is equivalent to ‘(file-info-type (file-info
     PATH))’.

 -- procedure: file-last-access-and-modification-times-set! PATH [ATIME
          [MTIME]]

     This procedure changes the last-access and last-modification times
     of the file whose location is given by the string PATH.  Time is
     specified either with a time object indicating an absolute point in
     time or a real number indicating the number of seconds relative to
     the moment the procedure is called.  When ATIME and MTIME are not
     specified, the last-access and last-modification times are set to
     the current time.  When MTIME is not specified, the last-access and
     last-modification times are set to ATIME.  Otherwise the
     last-access time is set to ATIME and the last-modification time is
     set to MTIME.

     For example:

          > (define (t path)
              (list (time->seconds (file-last-access-time path))
                    (time->seconds (file-last-modification-time path))))
          > (with-output-to-file "nl.txt" newline)
          > (t "nl.txt")
          (1429547027. 1429547027.)
          > (t "nl.txt")
          (1429547027. 1429547027.)
          > (file-last-access-and-modification-times-set! "nl.txt")
          > (t "nl.txt")
          (1429547039. 1429547039.)
          > (file-last-access-and-modification-times-set! "nl.txt" -60)
          > (t "nl.txt")
          (1429547006. 1429547006.)
          > (file-last-access-and-modification-times-set! "nl.txt" -60 0)
          > (t "nl.txt")
          (1429547049. 1429547109.)

13.9 Group information
======================

 -- procedure: group-info GROUP-NAME-OR-ID

     This procedure accesses the group database to get information about
     the group identified by GROUP-NAME-OR-ID, which is the group’s
     symbolic name (string) or the group’s GID (exact integer).  A
     group-information record is returned that contains the group’s
     symbolic name, the group’s id (GID), and the group’s members (list
     of symbolic user names).

     For example:

          > (group-info "staff")
          #<group-info #2 name: "staff" gid: 20 members: ("root")>
          > (group-info 29)
          #<group-info #3
             name: "certusers"
             gid: 29
             members: ("root" "jabber" "postfix" "cyrusimap")>
          > (group-info 5000)
          *** ERROR IN (console)@3.1 -- Resource temporarily unavailable
          (group-info 5000)

 -- procedure: group-info? OBJ

     This procedure returns ‘#t’ when OBJ is a group-information record
     and ‘#f’ otherwise.

     For example:

          > (group-info? (group-info "daemon"))
          #t
          > (group-info? 123)
          #f

 -- procedure: group-info-name GROUP-INFO

     Returns the symbolic name field of the group-information record
     GROUP-INFO.

     For example:

          > (group-info-name (group-info 29))
          "certusers"

 -- procedure: group-info-gid GROUP-INFO

     Returns the group id field of the group-information record
     GROUP-INFO.

     For example:

          > (group-info-gid (group-info "staff"))
          20

 -- procedure: group-info-members GROUP-INFO

     Returns the members field of the group-information record
     GROUP-INFO.

     For example:

          > (group-info-members (group-info "staff"))
          ("root")

13.10 User information
======================

 -- procedure: user-name

     This procedure returns the user’s name as a string.

     For example:

          > (user-name)
          "feeley"

 -- procedure: user-info USER-NAME-OR-ID

     This procedure accesses the user database to get information about
     the user identified by USER-NAME-OR-ID, which is the user’s
     symbolic name (string) or the user’s UID (exact integer).  A
     user-information record is returned that contains the user’s
     symbolic name, the user’s id (UID), the user’s group id (GID), the
     path to the user’s home directory, and the user’s login shell.

     For example:

          > (user-info "feeley")
          #<user-info #2
             name: "feeley"
             uid: 506
             gid: 506
             home: "/Users/feeley"
             shell: "/bin/bash">
          > (user-info 0)
          #<user-info #3 name: "root" uid: 0 gid: 0 home: "/var/root" shell: "/bin/sh">
          > (user-info 5000)
          *** ERROR IN (console)@3.1 -- Resource temporarily unavailable
          (user-info 5000)

 -- procedure: user-info? OBJ

     This procedure returns ‘#t’ when OBJ is a user-information record
     and ‘#f’ otherwise.

     For example:

          > (user-info? (user-info "feeley"))
          #t
          > (user-info? 123)
          #f

 -- procedure: user-info-name USER-INFO

     Returns the symbolic name field of the user-information record
     USER-INFO.

     For example:

          > (user-info-name (user-info 0))
          "root"

 -- procedure: user-info-uid USER-INFO

     Returns the user id field of the user-information record USER-INFO.

     For example:

          > (user-info-uid (user-info "feeley"))
          506

 -- procedure: user-info-gid USER-INFO

     Returns the group id field of the user-information record
     USER-INFO.

     For example:

          > (user-info-gid (user-info "feeley"))
          506

 -- procedure: user-info-home USER-INFO

     Returns the home directory field of the user-information record
     USER-INFO.

     For example:

          > (user-info-home (user-info 0))
          "/var/root"

 -- procedure: user-info-shell USER-INFO

     Returns the shell field of the user-information record USER-INFO.

     For example:

          > (user-info-shell (user-info 0))
          "/bin/sh"

13.11 Host information
======================

 -- procedure: host-name

     This procedure returns the machine’s host name as a string.

     For example:

          > (host-name)
          "mega.iro.umontreal.ca"

 -- procedure: host-info HOST-NAME

     This procedure accesses the internet host database to get
     information about the machine whose name is denoted by the string
     HOST-NAME.  A host-information record is returned that contains the
     official name of the machine, a list of aliases (alternative
     names), and a non-empty list of IP addresses for this machine.  An
     exception is raised when HOST-NAME does not appear in the database.

     For example:

          > (host-info "www.google.com")
          #<host-info #2
             name: "www.l.google.com"
             aliases: ("www.google.com")
             addresses: (#u8(66 249 85 99) #u8(66 249 85 104))>
          > (host-info "unknown.domain")
          *** ERROR IN (console)@2.1 -- Unknown host
          (host-info "unknown.domain")

 -- procedure: host-info? OBJ

     This procedure returns ‘#t’ when OBJ is a host-information record
     and ‘#f’ otherwise.

     For example:

          > (host-info? (host-info "www.google.com"))
          #t
          > (host-info? 123)
          #f

 -- procedure: host-info-name HOST-INFO

     Returns the official name field of the host-information record
     HOST-INFO.

     For example:

          > (host-info-name (host-info "www.google.com"))
          "www.l.google.com"

 -- procedure: host-info-aliases HOST-INFO

     Returns the aliases field of the host-information record HOST-INFO.
     This field is a possibly empty list of strings.

     For example:

          > (host-info-aliases (host-info "www.google.com"))
          ("www.google.com")

 -- procedure: host-info-addresses HOST-INFO

     Returns the addresses field of the host-information record
     HOST-INFO.  This field is a non-empty list of u8vectors denoting IP
     addresses.

     For example:

          > (host-info-addresses (host-info "www.google.com"))
          (#u8(66 249 85 99) #u8(66 249 85 104))

 -- procedure: address-infos [host: HOST] [service: SERVICE] [family:
          FAMILY] [socket-type: SOCKET-TYPE] [protocol: PROTOCOL]

     This procedure is an interface to the ‘getaddrinfo’ system call.
     It accesses the internet host database to get information about the
     machine whose name is denoted by the string HOST and service is
     denoted by the string SERVICE and network address family is FAMILY
     (‘INET’ or ‘INET6’) and network socket-type is SOCKET-TYPE
     (‘STREAM’ or ‘DGRAM’ or ‘RAW’) and network protocol is SOCKET-TYPE
     (‘TCP’ or ‘UDP’).  A list of address-information records is
     returned.

     For example:

          > (address-infos host: "ftp.at.debian.org")
          (#<address-info #2
              family: INET6
              socket-type: DGRAM
              protocol: UDP
              socket-info:
               #<socket-info #3
                  family: INET6
                  port-number: 0
                  address: #u16(8193 2136 2 1 0 0 0 16)>>
           #<address-info #4
              family: INET6
              socket-type: STREAM
              protocol: TCP
              socket-info:
               #<socket-info #5
                  family: INET6
                  port-number: 0
                  address: #u16(8193 2136 2 1 0 0 0 16)>>
           #<address-info #6
              family: INET
              socket-type: DGRAM
              protocol: UDP
              socket-info:
               #<socket-info #7
                  family: INET
                  port-number: 0
                  address: #u8(213 129 232 18)>>
           #<address-info #8
              family: INET
              socket-type: STREAM
              protocol: TCP
              socket-info:
               #<socket-info #9
                  family: INET
                  port-number: 0
                  address: #u8(213 129 232 18)>>)
          > (address-infos host: "ftp.at.debian.org"
                           family: 'INET
                           protocol: 'TCP)
          (#<address-info #10
              family: INET
              socket-type: STREAM
              protocol: TCP
              socket-info:
               #<socket-info #11
                  family: INET
                  port-number: 0
                  address: #u8(213 129 232 18)>>)
          > (address-infos host: "unknown.domain")
          *** ERROR IN (console)@5.1 -- nodename nor servname provided, or not known
          (address-infos host: "unknown.domain")

 -- procedure: address-info? OBJ

     This procedure returns ‘#t’ when OBJ is an address-information
     record and ‘#f’ otherwise.

     For example:

          > (map address-info?
                 (address-infos host: "ftp.at.debian.org"))
          (#t #t #t #t)
          > (address-info? 123)
          #f

 -- procedure: address-info-family ADDRESS-INFO

     Returns the family field of the address-information record
     ADDRESS-INFO.

     For example:

          > (map address-info-family
                 (address-infos host: "ftp.at.debian.org"))
          (INET6 INET6 INET INET)

 -- procedure: address-info-socket-type ADDRESS-INFO

     Returns the socket-type field of the address-information record
     ADDRESS-INFO.

     For example:

          > (map address-info-socket-type
                 (address-infos host: "ftp.at.debian.org"))
          (DGRAM STREAM DGRAM STREAM)

 -- procedure: address-info-protocol ADDRESS-INFO

     Returns the protocol field of the address-information record
     ADDRESS-INFO.

     For example:

          > (map address-info-protocol
                 (address-infos host: "ftp.at.debian.org"))
          (UDP TCP UDP TCP)

 -- procedure: address-info-socket-info ADDRESS-INFO

     Returns the socket-info field of the address-information record
     ADDRESS-INFO.

     For example:

          > (map address-info-socket-info
                 (address-infos host: "ftp.at.debian.org"))
          (#<socket-info #2
              family: INET6
              port-number: 0
              address: #u16(8193 2136 2 1 0 0 0 16)>
           #<socket-info #3
              family: INET6
              port-number: 0
              address: #u16(8193 2136 2 1 0 0 0 16)>
           #<socket-info #4
              family: INET
              port-number: 0
              address: #u8(213 129 232 18)>
           #<socket-info #5
              family: INET
              port-number: 0
              address: #u8(213 129 232 18)>)

13.12 Service information
=========================

 -- procedure: service-info SERVICE-NAME-OR-ID

     This procedure accesses the service database to get information
     about the service identified by SERVICE-NAME-OR-ID, which is the
     service’s symbolic name (string) or the service’s port number
     (exact integer).  A service-information record is returned that
     contains the service’s symbolic name, a list of aliases
     (alternative names), the port number (exact integer), and the
     protocol name (string).  An exception is raised when
     SERVICE-NAME-OR-ID does not appear in the database.

     For example:

          > (service-info "http")
          #<service-info #2
             name: "http"
             aliases: ("www" "www-http")
             port-number: 80
             protocol: "udp">
          > (service-info 80)
          #<service-info #3
             name: "http"
             aliases: ("www" "www-http")
             port-number: 80
             protocol: "udp">

 -- procedure: service-info? OBJ

     This procedure returns ‘#t’ when OBJ is a service-information
     record and ‘#f’ otherwise.

     For example:

          > (service-info? (service-info "http"))
          #t
          > (service-info? 123)
          #f

 -- procedure: service-info-name SERVICE-INFO

     Returns the symbolic name field of the service-information record
     SERVICE-INFO.

     For example:

          > (service-info-name (service-info 80))
          "http"

 -- procedure: service-info-aliases SERVICE-INFO

     Returns the aliases field of the service-information record
     SERVICE-INFO.  This field is a possibly empty list of strings.

     For example:

          > (service-info-aliases (service-info "http"))
          ("www" "www-http")

 -- procedure: service-info-port-number SERVICE-INFO

     Returns the service port number field of the service-information
     record SERVICE-INFO.

     For example:

          > (service-info-port-number (service-info "http"))
          80

 -- procedure: service-info-protocol SERVICE-INFO

     Returns the service protocol name field of the service-information
     record SERVICE-INFO.

     For example:

          > (service-info-protocol (service-info "http"))
          "udp"

13.13 Protocol information
==========================

 -- procedure: protocol-info PROTOCOL-NAME-OR-ID

     This procedure accesses the protocol database to get information
     about the protocol identified by PROTOCOL-NAME-OR-ID, which is the
     protocol’s symbolic name (string) or the protocol’s number (exact
     integer).  A protocol-information record is returned that contains
     the protocol’s symbolic name, a list of aliases (alternative
     names), and the protocol number (32 bit unsigned exact integer).
     An exception is raised when PROTOCOL-NAME-OR-ID does not appear in
     the database.

     For example:

          > (protocol-info "tcp")
          #<protocol-info #2 name: "tcp" aliases: ("TCP") number: 6>
          > (protocol-info 6)
          #<protocol-info #2 name: "tcp" aliases: ("TCP") number: 6>

 -- procedure: protocol-info? OBJ

     This procedure returns ‘#t’ when OBJ is a protocol-information
     record and ‘#f’ otherwise.

     For example:

          > (protocol-info? (protocol-info "tcp"))
          #t
          > (protocol-info? 123)
          #f

 -- procedure: protocol-info-name PROTOCOL-INFO

     Returns the symbolic name field of the protocol-information record
     PROTOCOL-INFO.

     For example:

          > (protocol-info-name (protocol-info 6))
          "tcp"

 -- procedure: protocol-info-aliases PROTOCOL-INFO

     Returns the aliases field of the protocol-information record
     PROTOCOL-INFO.  This field is a possibly empty list of strings.

     For example:

          > (protocol-info-aliases (protocol-info "tcp"))
          ("TCP")

 -- procedure: protocol-info-number PROTOCOL-INFO

     Returns the protocol number field of the protocol-information
     record PROTOCOL-INFO.

     For example:

          > (protocol-info-number (protocol-info "tcp"))
          6

13.14 Network information
=========================

 -- procedure: network-info NETWORK-NAME-OR-ID

     This procedure accesses the network database to get information
     about the network identified by NETWORK-NAME-OR-ID, which is the
     network’s symbolic name (string) or the network’s number (exact
     integer).  A network-information record is returned that contains
     the network’s symbolic name, a list of aliases (alternative names),
     and the network number (32 bit unsigned exact integer).  An
     exception is raised when NETWORK-NAME-OR-ID does not appear in the
     database.

     For example:

          > (network-info "loopback")
          #<network-info #2
             name: "loopback"
             aliases: ("loopback-net")
             number: 127>
          > (network-info 127)
          #<network-info #3
             name: "loopback"
             aliases: ("loopback-net")
             number: 127>

 -- procedure: network-info? OBJ

     This procedure returns ‘#t’ when OBJ is a network-information
     record and ‘#f’ otherwise.

     For example:

          > (network-info? (network-info "loopback"))
          #t
          > (network-info? 123)
          #f

 -- procedure: network-info-name NETWORK-INFO

     Returns the symbolic name field of the network-information record
     NETWORK-INFO.

     For example:

          > (network-info-name (network-info 127))
          "loopback"

 -- procedure: network-info-aliases NETWORK-INFO

     Returns the aliases field of the network-information record
     NETWORK-INFO.  This field is a possibly empty list of strings.

     For example:

          > (network-info-aliases (network-info "loopback"))
          ("loopback-net")

 -- procedure: network-info-number NETWORK-INFO

     Returns the network number field of the network-information record
     NETWORK-INFO.

     For example:

          > (network-info-number (network-info "loopback"))
          127

14 I/O and ports
****************

14.1 Unidirectional and bidirectional ports
===========================================

Unidirectional ports allow communication between a producer of
information and a consumer.  An input-port’s producer is typically a
resource managed by the operating system (such as a file, a process or a
network connection) and the consumer is the Scheme program.  The roles
are reversed for an output-port.

   Associated with each port are settings that affect I/O operations on
that port (encoding of characters to bytes, end-of-line encoding, type
of buffering, etc).  Port settings are specified when the port is
created.  Some port settings can be changed after a port is created.

   Bidirectional ports, also called input-output-ports, allow
communication in both directions.  They are best viewed as an object
that groups two separate unidirectional ports (one in each direction).
Each direction has its own port settings and can be closed independently
from the other direction.

14.2 Port classes
=================

The four classes of ports listed below form an inheritance hierarchy.
Operations possible for a certain class of port are also possible for
the subclasses.  Only device-ports are connected to a device managed by
the operating system.  For instance it is possible to create ports that
behave as a FIFO where the Scheme program is both the producer and
consumer of information (possibly one thread is the producer and another
thread is the consumer).

  1. An “object-port” (or simply a port) provides operations to read and
     write Scheme data (i.e.  any Scheme object) to/from the port.  It
     also provides operations to force output to occur, to change the
     way threads block on the port, and to close the port.  Note that
     the class of objects for which write/read invariance is guaranteed
     depends on the particular class of port.

  2. A “character-port” provides all the operations of an object-port,
     and also operations to read and write individual characters to/from
     the port.  When a Scheme object is written to a character-port, it
     is converted into the sequence of characters that corresponds to
     its external-representation.  When reading a Scheme object, an
     inverse conversion occurs.  Note that some Scheme objects do not
     have an external textual representation that can be read back.

  3. A “byte-port” provides all the operations of a character-port, and
     also operations to read and write individual bytes to/from the
     port.  When a character is written to a byte-port, some encoding of
     that character into a sequence of bytes will occur (for example,
     ‘#\newline’ will be encoded as the 2 bytes CR-LF when using
     ISO-8859-1 character encoding and ‘cr-lf’ end-of-line encoding, and
     a non-ASCII character will generate more than 1 byte when using
     UTF-8 character encoding).  When reading a character, a similar
     decoding occurs.

  4. A “device-port” provides all the operations of a byte-port, and
     also operations to control the operating system managed device
     (file, network connection, terminal, etc) that is connected to the
     port.

14.3 Port settings
==================

Some port settings are only valid for specific port classes whereas some
others are valid for all ports.  Port settings are specified when a port
is created.  The settings that are not specified will default to some
reasonable values.  Keyword objects are used to name the settings to be
set.  As a simple example, a device-port connected to the file ‘"foo"’
can be created using the call

     (open-input-file "foo")

   This will use default settings for the character encoding, buffering,
etc.  When a specific character encoding is desired, such as UTF-16BE,
the port can be opened using the call

     (open-input-file (list path: "foo" char-encoding: 'UTF-16BE))

   Here the argument of the procedure ‘open-input-file’ has been
replaced by a “port settings list” which specifies the value of each
port setting that should not be set to the default value.  Note that
some port settings have no useful default and it is therefore required
to specify a value for them, such as the ‘path:’ in the case of the file
opening procedures.  All port creation procedures (i.e.  named
‘open-...’) take a single argument that can either be a port settings
list or a value of a type that depends on the kind of port being created
(a path string for files, an IP port number for socket servers, etc).

14.4 Object-ports
=================

14.4.1 Object-port settings
---------------------------

The following is a list of port settings that are valid for all types of
ports.

   • ‘direction:’ ( ‘input’ | ‘output’ | ‘input-output’ )

     This setting controls the direction of the port.  The symbol
     ‘input’ indicates a unidirectional input-port, the symbol ‘output’
     indicates a unidirectional output-port, and the symbol
     ‘input-output’ indicates a bidirectional port.  The default value
     of this setting depends on the port creation procedure.

   • ‘buffering:’ ( ‘#f’ | ‘#t’ | ‘line’ )

     This setting controls the buffering of the port.  To set each
     direction separately the keywords ‘input-buffering:’ and
     ‘output-buffering:’ must be used instead of ‘buffering:’.  The
     value ‘#f’ selects unbuffered I/O, the value ‘#t’ selects fully
     buffered I/O, and the symbol ‘line’ selects line buffered I/O (the
     output buffer is drained when a ‘#\newline’ character is written).
     Line buffered I/O only applies to character-ports.  The default
     value of this setting is operating system dependent except consoles
     which are unbuffered.

14.4.2 Object-port operations
-----------------------------

 -- procedure: input-port? OBJ
 -- procedure: output-port? OBJ
 -- procedure: port? OBJ

     The procedure ‘input-port?’ returns ‘#t’ when OBJ is a
     unidirectional input-port or a bidirectional port and ‘#f’
     otherwise.

     The procedure ‘output-port?’ returns ‘#t’ when OBJ is a
     unidirectional output-port or a bidirectional port and ‘#f’
     otherwise.

     The procedure ‘port?’ returns ‘#t’ when OBJ is a port (either
     unidirectional or bidirectional) and ‘#f’ otherwise.

     For example:

          > (input-port? (current-input-port))
          #t
          > (call-with-input-string "some text" output-port?)
          #f
          > (port? (current-output-port))
          #t

 -- procedure: read [PORT]

     This procedure reads and returns the next Scheme datum from the
     input-port PORT.  The end-of-file object is returned when the end
     of the stream is reached.  If it is not specified, PORT defaults to
     the current input-port.

     For example:

          > (call-with-input-string "some text" read)
          some
          > (call-with-input-string "" read)
          #!eof

 -- procedure: read-all [PORT [READER]]

     This procedure repeatedly calls the procedure READER with PORT as
     the sole argument and accumulates a list of each value returned up
     to the end-of-file object.  The procedure ‘read-all’ returns the
     accumulated list without the end-of-file object.  If it is not
     specified, PORT defaults to the current input-port.  If it is not
     specified, READER defaults to the procedure ‘read’.

     For example:

          > (call-with-input-string "3,2,1\ngo!" read-all)
          (3 ,2 ,1 go!)
          > (call-with-input-string "3,2,1\ngo!"
                                    (lambda (p) (read-all p read-char)))
          (#\3 #\, #\2 #\, #\1 #\newline #\g #\o #\!)
          > (call-with-input-string "3,2,1\ngo!"
                                    (lambda (p) (read-all p read-line)))
          ("3,2,1" "go!")

 -- procedure: write OBJ [PORT]

     This procedure writes the Scheme datum OBJ to the output-port PORT
     and the value returned is unspecified.  If it is not specified,
     PORT defaults to the current output-port.

     For example:

          > (write (list 'compare (list 'quote '@x) 'and (list 'unquote '@x)))
          (compare '@x and , @x)>

 -- procedure: newline [PORT]

     This procedure writes an “object separator” to the output-port PORT
     and the value returned is unspecified.  The separator ensures that
     the next Scheme datum written with the ‘write’ procedure will not
     be confused with the latest datum that was written.  On
     character-ports this is done by writing the character ‘#\newline’.
     On ports where successive objects are implicitly distinct (such as
     “vector ports”) this procedure does nothing.

     Regardless of the class of a port P and assuming that the external
     textual representation of the object X is readable, the expression
     ‘(begin (write X P) (newline P))’ will write to P a representation
     of X that can be read back with the procedure ‘read’.  If it is not
     specified, PORT defaults to the current output-port.

     For example:

          > (begin (write 123) (newline) (write 456) (newline))
          123
          456

 -- procedure: force-output [PORT [LEVEL]]

     The procedure ‘force-output’ causes the data that was written to
     the output-port PORT to be moved closer to its destination
     according to LEVEL, an exact integer in the range 0 to 2.  If PORT
     is not specified, the current output-port is used.  If LEVEL is not
     specified, it defaults to 0.  Values of LEVEL above 0 are
     equivalent to LEVEL = 0 except for device ports as explained below.

     When LEVEL is 0, the output buffers of PORT which are managed in
     the Scheme process are drained (i.e.  the output operation that was
     delayed due to buffering is actually performed).  In the case of a
     device port the data is passed to the operating system and it
     becomes its responsibility to transmit the data to the device.  The
     operating system may implement its own buffering approach which
     delays the transmission of the data to the device.

     When LEVEL is 1, in addition to the operations for LEVEL = 0 and if
     the operating system supports the functionality, the operating
     system is requested to transmit the data to the device.  On UNIX
     this corresponds to a ‘fsync’ system call.

     When LEVEL is 2, in addition to the operations for LEVEL = 1 and if
     the operating system supports the functionality, the operating
     system is requested to wait until the device reports that the data
     was saved by the device (e.g.  actually written to disk in the case
     of a file).  This operation can take a long time on some operating
     systems.  On macOS this corresponds to a ‘fcntl’ system call with
     operation ‘F_FULLFSYNC’.

     For example:

          > (define p (open-tcp-client "www.iro.umontreal.ca:80"))
          > (display "GET /\n" p)
          > (force-output p)
          > (read-line p)
          "<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\""

 -- procedure: close-input-port PORT
 -- procedure: close-output-port PORT
 -- procedure: close-port PORT

     The PORT argument of these procedures must be a unidirectional or a
     bidirectional port.  For all three procedures the value returned is
     unspecified.

     The procedure ‘close-input-port’ closes the input-port side of
     PORT, which must not be a unidirectional output-port.

     The procedure ‘close-output-port’ closes the output-port side of
     PORT, which must not be a unidirectional input-port.  The ouput
     buffers are drained before PORT is closed.

     The procedure ‘close-port’ closes all sides of the PORT.  Unless
     PORT is a unidirectional input-port, the output buffers are drained
     before PORT is closed.

     For example:

          > (define p (open-tcp-client "www.iro.umontreal.ca:80"))
          > (display "GET /\n" p)
          > (close-output-port p)
          > (read-line p)
          "<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\""

 -- procedure: input-port-timeout-set! PORT TIMEOUT [THUNK]
 -- procedure: output-port-timeout-set! PORT TIMEOUT [THUNK]

     When a thread tries to perform an I/O operation on a port, the
     requested operation may not be immediately possible and the thread
     must wait.  For example, the thread may be trying to read a line of
     text from the console and the user has not typed anything yet, or
     the thread may be trying to write to a network connection faster
     than the network can handle.  In such situations the thread
     normally blocks until the operation becomes possible.

     It is sometimes necessary to guarantee that the thread will not
     block too long.  For this purpose, to each input-port and
     output-port is attached a “timeout” and “timeout-thunk”.  The
     timeout indicates the point in time beyond which the thread should
     stop waiting on an input and output operation respectively.  When
     the timeout is reached, the thread calls the port’s timeout-thunk.
     If the timeout-thunk returns ‘#f’ the thread abandons trying to
     perform the operation (in the case of an input operation an
     end-of-file is read and in the case of an output operation an
     exception is raised).  Otherwise, the thread will block again
     waiting for the operation to become possible (note that if the
     port’s timeout has not changed the thread will immediately call the
     timeout-thunk again).

     The procedure ‘input-port-timeout-set!’ sets the timeout of the
     input-port PORT to TIMEOUT and the timeout-thunk to THUNK.  The
     procedure ‘output-port-timeout-set!’ sets the timeout of the
     output-port PORT to TIMEOUT and the timeout-thunk to THUNK.  If it
     is not specified, the THUNK defaults to a thunk that returns ‘#f’.
     The TIMEOUT is either a time object indicating an absolute point in
     time, or it is a real number which indicates the number of seconds
     relative to the moment the procedure is called.  For both
     procedures the value returned is unspecified.

     When a port is created the timeout is set to infinity (‘+inf.0’).
     This causes the thread to wait as long as needed for the operation
     to become possible.  Setting the timeout to a point in the past
     (‘-inf.0’) will cause the thread to attempt the I/O operation and
     never block (i.e.  the timeout-thunk is called if the operation is
     not immediately possible).

     The following example shows how to cause the REPL to terminate when
     the user does not enter an expression within the next 60 seconds.

          > (input-port-timeout-set! (repl-input-port) 60)
          >
          *** EOF again to exit

14.5 Character-ports
====================

14.5.1 Character-port settings
------------------------------

The following is a list of port settings that are valid for
character-ports.

   • ‘readtable:’ READTABLE

     This setting determines the readtable attached to the
     character-port.  To set each direction separately the keywords
     ‘input-readtable:’ and ‘output-readtable:’ must be used instead of
     ‘readtable:’.  Readtables control the external textual
     representation of Scheme objects, that is the encoding of Scheme
     objects using characters.  The behavior of the ‘read’ procedure
     depends on the port’s input-readtable and the behavior of the
     procedures ‘write’, ‘pretty-print’, and related procedures is
     affected by the port’s output-readtable.  The default value of this
     setting is the value bound to the parameter object
     ‘current-readtable’.

   • ‘output-width:’ POSITIVE-INTEGER

     This setting indicates the width of the character output-port in
     number of characters.  This information is used by the
     pretty-printer.  The default value of this setting is 80.

14.5.2 Character-port operations
--------------------------------

 -- procedure: input-port-line PORT
 -- procedure: input-port-column PORT
 -- procedure: output-port-line PORT
 -- procedure: output-port-column PORT

     The current character location of a character input-port is the
     location of the next character to read.  The current character
     location of a character output-port is the location of the next
     character to write.  Location is denoted by a line number (the
     first line is line 1) and a column number, that is the location on
     the current line (the first column is column 1).  The procedures
     ‘input-port-line’ and ‘input-port-column’ return the line location
     and the column location respectively of the character input-port
     PORT.  The procedures ‘output-port-line’ and ‘output-port-column’
     return the line location and the column location respectively of
     the character output-port PORT.

     For example:

          > (call-with-output-string
              (lambda (p)
                (display "abc\n123def" p)
                (write (list (output-port-line p) (output-port-column p))
                       p)))
          "abc\n123def(2 7)"

 -- procedure: output-port-width PORT

     This procedure returns the width, in characters, of the character
     output-port PORT.  The value returned is the port’s output-width
     setting.

     For example:

          > (output-port-width (repl-output-port))
          80

 -- procedure: read-char [PORT]

     This procedure reads the character input-port PORT and returns the
     character at the current character location and advances the
     current character location to the next character, unless the PORT
     is already at end-of-file in which case ‘read-char’ returns the
     end-of-file object.  If it is not specified, PORT defaults to the
     current input-port.

     For example:

          > (call-with-input-string
              "some text"
              (lambda (p)
                (let ((a (read-char p))) (list a (read-char p)))))
          (#\s #\o)
          > (call-with-input-string "" read-char)
          #!eof

 -- procedure: peek-char [PORT]

     This procedure returns the same result as ‘read-char’ but it does
     not advance the current character location of the input-port PORT.
     If it is not specified, PORT defaults to the current input-port.

     For example:

          > (call-with-input-string
              "some text"
              (lambda (p)
                (let ((a (peek-char p))) (list a (read-char p)))))
          (#\s #\s)
          > (call-with-input-string "" peek-char)
          #!eof

 -- procedure: write-char CHAR [PORT]

     This procedure writes the character CHAR to the character
     output-port PORT and advances the current character location of
     that output-port.  The value returned is unspecified.  If it is not
     specified, PORT defaults to the current output-port.

     For example:

          > (write-char #\=)
          =>

 -- procedure: read-line [PORT [SEPARATOR [INCLUDE-SEPARATOR?
          [MAX-LENGTH]]]]

     This procedure reads characters from the character input-port PORT
     until a specific SEPARATOR or the end-of-file is encountered and
     returns a string containing the sequence of characters read.  If it
     is specified, MAX-LENGTH must be a nonnegative exact integer and it
     places an upper limit on the number of characters that are read.

     The SEPARATOR is included at the end of the string only if it was
     the last character read and INCLUDE-SEPARATOR? is not ‘#f’.  The
     SEPARATOR must be a character or ‘#f’ (in which case all the
     characters until the end-of-file are read).  If it is not
     specified, PORT defaults to the current input-port.  If it is not
     specified, SEPARATOR defaults to ‘#\newline’.  If it is not
     specified, INCLUDE-SEPARATOR? defaults to ‘#f’.

     For example:

          > (define (split sep)
              (lambda (str)
                (call-with-input-string
                  str
                  (lambda (p)
                    (read-all p (lambda (p) (read-line p sep)))))))
          > ((split #\,) "a,b,c")
          ("a" "b" "c")
          > (map (split #\,)
                 (call-with-input-string "1,2,3\n4,5"
                                         (lambda (p) (read-all p read-line))))
          (("1" "2" "3") ("4" "5"))
          > (read-line (current-input-port) #\newline #f 2)1234
          "12"
          > 34

 -- procedure: read-substring STRING START END [PORT [NEED]]
 -- procedure: write-substring STRING START END [PORT]

     These procedures support bulk character I/O. The part of the string
     STRING starting at index START and ending just before index END is
     used as a character buffer that will be the target of
     ‘read-substring’ or the source of the ‘write-substring’.  The
     ‘read-substring’ also accepts a NEED parameter which must be a
     nonnegative fixnum.  Up to END-START characters will be
     transferred.  The number of characters transferred, possibly zero,
     is returned by these procedures.  Fewer characters will be read by
     ‘read-substring’ if an end-of-file is read, or a timeout occurs
     before all the requested characters are transferred and the timeout
     thunk returns ‘#f’ (see the procedure ‘input-port-timeout-set!’),
     or NEED is specified and at least that many characters have been
     read (in other words the procedure does not block for more
     characters but may transfer more characters if they are immediately
     available).  Fewer characters will be written by ‘write-substring’
     if a timeout occurs before all the requested characters are
     transferred and the timeout thunk returns ‘#f’ (see the procedure
     ‘output-port-timeout-set!’).  If it is not specified, PORT defaults
     to the current input-port and current output-port respectively.

     For example:

          > (define s (make-string 10 #\x))
          > (read-substring s 2 5)123456789
          3
          > 456789
          > s
          "xx123xxxxx"
          > (read-substring s 2 10 (current-input-port) 3)abcd
          5
          > s
          "xxabcd\nxxx"

 -- procedure: input-port-readtable PORT
 -- procedure: output-port-readtable PORT

     These procedures return the readtable attached to the
     character-port PORT.  The PORT parameter of ‘input-port-readtable’
     must be an input-port.  The PORT parameter of
     ‘output-port-readtable’ must be an output-port.

 -- procedure: input-port-readtable-set! PORT READTABLE
 -- procedure: output-port-readtable-set! PORT READTABLE

     These procedures change the readtable attached to the
     character-port PORT to the readtable READTABLE.  The PORT parameter
     of ‘input-port-readtable-set!’ must be an input-port.  The PORT
     parameter of ‘output-port-readtable-set!’ must be an output-port.
     The value returned is unspecified.

14.6 Byte-ports
===============

14.6.1 Byte-port settings
-------------------------

The following is a list of port settings that are valid for byte-ports.

   • ‘char-encoding:’ ENCODING

     This setting controls the character encoding of the byte-port.  For
     bidirectional byte-ports, the character encoding for input and
     output is set.  To set each direction separately the keywords
     ‘input-char-encoding:’ and ‘output-char-encoding:’ must be used
     instead of ‘char-encoding:’.  The default value of this setting
     depends on how the runtime system was configured but typically
     UTF-8 is used.  The default can be overridden through various
     runtime options (*note Runtime options::), such as
     ‘-:file-settings=...’ and ‘-:io-settings=...’.  The following
     encodings are supported:

     ‘ISO-8859-1’
          ISO-8859-1 character encoding.  Each character is encoded by a
          single byte.  Only Unicode characters with a code in the range
          0 to 255 are allowed.

     ‘ASCII’
          ASCII character encoding.  Each character is encoded by a
          single byte.  In principle only Unicode characters with a code
          in the range 0 to 127 are allowed but most types of ports
          treat this exactly like ‘ISO-8859-1’.

     ‘UTF-8’
          UTF-8 character encoding.  Each character is encoded by a
          sequence of one to four bytes.  The minimum length UTF-8
          encoding is used.  If a BOM is needed at the beginning of the
          stream then it must be explicitly written.

     ‘UTF-16’
          UTF-16 character encoding.  Each character is encoded by one
          or two 16 bit integers (2 or 4 bytes).  The 16 bit integers
          may be encoded using little-endian encoding or big-endian
          encoding.  If the port is an input-port and the first two
          bytes read are a BOM (“Byte Order Mark” character with
          hexadecimal code FEFF) then the BOM will be discarded and the
          endianness will be set accordingly, otherwise the endianness
          depends on the operating system and how the Gambit runtime was
          compiled.  If the port is an output-port then a BOM will be
          output at the beginning of the stream and the endianness
          depends on the operating system and how the Gambit runtime was
          compiled.

     ‘UTF-16LE’
          UTF-16 character encoding with little-endian endianness.  It
          is like ‘UTF-16’ except the endianness is set to little-endian
          and there is no BOM processing.  If a BOM is needed at the
          beginning of the stream then it must be explicitly written.

     ‘UTF-16BE’
          UTF-16 character encoding with big-endian endianness.  It is
          like ‘UTF-16LE’ except the endianness is set to big-endian.

     ‘UTF / UTF-fallback-ASCII / UTF-fallback-ISO-8859-1 / UTF-fallback-UTF-16 / UTF-fallback-UTF-16LE / UTF-fallback-UTF-16BE’
          These encodings combine the UTF-8 and UTF-16 encodings.  When
          one of these character encodings is used for an output port,
          characters will be encoded using the UTF-8 encoding.  The
          first character, if there is one, is prefixed with a UTF-8 BOM
          (the three byte sequence EF BB BF in hexadecimal).  When one
          of these character encodings is used for an input port, the
          character encoding depends on the first few bytes.  If the
          first bytes of the stream are a UTF-16LE BOM (FF FE in
          hexadecimal), or a UTF-16BE BOM (FE FF in hexadecimal), or a
          UTF-8 BOM (EF BB BF in hexadecimal), then the BOM is discarded
          and the remaining bytes of the stream are decoded using the
          corresponding character encoding.  If a BOM is not present,
          then the stream is decoded using the fallback encoding
          specified.  The encoding ‘UTF’ is a synonym for
          ‘UTF-fallback-UTF-8’.  Note that the ‘UTF’ character encoding
          for input will correctly handle streams produced using the
          encodings ‘UTF’, ‘UTF-8’, ‘UTF-16’, ‘ASCII’, and if an
          explicit BOM is output, the encodings ‘UTF-16LE’, and
          ‘UTF-16BE’.

     ‘UCS-2’
          UCS-2 character encoding.  Each character is encoded by a 16
          bit integer (2 bytes).  The 16 bit integers may be encoded
          using little-endian encoding or big-endian encoding.  If the
          port is an input-port and the first two bytes read are a BOM
          (“Byte Order Mark” character with hexadecimal code FEFF) then
          the BOM will be discarded and the endianness will be set
          accordingly, otherwise the endianness depends on the operating
          system and how the Gambit runtime was compiled.  If the port
          is an output-port then a BOM will be output at the beginning
          of the stream and the endianness depends on the operating
          system and how the Gambit runtime was compiled.

     ‘UCS-2LE’
          UCS-2 character encoding with little-endian endianness.  It is
          like ‘UCS-2’ except the endianness is set to little-endian and
          there is no BOM processing.  If a BOM is needed at the
          beginning of the stream then it must be explicitly written.

     ‘UCS-2BE’
          UCS-2 character encoding with big-endian endianness.  It is
          like ‘UCS-2LE’ except the endianness is set to big-endian.

     ‘UCS-4’
          UCS-4 character encoding.  Each character is encoded by a 32
          integer (4 bytes).  The 32 bit integers may be encoded using
          little-endian encoding or big-endian encoding.  If the port is
          an input-port and the first four bytes read are a BOM (“Byte
          Order Mark” character with hexadecimal code 0000FEFF) then the
          BOM will be discarded and the endianness will be set
          accordingly, otherwise the endianness depends on the operating
          system and how the Gambit runtime was compiled.  If the port
          is an output-port then a BOM will be output at the beginning
          of the stream and the endianness depends on the operating
          system and how the Gambit runtime was compiled.

     ‘UCS-4LE’
          UCS-4 character encoding with little-endian endianness.  It is
          like ‘UCS-4’ except the endianness is set to little-endian and
          there is no BOM processing.  If a BOM is needed at the
          beginning of the stream then it must be explicitly written.

     ‘UCS-4BE’
          UCS-4 character encoding with big-endian endianness.  It is
          like ‘UCS-4LE’ except the endianness is set to big-endian.

   • ‘char-encoding-errors:’ ( ‘#f’ | ‘#t’ )

     This setting controls whether illegal character encodings are
     silently replaced with the Unicode character #xfffd (replacement
     character) or raise an error.  To set each direction separately the
     keywords ‘input-char-encoding-errors:’ and
     ‘output-char-encoding-errors:’ must be used instead of
     ‘char-encoding-errors:’.  The default value of this setting is
     ‘#t’.

   • ‘eol-encoding:’ ENCODING

     This setting controls the end-of-line encoding of the byte-port.
     To set each direction separately the keywords ‘input-eol-encoding:’
     and ‘output-eol-encoding:’ must be used instead of ‘eol-encoding:’.
     The default value of this setting is operating system dependent,
     but this can be overridden through the runtime options (*note
     Runtime options::).  Note that for output-ports the end-of-line
     encoding is applied before the character encoding, and for
     input-ports it is applied after.  The following encodings are
     supported:

     ‘lf’
          For an output-port, writing a ‘#\newline’ character outputs a
          ‘#\linefeed’ character to the stream (Unicode character code
          10).  For an input-port, a ‘#\newline’ character is read when
          a ‘#\linefeed’ character is encountered on the stream.  Note
          that ‘#\linefeed’ and ‘#\newline’ are two names for the same
          character, so this end-of-line encoding is actually the
          identity function.  Text files created by UNIX applications
          typically use this end-of-line encoding.

     ‘cr’
          For an output-port, writing a ‘#\newline’ character outputs a
          ‘#\return’ character to the stream (Unicode character code
          13).  For an input-port, a ‘#\newline’ character is read when
          a ‘#\linefeed’ character or a ‘#\return’ character is
          encountered on the stream.  Text files created by Classic Mac
          OS applications typically use this end-of-line encoding.

     ‘cr-lf’
          For an output-port, writing a ‘#\newline’ character outputs to
          the stream a ‘#\return’ character followed by a ‘#\linefeed’
          character.  For an input-port, a ‘#\newline’ character is read
          when a ‘#\linefeed’ character or a ‘#\return’ character is
          encountered on the stream.  Moreover, if this character is
          immediately followed by the opposite character (‘#\linefeed’
          followed by ‘#\return’ or ‘#\return’ followed by ‘#\linefeed’)
          then the second character is ignored.  In other words, all
          four possible end-of-line encodings are read as a single
          ‘#\newline’ character.  Text files created by DOS and
          Microsoft Windows applications typically use this end-of-line
          encoding.

14.6.2 Byte-port operations
---------------------------

 -- procedure: read-u8 [PORT]
 -- procedure: peek-u8 [PORT]

     These procedures read the byte input-port PORT and return the byte
     at the current byte location unless the PORT is already at
     end-of-file in which case the end-of-file object is returned.  If
     the end-of-file is not reached then the procedure ‘read-u8’
     advances the current byte location to the next byte.  The procedure
     ‘peek-u8’ does not advance the port’s current byte location.  If it
     is not specified, PORT defaults to the current input-port.

     One way to ensure that the port’s input character buffer is empty
     is to call ‘peek-u8’ strictly before any use of the port in a
     character input operation (i.e.  a call to the procedures ‘read’,
     ‘read-char’, ‘peek-char’, etc).  Alternatively
     ‘input-port-characters-buffered’ can be used to get the number of
     characters in the port’s input character buffer, and to empty the
     buffer with calls to ‘read-char’ or ‘read-substring’.

     For example:

          > (call-with-input-u8vector
              '#u8(11 22 33 44)
              (lambda (p)
                (let ((a (read-u8 p))) (list a (read-u8 p)))))
          (11 22)
          > (call-with-input-u8vector '#u8() read-u8)
          #!eof
          > (with-input-from-u8vector '#u8(1 5) (lambda () (+ (peek-u8) (peek-u8))))
          2

 -- procedure: write-u8 N [PORT]

     This procedure writes the byte N to the byte output-port PORT and
     advances the current byte location of that output-port.  The value
     returned is unspecified.  If it is not specified, PORT defaults to
     the current output-port.

     For example:

          > (call-with-output-u8vector (lambda (p) (write-u8 33 p)))
          #u8(33)

 -- procedure: read-subu8vector U8VECTOR START END [PORT [NEED]]
 -- procedure: write-subu8vector U8VECTOR START END [PORT]

     These procedures support bulk byte I/O. The part of the u8vector
     U8VECTOR starting at index START and ending just before index END
     is used as a byte buffer that will be the target of
     ‘read-subu8vector’ or the source of the ‘write-subu8vector’.  The
     ‘read-subu8vector’ also accepts a NEED parameter which must be a
     nonnegative fixnum.  Up to END-START bytes will be transferred.
     The number of bytes transferred, possibly zero, is returned by
     these procedures.  Fewer bytes will be read by ‘read-subu8vector’
     if an end-of-file is read, or a timeout occurs before all the
     requested bytes are transferred and the timeout thunk returns ‘#f’
     (see the procedure ‘input-port-timeout-set!’), or NEED is specified
     and at least that many bytes have been read (in other words the
     procedure does not block for more bytes but may transfer more bytes
     if they are immediately available).  Fewer bytes will be written by
     ‘write-subu8vector’ if a timeout occurs before all the requested
     bytes are transferred and the timeout thunk returns ‘#f’ (see the
     procedure ‘output-port-timeout-set!’).  If it is not specified,
     PORT defaults to the current input-port and current output-port
     respectively.

     The procedure ‘read-subu8vector’ must be called before any use of
     the port in a character input operation (i.e.  a call to the
     procedures ‘read’, ‘read-char’, ‘peek-char’, etc) because otherwise
     the character-stream and byte-stream may be out of sync due to the
     port buffering.

     For example:

          > (define v (make-u8vector 10))
          > (read-subu8vector v 2 5)123456789
          3
          > 456789
          > v
          #u8(0 0 49 50 51 0 0 0 0 0)
          > (read-subu8vector v 2 10 (current-input-port) 3)abcd
          5
          > v
          #u8(0 0 97 98 99 100 10 0 0 0)

14.7 Device-ports
=================

14.7.1 Filesystem devices
-------------------------

 -- procedure: open-file PATH-OR-SETTINGS
 -- procedure: open-input-file PATH-OR-SETTINGS
 -- procedure: open-output-file PATH-OR-SETTINGS
 -- procedure: call-with-input-file PATH-OR-SETTINGS PROC
 -- procedure: call-with-output-file PATH-OR-SETTINGS PROC
 -- procedure: with-input-from-file PATH-OR-SETTINGS THUNK
 -- procedure: with-output-to-file PATH-OR-SETTINGS THUNK

     All of these procedures create a port to interface to a byte-stream
     device (such as a file, console, serial port, named pipe, etc)
     whose name is given by a path of the filesystem.  The ‘direction:’
     setting will default to the value ‘input’ for the procedures
     ‘open-input-file’, ‘call-with-input-file’ and
     ‘with-input-from-file’, to the value ‘output’ for the procedures
     ‘open-output-file’, ‘call-with-output-file’ and
     ‘with-output-to-file’, and to the value ‘input-output’ for the
     procedure ‘open-file’.

     The procedures ‘open-file’, ‘open-input-file’ and
     ‘open-output-file’ return the port that is created.  The procedures
     ‘call-with-input-file’ and ‘call-with-output-file’ call the
     procedure PROC with the port as single argument, and then return
     the value(s) of this call after closing the port.  The procedures
     ‘with-input-from-file’ and ‘with-output-to-file’ dynamically bind
     the current input-port and current output-port respectively to the
     port created for the duration of a call to the procedure THUNK with
     no argument.  The value(s) of the call to THUNK are returned after
     closing the port.

     The first argument of these procedures is either a string denoting
     a filesystem path or a list of port settings which must contain a
     ‘path:’ setting.  Here are the settings allowed in addition to the
     generic settings of byte-ports:

        • ‘path:’ STRING

          This setting indicates the location of the file in the
          filesystem.  There is no default value for this setting.

        • ‘append:’ ( ‘#f’ | ‘#t’ )

          This setting controls whether output will be added to the end
          of the file.  This is useful for writing to log files that
          might be open by more than one process.  The default value of
          this setting is ‘#f’.

        • ‘create:’ ( ‘#f’ | ‘#t’ | ‘maybe’ )

          This setting controls whether the file will be created when it
          is opened.  A setting of ‘#f’ requires that the file exist
          (otherwise an exception is raised).  A setting of ‘#t’
          requires that the file does not exist (otherwise an exception
          is raised).  A setting of ‘maybe’ will create the file if it
          does not exist.  The default value of this setting is ‘maybe’
          for output-ports and ‘#f’ for input-ports and bidirectional
          ports.

        • ‘permissions:’ 12-BIT-EXACT-INTEGER

          This setting controls the UNIX permissions that will be
          attached to the file if it is created.  The default value of
          this setting is ‘#o666’.

        • ‘truncate:’ ( ‘#f’ | ‘#t’ )

          This setting controls whether the file will be truncated when
          it is opened.  For input-ports and bidirectional ports, the
          default value of this setting is ‘#f’.  For output-ports, the
          default value of this setting is ‘#t’ when the ‘append:’
          setting is ‘#f’, and ‘#f’ otherwise.

     For example:

          > (with-output-to-file
              (list path: "nofile"
                    create: #f)
              (lambda ()
                (display "hello world!\n")))
          *** ERROR IN (console)@1.1 -- No such file or directory
          (with-output-to-file '(path: "nofile" create: #f) '#<procedure #2>)

 -- procedure: input-port-byte-position PORT [POSITION [WHENCE]]
 -- procedure: output-port-byte-position PORT [POSITION [WHENCE]]

     When called with a single argument these procedures return the byte
     position where the next I/O operation would take place in the file
     attached to the given PORT (relative to the beginning of the file).
     When called with two or three arguments, the byte position for
     subsequent I/O operations on the given PORT is changed to POSITION,
     which must be an exact integer.  When WHENCE is omitted or is 0,
     the POSITION is relative to the beginning of the file.  When WHENCE
     is 1, the POSITION is relative to the current byte position of the
     file.  When WHENCE is 2, the POSITION is relative to the end of the
     file.  The return value is the new byte position.  On most
     operating systems the byte position for reading and writing of a
     given bidirectional port are the same.

     When ‘input-port-byte-position’ is called to change the byte
     position of an input-port, all input buffers will be flushed so
     that the next byte read will be the one at the given position.

     When ‘output-port-byte-position’ is called to change the byte
     position of an output-port, there is an implicit call to
     ‘force-output’ before the position is changed.

     For example:

          > (define p  ; p is an input-output-port
              (open-file '(path: "test" char-encoding: ISO-8859-1 create: maybe)))
          > (list (input-port-byte-position p) (output-port-byte-position p))
          (0 0)
          > (display "abcdefghij\n" p)
          > (list (input-port-byte-position p) (output-port-byte-position p))
          (0 0)
          > (force-output p)
          > (list (input-port-byte-position p) (output-port-byte-position p))
          (11 11)
          > (input-port-byte-position p 2)
          2
          > (list (input-port-byte-position p) (output-port-byte-position p))
          (2 2)
          > (peek-char p)
          #\c
          > (list (input-port-byte-position p) (output-port-byte-position p))
          (11 11)
          > (output-port-byte-position p -7 2)
          4
          > (list (input-port-byte-position p) (output-port-byte-position p))
          (4 4)
          > (write-char #\! p)
          > (list (input-port-byte-position p) (output-port-byte-position p))
          (4 4)
          > (force-output p)
          > (list (input-port-byte-position p) (output-port-byte-position p))
          (5 5)
          > (input-port-byte-position p 1)
          1
          > (read p)
          bcd!fghij

14.7.2 Process devices
----------------------

 -- procedure: open-process PATH-OR-SETTINGS
 -- procedure: open-input-process PATH-OR-SETTINGS
 -- procedure: open-output-process PATH-OR-SETTINGS
 -- procedure: call-with-input-process PATH-OR-SETTINGS PROC
 -- procedure: call-with-output-process PATH-OR-SETTINGS PROC
 -- procedure: with-input-from-process PATH-OR-SETTINGS THUNK
 -- procedure: with-output-to-process PATH-OR-SETTINGS THUNK

     All of these procedures start a new operating system process and
     create a bidirectional port which allows communication with that
     process on its standard input and standard output.  The
     ‘direction:’ setting will default to the value ‘input’ for the
     procedures ‘open-input-process’, ‘call-with-input-process’ and
     ‘with-input-from-process’, to the value ‘output’ for the procedures
     ‘open-output-process’, ‘call-with-output-process’ and
     ‘with-output-to-process’, and to the value ‘input-output’ for the
     procedure ‘open-process’.  If the ‘direction:’ setting is ‘input’,
     the output-port side is closed.  If the ‘direction:’ setting is
     ‘output’, the input-port side is closed.

     The procedures ‘open-process’, ‘open-input-process’ and
     ‘open-output-process’ return the port that is created.  The
     procedures ‘call-with-input-process’ and ‘call-with-output-process’
     call the procedure PROC with the port as single argument, and then
     return the value(s) of this call after closing the port and waiting
     for the process to terminate.  The procedures
     ‘with-input-from-process’ and ‘with-output-to-process’ dynamically
     bind the current input-port and current output-port respectively to
     the port created for the duration of a call to the procedure THUNK
     with no argument.  The value(s) of the call to THUNK are returned
     after closing the port and waiting for the process to terminate.

     The first argument of this procedure is either a string denoting a
     filesystem path of an executable program or a list of port settings
     which must contain a ‘path:’ setting.  Here are the settings
     allowed in addition to the generic settings of byte-ports:

        • ‘path:’ STRING

          This setting indicates the location of the executable program
          in the filesystem.  There is no default value for this
          setting.

        • ‘arguments:’ LIST-OF-STRINGS

          This setting indicates the string arguments that are passed to
          the program.  The default value of this setting is the empty
          list (i.e.  no arguments).

        • ‘environment:’ LIST-OF-STRINGS

          This setting indicates the set of environment variable
          bindings that the process receives.  Each element of the list
          is a string of the form “‘VAR=VALUE’”, where ‘VAR’ is the name
          of the variable and ‘VALUE’ is its binding.  When
          LIST-OF-STRINGS is ‘#f’, the process inherits the environment
          variable bindings of the Scheme program.  The default value of
          this setting is ‘#f’.

        • ‘directory:’ DIR

          This setting indicates the current working directory of the
          process.  When DIR is ‘#f’, the process uses the value of
          ‘(current-directory)’.  The default value of this setting is
          ‘#f’.

        • ‘stdin-redirection:’ ( ‘#f’ | ‘#t’ )

          This setting indicates how the standard input of the process
          is redirected.  A setting of ‘#t’ will redirect the standard
          input from the process-port (i.e.  what is written to the
          process-port will be available on the standard input).  A
          setting of ‘#f’ will leave the standard input as-is, which
          typically results in input coming from the console.  The
          default value of this setting is ‘#t’.

        • ‘stdout-redirection:’ ( ‘#f’ | ‘#t’ )

          This setting indicates how the standard output of the process
          is redirected.  A setting of ‘#t’ will redirect the standard
          output to the process-port (i.e.  all output to standard
          output can be read from the process-port).  A setting of ‘#f’
          will leave the standard output as-is, which typically results
          in the output going to the console.  The default value of this
          setting is ‘#t’.

        • ‘stderr-redirection:’ ( ‘#f’ | ‘#t’ )

          This setting indicates how the standard error of the process
          is redirected.  A setting of ‘#t’ will redirect the standard
          error to the process-port (i.e.  all output to standard error
          can be read from the process-port).  A setting of ‘#f’ will
          leave the standard error as-is, which typically results in
          error messages being output to the console.  The default value
          of this setting is ‘#f’.

        • ‘pseudo-terminal:’ ( ‘#f’ | ‘#t’ )

          This setting applies to UNIX. It indicates what type of device
          will be bound to the process’ standard input and standard
          output.  A setting of ‘#t’ will use a pseudo-terminal device
          (this is a device that behaves like a tty device even though
          there is no real terminal or user directly involved).  A
          setting of ‘#f’ will use a pair of pipes.  The difference is
          important for programs which behave differently when they are
          used interactively, for example shells.  The default value of
          this setting is ‘#f’.

        • ‘show-console:’ ( ‘#f’ | ‘#t’ )

          This setting applies to Microsoft Windows.  It controls
          whether the process’ console window will be hidden or visible.
          The default value of this setting is ‘#t’ (i.e.  show the
          console window).

     For example:

          > (with-input-from-process "date" read-line)
          "Sun Jun 14 15:06:41 EDT 2009"
          > (define p (open-process (list path: "ls"
                                          arguments: '("../examples"))))
          > (read-line p)
          "README"
          > (read-line p)
          "Xlib-simple"
          > (close-port p)
          > (define p (open-process "/usr/bin/dc"))
          > (display "2 100 ^ p\n" p)
          > (force-output p)
          > (read-line p)
          "1267650600228229401496703205376"

 -- procedure: process-pid PROCESS-PORT

     This procedure returns the PID (Process Identifier) of the process
     of PROCESS-PORT.  The PID is a small exact integer.

     For example:

          > (let ((p (open-process "sort")))
              (process-pid p))
          318

 -- procedure: process-status PROCESS-PORT [TIMEOUT [TIMEOUT-VAL]]

     This procedure causes the current thread to wait until the process
     of PROCESS-PORT terminates (normally or not) or until the timeout
     is reached if TIMEOUT is supplied.  If the timeout is reached,
     PROCESS-STATUS returns TIMEOUT-VAL if it is supplied, otherwise an
     unterminated-process-exception object is raised.  The procedure
     returns the process exit status as encoded by the operating system.
     Typically, if the process exited normally the return value is the
     process exit status multiplied by 256.

     For example:

          > (let ((p (open-process "sort")))
              (for-each (lambda (x) (pretty-print x p))
                        '(22 11 33))
              (close-output-port p)
              (let ((r (read-all p)))
                (close-input-port p)
                (list (process-status p) r)))
          (0 (11 22 33))

 -- procedure: unterminated-process-exception? OBJ
 -- procedure: unterminated-process-exception-procedure EXC
 -- procedure: unterminated-process-exception-arguments EXC

     Unterminated-process-exception objects are raised when a call to
     the ‘process-status’ procedure reaches its timeout before the
     target process terminates and a timeout-value parameter is not
     specified.  The parameter EXC must be an
     unterminated-process-exception object.

     The procedure ‘unterminated-process-exception?’ returns ‘#t’ when
     OBJ is an unterminated-process-exception object and ‘#f’ otherwise.

     The procedure ‘unterminated-process-exception-procedure’ returns
     the procedure that raised EXC.

     The procedure ‘unterminated-process-exception-arguments’ returns
     the list of arguments of the procedure that raised EXC.

     For example:

          > (define (handler exc)
              (if (unterminated-process-exception? exc)
                  (list (unterminated-process-exception-procedure exc)
                        (unterminated-process-exception-arguments exc))
                  'not-unterminated-process-exception))
          > (with-exception-catcher
              handler
              (lambda ()
                (let ((p (open-process "sort")))
                  (process-status p 1))))
          (#<procedure #2 process-status> (#<input-output-port #3 (process "sort")>))

14.7.3 Network devices
----------------------

 -- procedure: open-tcp-client PORT-NUMBER-OR-ADDRESS-OR-SETTINGS

     This procedure opens a network connection to a socket server and
     returns a tcp-client-port (a subtype of device-port) that
     represents this connection and allows communication with that
     server.  The default value of the ‘direction:’ setting is
     ‘input-output’, i.e.  the Scheme program can send information to
     the server and receive information from the server.  The sending
     direction can be “shutdown” using the ‘close-output-port’ procedure
     and the receiving direction can be “shutdown” using the
     ‘close-input-port’ procedure.  The ‘close-port’ procedure closes
     both directions of the connection.

     The parameter of this procedure is an IP port number (16-bit
     nonnegative exact integer), a string of the form ‘"HOST:PORT"’ or a
     list of port settings.  When the parameter is the number PORT it is
     handled as if it was the setting ‘port-number:’ PORT.  When the
     parameter is the string ‘"HOST:PORT"’ it is handled as if it was
     the setting ‘address:’ ‘"HOST:PORT"’.

     Here are the settings allowed in addition to the generic settings
     of byte-ports:

        • ‘address:’ STRING-OR-IP-ADDRESS

          This setting indicates the internet address of the server, and
          possibly the IP port number.  When this parameter is not
          specified or is ‘""’, the connection requests are sent to the
          loopback interface (with IP address 127.0.0.1).  The parameter
          can be a string denoting a host name, which will be translated
          to an IP address by the ‘host-info’ procedure, or a 4 element
          u8vector which contains the 32-bit IPv4 address or an 8
          element u16vector which contains the 128-bit IPv6 address.  A
          string of the form ‘"HOST:PORT"’ is handled as if it was the
          combination of settings ‘address:’ ‘"HOST"’ ‘port-number:’
          PORT.

        • ‘port-number:’ 16-BIT-EXACT-INTEGER

          This setting indicates the IP port number of the server to
          connect to (e.g.  80 for the standard HTTP server, 23 for the
          standard telnet server).  There is no default value for this
          setting.

        • ‘local-address:’ STRING-OR-IP-ADDRESS

          This setting indicates the internet address of the local
          network interface on which connections requests are initiated,
          and possibly the IP port number.  When this parameter is not
          specified or is ‘"*"’, the connection requests are initiated
          on any network interface (i.e.  address INADDR_ANY). When this
          parameter is ‘""’, the connection requests are initiated only
          on the loopback interface (with IP address 127.0.0.1).  The
          parameter can be a string denoting a host name, which will be
          translated to an IP address by the ‘host-info’ procedure, or a
          4 element u8vector which contains the 32-bit IPv4 address or
          an 8 element u16vector which contains the 128-bit IPv6
          address.  A string of the form ‘"INTF:PORT"’ is handled as if
          it was the combination of settings ‘local-address:’ ‘"INTF"’
          ‘local-port-number:’ PORT.

        • ‘local-port-number:’ 16-BIT-EXACT-INTEGER

          This setting indicates the IP port number assigned to the
          socket which initiates connection requests.  The special value
          0 requests that a currently unused port number be assigned to
          the socket.  This is the default value for this setting.

        • ‘keep-alive:’ ( ‘#f’ | ‘#t’ )

          This setting controls the use of the “keep alive” option on
          the connection.  The “keep alive” option will periodically
          send control packets on otherwise idle network connections to
          ensure that the server host is active and reachable.  The
          default value of this setting is ‘#f’.

        • ‘coalesce:’ ( ‘#f’ | ‘#t’ )

          This setting controls the use of TCP’s “Nagle algorithm” which
          reduces the number of small packets by delaying their
          transmission and coalescing them into larger packets.  A
          setting of ‘#t’ will coalesce small packets into larger ones.
          A setting of ‘#f’ will transmit packets as soon as possible.
          The default value of this setting is ‘#t’.  Note that this
          setting does not affect the buffering of the port.

        • ‘tls-context:’ ( ‘#f’ | TLS-CONTEXT )

          This setting controls the use of TLS encryption.  If provided,
          the client will use this configuration for setting up a TCP
          connection with TLS encryption, otherwise it will use a plain
          TCP connection as usual.  Please note that Gambit must be
          compiled with TLS support for this option to be implemented.
          See ‘make-tls-context’ for futher information.  The default
          value of this setting is ‘#f’.

     Below is an example of the client-side code that opens a connection
     to an HTTP server on port 8080 of the loopback interface (with IP
     address 127.0.0.1).  For the server-side code see the example for
     the procedure ‘open-tcp-server’.

          > (define p (open-tcp-client (list port-number: 8080
                                             eol-encoding: 'cr-lf)))
          > p
          #<input-output-port #2 (tcp-client #u8(127 0 0 1) 8080)>
          > (display "GET /\n" p)
          > (force-output p)
          > (read-line p)
          "<HTML>"

 -- procedure: open-tcp-server PORT-NUMBER-OR-ADDRESS-OR-SETTINGS

     This procedure sets up a socket to accept network connection
     requests from clients and returns a tcp-server-port from which
     network connections to clients are obtained.  Tcp-server-ports are
     a direct subtype of object-ports (i.e.  they are not
     character-ports) and are input-ports.  Reading from a
     tcp-server-port with the ‘read’ procedure will block until a
     network connection request is received from a client.  The ‘read’
     procedure will then return a tcp-client-port (a subtype of
     device-port) that represents this connection and allows
     communication with that client.  Closing a tcp-server-port with
     either the ‘close-input-port’ or ‘close-port’ procedures will cause
     the network subsystem to stop accepting connections on that socket.

     The parameter of this procedure is an IP port number (16-bit
     nonnegative exact integer), a string of the form ‘"INTF:PORT"’ or a
     list of port settings which must contain a ‘local-port-number:’
     setting.  When the parameter is the number PORT it is handled as if
     it was the setting ‘local-port-number:’ PORT.  When the parameter
     is the string ‘"INTF:PORT"’ it is handled as if it was the setting
     ‘local-address:’ ‘"INTF:PORT"’.

     Below is a list of the settings allowed in addition to the settings
     ‘keep-alive:’ and ‘coalesce:’ allowed by the ‘open-tcp-client’
     procedure and the generic settings of byte-ports.  The settings
     which are not listed below apply to the tcp-client-port that is
     returned by ‘read’ when a connection is accepted and have the same
     meaning as if they were used in a call to the ‘open-tcp-client’
     procedure.

        • ‘local-address:’ STRING-OR-IP-ADDRESS

          This setting indicates the internet address of the local
          network interface on which connections requests are accepted,
          and possibly the IP port number.  When this parameter is not
          specified or is ‘""’, the connection requests are accepted
          only on the loopback interface (with IP address 127.0.0.1).
          When this parameter is ‘"*"’, the connection requests are
          accepted on all network interfaces (i.e.  address INADDR_ANY).
          The parameter can be a string denoting a host name, which will
          be translated to an IP address by the ‘host-info’ procedure,
          or a 4 element u8vector which contains the 32-bit IPv4 address
          or an 8 element u16vector which contains the 128-bit IPv6
          address.  A string of the form ‘"INTF:PORT"’ is handled as if
          it was the combination of settings ‘local-address:’ ‘"INTF"’
          ‘local-port-number:’ PORT.

        • ‘local-port-number:’ 16-BIT-EXACT-INTEGER

          This setting indicates the IP port number assigned to the
          socket which accepts connection requests from clients.  So
          called “well-known ports”, which are reserved for standard
          services, have a port number below 1024 and can only be
          assigned to a socket by a process with superuser priviledges
          (e.g.  80 for the HTTP service, 23 for the telnet service).
          No special priviledges are needed to assign higher port
          numbers to a socket.  The special value 0 requests that a
          currently unused port number be assigned to the socket (the
          port number assigned can be retrieved using the procedure
          ‘tcp-server-socket-info’).  There is no default value for this
          setting.

        • ‘backlog:’ POSITIVE-EXACT-INTEGER

          This setting indicates the maximum number of connection
          requests that can be waiting to be accepted by a call to
          ‘read’ (technically it is the value passed as the second
          argument of the UNIX ‘listen()’ function).  The default value
          of this setting is 128.

        • ‘reuse-address:’ ( ‘#f’ | ‘#t’ )

          This setting controls whether it is possible to assign a port
          number that is currently active.  Note that when a server
          process terminates, the socket it was using to accept
          connection requests does not become inactive immediately.
          Instead it remains active for a few minutes to ensure clean
          termination of the connections.  A setting of ‘#f’ will cause
          an exception to be raised in that case.  A setting of ‘#t’
          will allow a port number to be used even if it is active.  The
          default value of this setting is ‘#t’.

        • ‘tls-context:’ ( ‘#f’ | TLS-CONTEXT )

          This setting controls the use of TLS encryption.  If provided,
          the server will use this configuration for accepting TCP
          connections with TLS encryption, otherwise it will accept
          plain TCP connections as usual.  Please note that Gambit must
          be compiled with TLS support for this option to be
          implemented.  See ‘make-tls-context’ for futher information.
          The default value of this setting is ‘#f’.

     Below is an example of the server-side code that accepts
     connections on port 8080 of any network interface.  For the
     client-side code see the example for the procedure
     ‘open-tcp-client’.

          > (define s (open-tcp-server (list local-address: "*"
                                             local-port-number: 8080
                                             eol-encoding: 'cr-lf)))
          > (define p (read s))  ; blocks until client connects
          > p
          #<input-output-port #2 (tcp-client 8080)>
          > (read-line p)
          "GET /"
          > (display "<HTML>\n" p)
          > (force-output p)

 -- procedure: tcp-service-register! PORT-NUMBER-OR-ADDRESS-OR-SETTINGS
          THUNK [THREAD-GROUP]
 -- procedure: tcp-service-unregister!
          PORT-NUMBER-OR-ADDRESS-OR-SETTINGS

     The procedure ‘tcp-service-register!’ sets up a socket to accept
     network connection requests from clients and creates a “service”
     thread which processes the incoming connections and returns this
     thread.  The parameter PORT-NUMBER-OR-ADDRESS-OR-SETTINGS has the
     same meaning as for the procedure ‘open-tcp-server’.

     For each connection established the service thread creates a
     “handler” thread which executes a call to the procedure THUNK with
     no argument.  The handler thread’s current input-port and current
     output-port are both set to the tcp-client-port created for the
     connection.  There is no need for the THUNK to close the
     tcp-client-port, as this is done by the handler thread when the
     THUNK returns normally.

     The procedure ‘tcp-service-unregister!’ terminates the service
     thread which was registered by ‘tcp-service-register!’ with the
     same network interface and port number (if a service thread is
     still registered).  The procedure ‘tcp-service-register!’
     implicitly calls ‘tcp-service-unregister!’ before registering the
     new service thread.

          > (tcp-service-register!
             8000
             (lambda () (display "hello\n")))
          > (define p (open-tcp-client 8000))
          > (read-line p)
          "hello"
          > (tcp-service-unregister! 8000)

 -- procedure: make-tls-context [OPTIONS]

     This procedure requires Gambit to be compiled with TLS support,
     which is currently provided by OpenSSL. The ‘--enable-openssl’ flag
     of the configure script will activate it, provided that you have
     the OpenSSL library and headers installed.  It is strongly
     recommended that versions above 1.x are used.  On OSX, this means
     updating the OpenSSL bundled by default.  This can be achieved
     using Homebrew, but manual installation or any other package
     manager will do.  Some notes on Windows with MinGW are also
     relevant here.  Once you have a sane MinGW environment, remember to
     decompress the OpenSSL tarball with the tar utility, otherwise
     links to files won’t work during the compilation process.  The
     recommended build procedure for MinGW is as follows.

     Configure OpenSSL on MinGW 32 bits:

          perl Configure mingw no-asm --prefix=/usr/local --openssldir=/usr/local/openssl

     Configure OpenSSL on MinGW 64 bits:

          perl Configure mingw64 no-asm --prefix=/usr/local --openssldir=/usr/local/openssl

     Build and install with the following commands:

          make depend
          make
          make install

     A TLS context describes the options that will be used for setting
     up a TLS connection.  If no TLS context is provided to
     ‘open-tcp-client’ or ‘open-tcp-server’, regular TCP connections
     without encryption will be used instead.  The result of this
     procedure is a ‘SSL_CTX’ pointer, which can be further manipulated
     with custom OpenSSL bindings.  The configuration options are:

        • ‘min-version:’ SYMBOL

          Establish a minimum TLS version for the connection.  If the
          other peer doesn’t support or agree with it, the connection
          will fail.  Possible options (support depends on linked
          OpenSSL version): ‘ssl-v2’, ‘ssl-v3’, ‘tls-v1’, ‘tls-v1.1’,
          ‘tls-v1.2’.

        • ‘options:’ LIST-OF-SYMBOLS

          A list of flags enabling/disabling TLS options.  ‘server-mode’
          is required for using the TLS context with ‘open-tcp-server’.
          ‘use-diffie-hellman’ enables the Diffie-Hellman key exchange.
          ‘use-elliptic-curves’ enables Elliptic Curves.  If no curve
          name is provided (with ‘elliptic-curve:’), ‘prime256v1’ will
          be used.  ‘request-client-authentication’ is used by a server
          to enable request of authentication to clients.
          ‘insert-empty-fragments’ enables a countermeasure against a
          SSL 3.0/TLS 1.0 protocol vulnerability affecting CBC ciphers.
          If used, the resulting connection may not be handled by some
          broken SSL implementations.  This option has no effect for
          connections using other ciphers.

        • ‘certificate:’ PATH

          Path to PEM Certificate file.  This is a recommended option.
          If not provided OpenSSL will try to use anonymous cipher
          suites.

        • ‘private-key:’ PATH

          Path to PEM Private Key file.  If not provided, the
          Certificate path will be used instead.

        • ‘client-ca:’ PATH

          Path to PEM file containing Certificate Authorities allowed
          for client authentication.  Used only if
          ‘request-client-authentication’ option is enabled.

        • ‘elliptic-curve:’ STRING

          Name of the Elliptic Curve to use, according to RFC 4492.
          Used only if ‘use-elliptic-curves’ option is enabled.

     TCP Client example with TLS encryption.

          (define ctx (make-tls-context))
          (define c (open-tcp-client (list address: "twitter.com"
                                           port-number: 443
                                           tls-context: ctx)))
          (display "GET / HTTP/1.1\nHost: twitter.com\n\n" c)
          (force-output c)
          (read-line c)

     TCP Server example with several options.  These are not mandatory,
     except for ‘server-mode’.

          (define ctx (make-tls-context options: '(server-mode
                                                   use-diffie-hellman
                                                   use-elliptic-curves)
                                        certificate: "server.pem"
                                        diffie-hellman-parameters: "dh_param_1024.pem"
                                        elliptic-curve: "prime256v1"))

          (define s (open-tcp-server (list local-address: "localhost"
                                           local-port-number: 1443
                                           tls-context: ctx)))
          (define p (read s))
          (display "<HTML></HTML>\n" p)
          (force-output p)

     A practical way of testing TLS options are the ‘s_server’ and
     ‘s_client’ commands of the ‘openssl’ tool.

 -- procedure: open-udp PORT-NUMBER-OR-ADDRESS-OR-SETTINGS

     This procedure opens a socket for doing network communication with
     the UDP protocol.  The default value of the ‘direction:’ setting is
     ‘input-output’, i.e.  the Scheme program can send information and
     receive information on the socket.  The sending direction can be
     closed using the ‘close-output-port’ procedure and the receiving
     direction can be closed using the ‘close-input-port’ procedure.
     The ‘close-port’ procedure closes both directions.

     The resulting port designates a UDP socket.  Each call to ‘read’
     and ‘udp-read-subu8vector’ causes the reception of a single
     datagram on the designated UDP socket, and each call to ‘write’ and
     ‘udp-write-subu8vector’ sends a single datagram.  UDP ports are a
     direct subtype of object-ports (i.e.  they are not character-ports)
     and ‘read’ and ‘write’ transfer u8vectors.  If ‘read’ is called and
     a timeout occurs before a datagram is transferred and the timeout
     thunk returns ‘#f’ (see the procedure ‘input-port-timeout-set!’)
     then the end-of-file object is returned.

     The parameter of this procedure is an IP port number (16-bit
     nonnegative exact integer), a string of the form ‘"HOST:PORT"’ or a
     list of port settings.  When the parameter is the number PORT it is
     handled as if it was the setting ‘local-port-number:’ PORT.  When
     the parameter is the string ‘"HOST:PORT"’ it is handled as if it
     was the setting ‘local-address:’ ‘"HOST:PORT"’.

     Here are the settings allowed:

        • ‘direction:’ ( ‘input’ | ‘output’ | ‘input-output’ )

          This setting controls the direction of the port.  The symbol
          ‘input’ indicates a unidirectional input-port, the symbol
          ‘output’ indicates a unidirectional output-port, and the
          symbol ‘input-output’ indicates a bidirectional port.  The
          default value of this setting is ‘input-output’.

        • ‘local-address:’ STRING-OR-IP-ADDRESS

          This setting indicates the internet address of the local
          network interface on which the socket is open and possibly the
          IP port number.  When this parameter is not specified or is
          ‘""’, the socket is open on the loopback interface (with IP
          address 127.0.0.1).  When this parameter is ‘"*"’ the socket
          is open on all network interfaces (i.e.  address INADDR_ANY).
          The parameter can be a string denoting a host name, which will
          be translated to an IP address by the ‘host-info’ procedure,
          or a 4 element u8vector which contains the 32-bit IPv4 address
          or an 8 element u16vector which contains the 128-bit IPv6
          address.  A string of the form ‘"INTF:PORT"’ is handled as if
          it was the combination of settings ‘local-address:’ ‘"INTF"’
          ‘local-port-number:’ PORT.

        • ‘local-port-number:’ 16-BIT-EXACT-INTEGER

          This setting indicates the IP port number assigned to the
          socket.  The special value 0 requests that a currently unused
          port number be assigned to the socket.  This is the default
          value for this setting.

        • ‘address:’ STRING-OR-IP-ADDRESS

          This setting indicates the initial destination internet
          address of the datagrams, and possibly the IP port number.
          When this parameter is not specified the destination is set to
          the local address if it is not all network interfaces (i.e.
          ‘"*"’ = address INADDR_ANY). When this parameter is ‘""’ or
          this parameter is not specified and the local address is all
          network interfaces, the destination is set to the loopback
          interface (with IP address 127.0.0.1).  The parameter can be a
          string denoting a host name, which will be translated to an IP
          address by the ‘host-info’ procedure, or a 4 element u8vector
          which contains the 32-bit IPv4 address or an 8 element
          u16vector which contains the 128-bit IPv6 address.  A string
          of the form ‘"HOST:PORT"’ is handled as if it was the
          combination of settings ‘address:’ ‘"HOST"’ ‘port-number:’
          PORT.

        • ‘port-number:’ 16-BIT-EXACT-INTEGER

          This setting indicates the initial destination IP port number
          of the datagrams.  It defaults to the local port number.

 -- procedure: udp-destination-set! ADDRESS PORT-NUMBER [UDP-PORT]

     This procedure sets the destination address and port-number for the
     next datagram sent on the UDP socket designated by UDP-PORT,
     obtained with a call to ‘open-udp’.  If it is not specified,
     UDP-PORT defaults to the current output-port.

 -- procedure: udp-read-u8vector [UDP-PORT]
 -- procedure: udp-write-u8vector U8VECTOR [UDP-PORT]
 -- procedure: udp-read-subu8vector U8VECTOR START END [UDP-PORT]
 -- procedure: udp-write-subu8vector U8VECTOR START END [UDP-PORT]

     These procedures receive and send datagrams on the UDP socket
     designated by UDP-PORT, obtained with a call to ‘open-udp’.  If it
     is not specified, UDP-PORT defaults to the current input-port for
     ‘udp-read-u8vector’ and ‘udp-read-subu8vector’ and to the current
     output-port for ‘udp-write-u8vector’ and ‘udp-write-subu8vector’.

     These procedures are similar in function to ‘read-subu8vector’ and
     ‘write-subu8vector’, but because they read or write a group of
     bytes at a time rather than a stream of bytes, they are distinct
     procedures with slightly different APIs.

     The procedure ‘udp-read-u8vector’ receives the next datagram and
     returns it in a fresh u8vector.  If a timeout occurs before a
     datagram is transferred and the timeout thunk returns ‘#f’ (see the
     procedure ‘input-port-timeout-set!’) then ‘#f’ is returned.

     The procedure ‘udp-write-u8vector’ sends as a datagram the u8vector
     U8VECTOR.

     For the procedures ‘udp-read-subu8vector’ and
     ‘udp-write-subu8vector’, the part of the u8vector U8VECTOR starting
     at index START and ending just before index END is used as a byte
     buffer that will be the target of ‘udp-read-subu8vector’ or the
     source of the ‘udp-write-subu8vector’.  Up to END-START bytes will
     be transferred.  The number of bytes transferred, possibly zero, is
     returned by these procedures, unless a timeout occurs (see below).
     Fewer bytes will be read by ‘udp-read-subu8vector’ if the received
     datagram’s length is less than END-START.  ‘udp-write-subu8vector’
     always transfers END-START bytes, but note that this must be less
     than 65536 bytes, and some operating systems have a lower limit
     (for example macOS limits the number of bytes to 9216 by default).
     If a timeout occurs before a datagram is transferred and the
     timeout thunk returns ‘#f’ (see the procedure
     ‘input-port-timeout-set!’) then ‘#f’ is returned by these
     procedures (this is different from the procedures
     ‘read-subu8vector’ and ‘write-subu8vector’ which return 0).

     For ‘udp-write-u8vector’ and ‘udp-write-subu8vector’ the datagram’s
     destination is the address initially supplied when ‘open-udp’ was
     called, or the latest address set when ‘udp-destination-set!’ was
     called.

     Here is an example of sending a 3 byte datagram to port 5678 of the
     loopback interface (with IP address 127.0.0.1):

          > (define p (open-udp (list address: '#u8(127 0 0 1) port-number: 5678)))
          > (write '#u8(11 22 33) p)

     An alternative approach is to use ‘udp-destination-set!’:

          > (define p (open-udp))
          > (udp-destination-set! '#u8(127 0 0 1) 5678 p)
          > (write '#u8(11 22 33) p)

     Another approach is to use ‘udp-write-subu8vector’:

          > (define p (open-udp))
          > (udp-destination-set! '#u8(127 0 0 1) 5678 p)
          > (define v '#u8(11 22 33))
          > (udp-write-subu8vector v 0 3 p)
          3

     Note that by default the internet address of the local network
     interface is the loopback interface, which is not connected to the
     internet.  To contact an external socket the address of the local
     network interface must be specified, for example ‘"*"’ will select
     all interfaces.  The following example shows how to connect to a
     Time Protocol server to get the current time:

          > (define p (open-udp (list local-address: "*" address: "time.nist.gov:37")))
          > (write '#u8() p)
          > (read p)
          #u8(222 27 158 226)

     Here is an example of receiving a 3 byte datagram on port 5678 of
     the loopback interface:

          > (define p (open-udp 5678))
          > (read p)
          #u8(11 22 33)

     An alternative approach is to use ‘udp-read-subu8vector’:

          > (define p (open-udp 5678))
          > (define v (make-u8vector 10000))
          > (udp-read-subu8vector v 0 10000 p)
          3
          > (subu8vector v 0 3)
          #u8(11 22 33)

     Note that using ‘udp-read-subu8vector’ and ‘udp-write-subu8vector’
     is typically more efficient than ‘read’ and ‘write’ because it
     avoids having to construct a new u8vector for each datagram
     transferred.

 -- procedure: udp-local-socket-info UDP-PORT
 -- procedure: udp-source-socket-info UDP-PORT

     The procedure ‘udp-local-socket-info’ returns the local socket-info
     of the UDP socket designated by UDP-PORT.

     The procedure ‘udp-source-socket-info’ returns the socket-info of
     the source of the latest datagram received on the UDP socket
     designated by UDP-PORT.  When a datagram hasn’t been received yet,
     ‘#f’ is returned.

     For example:

          > (define p (open-udp (list local-address: "*" address: "time.nist.gov:37")))
          > (udp-local-socket-info p)
          #<socket-info #2 family: INET port-number: 64716 address: #f>
          > (udp-source-socket-info p)
          #f
          > (write '#u8() p)
          > (read p)
          #u8(222 27 162 109)
          > (udp-source-socket-info p)
          #<socket-info #3 family: INET port-number: 37 address: #u8(132 163 97 4)>

14.8 Directory-ports
====================

 -- procedure: open-directory PATH-OR-SETTINGS

     This procedure opens a directory of the filesystem for reading its
     entries and returns a directory-port from which the entries can be
     enumerated.  Directory-ports are a direct subtype of object-ports
     (i.e.  they are not character-ports) and are input-ports.  Reading
     from a directory-port with the ‘read’ procedure returns the next
     file name in the directory as a string.  The end-of-file object is
     returned when all the file names have been enumerated.  Another way
     to get the list of all files in a directory is the
     ‘directory-files’ procedure which returns a list of the files in
     the directory.  The advantage of using directory-ports is that it
     allows iterating over the files in a directory in constant space,
     which is interesting when the number of files in the directory is
     not known in advance and may be large.  Note that the order in
     which the names are returned is operating-system dependent.

     The parameter of this procedure is either a string denoting a
     filesystem path to a directory or a list of port settings which
     must contain a ‘path:’ setting.  Here are the settings allowed in
     addition to the generic settings of object-ports:

        • ‘path:’ STRING

          This setting indicates the location of the directory in the
          filesystem.  There is no default value for this setting.

        • ‘ignore-hidden:’ ( ‘#f’ | ‘#t’ | ‘dot-and-dot-dot’ )

          This setting controls whether hidden-files will be returned.
          Under UNIX and macOS hidden-files are those that start with a
          period (such as ‘.’, ‘..’, and ‘.profile’).  Under Microsoft
          Windows hidden files are the ‘.’ and ‘..’ entries and the
          files whose “hidden file” attribute is set.  A setting of ‘#f’
          will enumerate all the files.  A setting of ‘#t’ will only
          enumerate the files that are not hidden.  A setting of
          ‘dot-and-dot-dot’ will enumerate all the files except for the
          ‘.’ and ‘..’ hidden files.  The default value of this setting
          is ‘#t’.

     For example:

          > (let ((p (open-directory (list path: "../examples"
                                           ignore-hidden: #f))))
              (let loop ()
                (let ((fn (read p)))
                  (if (string? fn)
                      (begin
                        (pp (path-expand fn))
                        (loop)))))
              (close-input-port p))
          "/u/feeley/examples/."
          "/u/feeley/examples/.."
          "/u/feeley/examples/complex"
          "/u/feeley/examples/README"
          "/u/feeley/examples/simple"
          > (define x (open-directory "../examples"))
          > (read-all x)
          ("complex" "README" "simple")

14.9 Vector-ports
=================

 -- procedure: open-vector [VECTOR-OR-SETTINGS]
 -- procedure: open-input-vector [VECTOR-OR-SETTINGS]
 -- procedure: open-output-vector [VECTOR-OR-SETTINGS]
 -- procedure: call-with-input-vector VECTOR-OR-SETTINGS PROC
 -- procedure: call-with-output-vector [VECTOR-OR-SETTINGS] PROC
 -- procedure: with-input-from-vector VECTOR-OR-SETTINGS THUNK
 -- procedure: with-output-to-vector [VECTOR-OR-SETTINGS] THUNK

     Vector-ports represent streams of Scheme objects.  They are a
     direct subtype of object-ports (i.e.  they are not
     character-ports).  All of these procedures create vector-ports that
     are either unidirectional or bidirectional.  The ‘direction:’
     setting will default to the value ‘input’ for the procedures
     ‘open-input-vector’, ‘call-with-input-vector’ and
     ‘with-input-from-vector’, to the value ‘output’ for the procedures
     ‘open-output-vector’, ‘call-with-output-vector’ and
     ‘with-output-to-vector’, and to the value ‘input-output’ for the
     procedure ‘open-vector’.  Bidirectional vector-ports behave like
     FIFOs: data written to the port is added to the end of the stream
     that is read.  It is only when a bidirectional vector-port’s
     output-side is closed with a call to the ‘close-output-port’
     procedure that the stream’s end is known (when the stream’s end is
     reached, reading the port returns the end-of-file object).

     The procedures ‘open-vector’, ‘open-input-vector’ and
     ‘open-output-vector’ return the port that is created.  The
     procedures ‘call-with-input-vector’ and ‘call-with-output-vector’
     create a vector port, call the procedure PROC with the port as
     single argument and then close the port.  The procedures
     ‘with-input-from-vector’ and ‘with-output-to-vector’ create a
     vector port, dynamically bind the current input-port and current
     output-port respectively to the port created for the duration of a
     call to the procedure THUNK with no argument, and then close the
     port.  The procedures ‘call-with-input-vector’ and
     ‘with-input-from-vector’ return the value returned by the
     procedures PROC and THUNK respectively.  The procedures
     ‘call-with-output-vector’ and ‘with-output-to-vector’ return the
     vector accumulated in the port (see ‘get-output-vector’).

     The VECTOR-OR-SETTINGS parameter of these procedures is either a
     vector of the elements used to initialize the stream or a list of
     port settings.  If it is not specified, the parameter of the
     ‘open-vector’, ‘open-input-vector’, ‘open-output-vector’,
     ‘with-output-to-vector’, and ‘call-with-output-vector’ procedures
     defaults to an empty list of port settings.  Here are the settings
     allowed in addition to the generic settings of object-ports:

        • ‘init:’ VECTOR

          This setting indicates the initial content of the stream.  The
          default value of this setting is an empty vector.

        • ‘permanent-close:’ ( ‘#f’ | ‘#t’ )

          This setting controls whether a call to the procedures
          ‘close-output-port’ will close the output-side of a
          bidirectional vector-port permanently or not.  A permanently
          closed bidirectional vector-port whose end-of-file has been
          reached on the input-side will return the end-of-file object
          for all subsequent calls to the ‘read’ procedure.  A
          non-permanently closed bidirectional vector-port will return
          to its opened state when its end-of-file is read.  The default
          value of this setting is ‘#t’.

     For example:

          > (define p (open-vector))
          > (write 1 p)
          > (write 2 p)
          > (write 3 p)
          > (force-output p)
          > (read p)
          1
          > (read p)
          2
          > (close-output-port p)
          > (read p)
          3
          > (read p)
          #!eof
          > (with-output-to-vector (lambda () (write 1) (write 2)))
          #(1 2)

 -- procedure: open-vector-pipe [VECTOR-OR-SETTINGS1
          [VECTOR-OR-SETTINGS2]]

     The procedure ‘open-vector-pipe’ creates two vector-ports and
     returns these two ports.  The two ports are interrelated as
     follows: the first port’s output-side is connected to the second
     port’s input-side and the first port’s input-side is connected to
     the second port’s output-side.  The value VECTOR-OR-SETTINGS1 is
     used to setup the first vector-port and VECTOR-OR-SETTINGS2 is used
     to setup the second vector-port.  The same settings as for
     ‘open-vector’ are allowed.  The default ‘direction:’ setting is
     ‘input-output’ (i.e.  a bidirectional port is created).  If it is
     not specified VECTOR-OR-SETTINGS1 defaults to the empty list.  If
     it is not specified VECTOR-OR-SETTINGS2 defaults to
     VECTOR-OR-SETTINGS1 but with the ‘init:’ setting set to the empty
     vector and with the input and output settings exchanged (e.g.  if
     the first port is an input-port then the second port is an
     output-port, if the first port’s input-side is non-buffered then
     the second port’s output-side is non-buffered).

     For example:

          > (define (server op)
              (receive (c s) (open-vector-pipe)  ; client-side and server-side ports
                (thread-start!
                  (make-thread
                    (lambda ()
                      (let loop ()
                        (let ((request (read s)))
                          (if (not (eof-object? request))
                              (begin
                                (write (op request) s)
                                (newline s)
                                (force-output s)
                                (loop))))))))
                c))
          > (define a (server (lambda (x) (expt 2 x))))
          > (define b (server (lambda (x) (expt 10 x))))
          > (write 100 a)
          > (write 30 b)
          > (read a)
          1267650600228229401496703205376
          > (read b)
          1000000000000000000000000000000

 -- procedure: get-output-vector VECTOR-PORT

     The procedure ‘get-output-vector’ takes an output vector-port or a
     bidirectional vector-port as parameter and removes all the objects
     currently on the output-side, returning them in a vector.  The port
     remains open and subsequent output to the port and calls to the
     procedure ‘get-output-vector’ are possible.

     For example:

          > (define p (open-vector '#(1 2 3)))
          > (write 4 p)
          > (get-output-vector p)
          #(1 2 3 4)
          > (write 5 p)
          > (write 6 p)
          > (get-output-vector p)
          #(5 6)

14.10 String-ports
==================

 -- procedure: open-string [STRING-OR-SETTINGS]
 -- procedure: open-input-string [STRING-OR-SETTINGS]
 -- procedure: open-output-string [STRING-OR-SETTINGS]
 -- procedure: call-with-input-string STRING-OR-SETTINGS PROC
 -- procedure: call-with-output-string [STRING-OR-SETTINGS] PROC
 -- procedure: with-input-from-string STRING-OR-SETTINGS THUNK
 -- procedure: with-output-to-string [STRING-OR-SETTINGS] THUNK
 -- procedure: open-string-pipe [STRING-OR-SETTINGS1
          [STRING-OR-SETTINGS2]]
 -- procedure: get-output-string STRING-PORT

     String-ports represent streams of characters.  They are a direct
     subtype of character-ports.  These procedures are the string-port
     analog of the procedures specified in the vector-ports section.
     Note that these procedures are a superset of the procedures
     specified in the “Basic String Ports SRFI” (SRFI 6).

     For example:

          > (define p (open-string))
          > (write 1 p)
          > (write 2 p)
          > (write 3 p)
          > (force-output p)
          > (read-char p)
          #\1
          > (read-char p)
          #\2
          > (close-output-port p)
          > (read-char p)
          #\3
          > (read-char p)
          #!eof
          > (with-output-to-string (lambda () (write 1) (write 2)))
          "12"

 -- procedure: object->string OBJ [N]

     This procedure converts the object OBJ to its external
     representation and returns it in a string.  The parameter N
     specifies the maximal width of the resulting string.  If the
     external representation is wider than N, the resulting string will
     be truncated to N characters and the last 3 characters will be set
     to periods.  Note that the current readtable is used.

     For example:

          > (object->string (expt 2 100))
          "1267650600228229401496703205376"
          > (object->string (expt 2 100) 30)
          "126765060022822940149670320..."
          > (object->string (cons car cdr))
          "(#<procedure #2 car> . #<procedure #3 cdr>)"

14.11 U8vector-ports
====================

 -- procedure: open-u8vector [U8VECTOR-OR-SETTINGS]
 -- procedure: open-input-u8vector [U8VECTOR-OR-SETTINGS]
 -- procedure: open-output-u8vector [U8VECTOR-OR-SETTINGS]
 -- procedure: call-with-input-u8vector U8VECTOR-OR-SETTINGS PROC
 -- procedure: call-with-output-u8vector [U8VECTOR-OR-SETTINGS] PROC
 -- procedure: with-input-from-u8vector U8VECTOR-OR-SETTINGS THUNK
 -- procedure: with-output-to-u8vector [U8VECTOR-OR-SETTINGS] THUNK
 -- procedure: open-u8vector-pipe [U8VECTOR-OR-SETTINGS1
          [U8VECTOR-OR-SETTINGS2]]
 -- procedure: get-output-u8vector U8VECTOR-PORT

     U8vector-ports represent streams of bytes.  They are a direct
     subtype of byte-ports.  These procedures are the u8vector-port
     analog of the procedures specified in the vector-ports section.

     For example:

          > (define p (open-u8vector))
          > (write 1 p)
          > (write 2 p)
          > (write 3 p)
          > (force-output p)
          > (read-u8 p)
          49
          > (read-u8 p)
          50
          > (close-output-port p)
          > (read-u8 p)
          51
          > (read-u8 p)
          #!eof
          > (with-output-to-u8vector (lambda () (write 1) (write 2)))
          #u8(49 50)

14.12 Other procedures related to I/O
=====================================

 -- procedure: current-input-port [NEW-VALUE]
 -- procedure: current-output-port [NEW-VALUE]
 -- procedure: current-error-port [NEW-VALUE]
 -- procedure: current-readtable [NEW-VALUE]

     These procedures are parameter objects which represent
     respectively: the current input-port, the current output-port, the
     current error-port, and the current readtable.

 -- procedure: print [port: PORT] OBJ...
 -- procedure: println [port: PORT] OBJ...

     The ‘print’ procedure writes a representation of each OBJ, from
     left to right, to PORT.  When a compound object is encountered
     (pair, list, vector, box) the elements of that object are
     recursively written without the surrounding tokens (parentheses,
     spaces, dots, etc).  Strings, symbols, keywords and characters are
     written like the ‘display’ procedure.  If there are more than one
     OBJ, the first OBJ must not be a keyword object.  If it is not
     specified, PORT defaults to the current output-port.  The procedure
     ‘print’ returns an unspecified value.

     The ‘println’ procedure does the same thing as the ‘print’
     procedure and then writes an end of line to PORT.

     For example:

          > (println "2*2 is " (* 2 2) " and 2+2 is " (+ 2 2))
          2*2 is 4 and 2+2 is 4
          > (define x (list "<i>" (list "<tt>" 123 "</tt>") "</i>"))
          > (println x)
          <i><tt>123</tt></i>
          > (define p (open-output-string))
          > (print port: p 1 #\2 "345")
          > (get-output-string p)
          "12345"

 -- procedure: read-file-string PATH-OR-SETTINGS
 -- procedure: read-file-string-list PATH-OR-SETTINGS
 -- procedure: read-file-u8vector PATH-OR-SETTINGS

     These procedures open the file designated by PATH-OR-SETTINGS and
     read the whole content.  They respectively return a string (the
     characters in the file), a list of strings (the lines in the file),
     and a u8vector (the bytes in the file).

     The PATH-OR-SETTINGS parameter is either a string denoting a
     filesystem path or a list of port settings which must contain a
     ‘path:’ setting.  The same settings as ‘open-input-file’ are
     allowed, and the same default settings are used.  The default value
     of the ‘char-encoding:’ setting (which is relevant for
     ‘read-file-string’ and ‘read-file-string-list’) depends on how the
     runtime system was configured but typically UTF-8 is used.  The
     default can be overridden through various runtime options (*note
     Runtime options::), such as ‘-:file-settings=...’ and
     ‘-:io-settings=...’.

     For example:

          > (with-output-to-file
              "test"
              (lambda () (for-each pretty-print (map square (iota 5)))))
          > (read-file-string "test")
          "0\n1\n4\n9\n16\n"
          > (read-file-string-list "test")
          ("0" "1" "4" "9" "16")
          > (read-file-u8vector "test")
          #u8(48 10 49 10 52 10 57 10 49 54 10)
          > (utf8->string (read-file-u8vector "test"))
          "0\n1\n4\n9\n16\n"

 -- procedure: write-file-string PATH-OR-SETTINGS STRING
 -- procedure: write-file-string-list PATH-OR-SETTINGS STRING-LIST
 -- procedure: write-file-u8vector PATH-OR-SETTINGS U8VECTOR

     These procedures open the file designated by PATH-OR-SETTINGS and
     write the data specified by the second parameter.  They
     respectively write a string (the characters to write to the file),
     a list of strings (the lines to write to the file), and a u8vector
     (the bytes to write to the file).  These procedures return the void
     object.

     The PATH-OR-SETTINGS parameter is either a string denoting a
     filesystem path or a list of port settings which must contain a
     ‘path:’ setting.  The same settings as ‘open-output-file’ are
     allowed, and the same default settings are used.  The default value
     of the ‘char-encoding:’ setting (which is relevant for
     ‘write-file-string’ and ‘write-file-string-list’) depends on how
     the runtime system was configured but typically UTF-8 is used.  The
     default can be overridden through various runtime options (*note
     Runtime options::), such as ‘-:file-settings=...’ and
     ‘-:io-settings=...’.

     For example:

          > (write-file-string "test" "1\n2\n3\n")
          > (read-file-u8vector "test")
          #u8(49 10 50 10 51 10)
          > (write-file-string-list "test" (list "1" "2" "3"))
          > (read-file-u8vector "test")
          #u8(49 10 50 10 51 10)
          > (write-file-u8vector "test" (u8vector 97 98 99))
          > (read-file-string "test")
          "abc"

15 Lexical syntax and readtables
********************************

15.1 Readtables
===============

Readtables control the external textual representation of Scheme
objects, that is the encoding of Scheme objects using characters.
Readtables affect the behavior of the reader (i.e.  the ‘read’ procedure
and the parser used by the ‘load’ procedure and the interpreter and
compiler) and the printer (i.e.  the procedures ‘write’, ‘display’,
‘print’, ‘println’, ‘pretty-print’, and ‘pp’, and the procedure used by
the REPL to print results).  To preserve write/read invariance the
printer and reader must be using compatible readtables.  For example a
symbol which contains upper case letters will be printed with special
escapes if the readtable indicates that the reader is case-insensitive.

   Readtables are immutable records whose fields specify various textual
representation aspects.  There are accessor procedures to retrieve the
content of specific fields.  There are also functional update procedures
that create a copy of a readtable, with a specific field set to a new
value.

 -- procedure: readtable? OBJ

     This procedure returns ‘#t’ when OBJ is a readtable and ‘#f’
     otherwise.

     For example:

          > (readtable? (current-readtable))
          #t
          > (readtable? 123)
          #f

 -- procedure: readtable-case-conversion? READTABLE
 -- procedure: readtable-case-conversion?-set READTABLE NEW-VALUE

     The procedure ‘readtable-case-conversion?’ returns the content of
     the ‘case-conversion?’ field of READTABLE.  When the content of
     this field is ‘#f’, the reader preserves the case of symbols,
     keyword and named characters that are read (i.e.  ‘Ice’ and ‘ice’
     are distinct symbols).  When the content of this field is the
     symbol ‘upcase’, the reader converts to uppercase (i.e.  ‘Ice’ is
     read as the symbol ‘(string->symbol "ICE")’).  Otherwise the reader
     converts using ‘string-foldcase’, which for many letters converts
     them to lowercase (i.e.  ‘Ice’ is read as the symbol
     ‘(string->symbol "ice")’).

     The procedure ‘readtable-case-conversion?-set’ returns a copy of
     READTABLE where only the ‘case-conversion?’ field has been changed
     to NEW-VALUE.

     For example:

          > (output-port-readtable-set!
              (repl-output-port)
              (readtable-case-conversion?-set
                (output-port-readtable (repl-output-port))
                #f))
          > (input-port-readtable-set!
              (repl-input-port)
              (readtable-case-conversion?-set
                (input-port-readtable (repl-input-port))
                #f))
          > 'Ice
          Ice
          > (input-port-readtable-set!
              (repl-input-port)
              (readtable-case-conversion?-set
                (input-port-readtable (repl-input-port))
                #t))
          > 'Ice
          ice
          > (input-port-readtable-set!
              (repl-input-port)
              (readtable-case-conversion?-set
                (input-port-readtable (repl-input-port))
                'upcase))
          > 'Ice
          ICE

 -- procedure: readtable-keywords-allowed? READTABLE
 -- procedure: readtable-keywords-allowed?-set READTABLE NEW-VALUE

     The procedure ‘readtable-keywords-allowed?’ returns the content of
     the ‘keywords-allowed?’ field of READTABLE.  When the content of
     this field is ‘#f’, the reader does not recognize keyword objects
     (i.e.  ‘:foo’ and ‘foo:’ are read as the symbols ‘(string->symbol
     ":foo")’ and ‘(string->symbol "foo:")’ respectively).  When the
     content of this field is the symbol ‘prefix’, the reader recognizes
     keyword objects that start with a colon, as in Common Lisp (i.e.
     ‘:foo’ is read as the keyword ‘(string->keyword "foo")’).
     Otherwise the reader recognizes keyword objects that end with a
     colon, as in DSSSL (i.e.  ‘foo:’ is read as the symbol
     ‘(string->symbol "foo")’).

     The procedure ‘readtable-keywords-allowed?-set’ returns a copy of
     READTABLE where only the ‘keywords-allowed?’ field has been changed
     to NEW-VALUE.

     For example:

          > (input-port-readtable-set!
              (repl-input-port)
              (readtable-keywords-allowed?-set
                (input-port-readtable (repl-input-port))
                #f))
          > (map keyword? '(foo :foo foo:))
          (#f #f #f)
          > (input-port-readtable-set!
              (repl-input-port)
              (readtable-keywords-allowed?-set
                (input-port-readtable (repl-input-port))
                #t))
          > (map keyword? '(foo :foo foo:))
          (#f #f #t)
          > (input-port-readtable-set!
              (repl-input-port)
              (readtable-keywords-allowed?-set
                (input-port-readtable (repl-input-port))
                'prefix))
          > (map keyword? '(foo :foo foo:))
          (#f #t #f)

 -- procedure: readtable-sharing-allowed? READTABLE
 -- procedure: readtable-sharing-allowed?-set READTABLE NEW-VALUE

     The procedure ‘readtable-sharing-allowed?’ returns the content of
     the ‘sharing-allowed?’ field of READTABLE.  The reader recognizes
     the ‘#N#’ and ‘#N=DATUM’ notation for shared and circular
     structures and the printer uses this notation depending on the
     content of the ‘sharing-allowed?’ field and the printing primitive
     used.  When the content of the ‘sharing-allowed?’ field is the
     symbol ‘default’, the procedure ‘write-shared’ will use this
     notation for shared and circular structures, the procedures
     ‘write’, ‘display’, ‘pp’, ‘pretty-print’, ‘print’, and ‘println’
     will use this notation for circular structures, and the procedure
     ‘write-simple’ does not use this notation.  When the content of the
     ‘sharing-allowed?’ field is ‘#f’, the printing procedures will not
     use this notation.  When the content of the ‘sharing-allowed?’
     field is ‘#t’, the printing procedures will use this notation for
     shared and circular structures.  Finally, when the content of the
     ‘sharing-allowed?’ field is the symbol ‘serialize’, the printer
     uses a special external representation that the reader understands
     and that extends write/read invariance to the following types:
     records, procedures and continuations.  Note that an object can be
     serialized and deserialized if and only if all of its components
     are serializable.

     The procedure ‘readtable-sharing-allowed?-set’ returns a copy of
     READTABLE where only the ‘sharing-allowed?’ field has been changed
     to NEW-VALUE.

     Here is a simple example:

          > (define (wr obj allow?)
              (call-with-output-string
                (lambda (p)
                  (output-port-readtable-set!
                    p
                    (readtable-sharing-allowed?-set
                      (output-port-readtable p)
                      allow?))
                  (write obj p))))
          > (define (rd str allow?)
              (call-with-input-string
                str
                (lambda (p)
                  (input-port-readtable-set!
                    p
                    (readtable-sharing-allowed?-set
                      (input-port-readtable p)
                      allow?))
                  (read p))))
          > (define x (list 1 2 3))
          > (set-car! (cdr x) (cddr x))
          > (wr x #f)
          "(1 (3) 3)"
          > (wr x #t)
          "(1 #0=(3) . #0#)"
          > (define y (rd (wr x #t) #t))
          > y
          (1 (3) 3)
          > (eq? (cadr y) (cddr y))
          #t
          > (define f #f)
          > (let ((free (expt 2 10)))
            (set! f (lambda (x) (+ x free))))
          > (define s (wr f 'serialize))
          > (string-length s)
          4196
          > (define g (rd s 'serialize))
          > (eq? f g)
          #f
          > (g 4)
          1028

     Continuations are tricky to serialize because they contain a
     dynamic environment and this dynamic environment may contain
     non-serializable objects, in particular ports attached to
     operating-system streams such as files, the console or standard
     input/output.  Indeed, all dynamic environments contain a binding
     for the ‘current-input-port’ and ‘current-output-port’.  Moreover,
     any thread that has started a REPL has a continuation which refers
     to the “repl-context” object in its dynamic environment.  A
     repl-context object contains the interaction channel, which is
     typically connected to a non-serializable port, such as the
     console.  Another problem is that the ‘parameterize’ form saves the
     old binding of the parameter in the continuation, so it is not
     possible to eliminate the references to these ports in the
     continuation by using the ‘parameterize’ form alone.

     Serialization of continuations can be achieved dependably by taking
     advantage of string-ports, which are serializable objects (unless
     there is a blocked thread), and the following features of threads:
     they inherit the dynamic environment of the parent thread and they
     start with an initial continuation that contains only serializable
     objects.  So a thread created in a dynamic environment where
     ‘current-input-port’ and ‘current-output-port’ are bound to a dummy
     string-port has a serializable continuation.

     Here is an example where continuations are serialized:

          > (define (wr obj)
              (call-with-output-string
               (lambda (p)
                 (output-port-readtable-set!
                  p
                  (readtable-sharing-allowed?-set
                   (output-port-readtable p)
                   'serialize))
                 (write obj p))))
          > (define (rd str)
              (call-with-input-string
               str
               (lambda (p)
                 (input-port-readtable-set!
                  p
                  (readtable-sharing-allowed?-set
                   (input-port-readtable p)
                   'serialize))
                 (read p))))
          > (define fifo (open-vector))
          > (define (suspend-and-die!)
              (call-with-current-continuation
               (lambda (k)
                 (write (wr k) fifo)
                 (newline fifo)
                 (force-output fifo)
                 (thread-terminate! (current-thread)))))
          > (let ((dummy-port (open-string)))
              (parameterize ((current-input-port dummy-port)
                             (current-output-port dummy-port))
                (thread-start!
                 (make-thread
                  (lambda ()
                    (* 100
                       (suspend-and-die!)))))))
          #<thread #2>
          > (define s (read fifo))
          > (thread-join!
              (thread-start!
                (make-thread
                  (lambda ()
                    ((rd s) 111)))))
          11100
          > (thread-join!
              (thread-start!
                (make-thread
                  (lambda ()
                    ((rd s) 222)))))
          22200
          > (string-length s)
          13114

 -- procedure: readtable-eval-allowed? READTABLE
 -- procedure: readtable-eval-allowed?-set READTABLE NEW-VALUE

     The procedure ‘readtable-eval-allowed?’ returns the content of the
     ‘eval-allowed?’ field of READTABLE.  The reader recognizes the
     ‘#.EXPRESSION’ notation for read-time evaluation if and only if the
     content of the ‘eval-allowed?’ field is not ‘#f’.

     The procedure ‘readtable-eval-allowed?-set’ returns a copy of
     READTABLE where only the ‘eval-allowed?’ field has been changed to
     NEW-VALUE.

     For example:

          > (input-port-readtable-set!
              (repl-input-port)
              (readtable-eval-allowed?-set
                (input-port-readtable (repl-input-port))
                #t))
          > '(5 plus 7 is #.(+ 5 7))
          (5 plus 7 is 12)
          > '(buf = #.(make-u8vector 25))
          (buf = #u8(0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0))

 -- procedure: readtable-write-cdr-read-macros? READTABLE
 -- procedure: readtable-write-cdr-read-macros?-set READTABLE NEW-VALUE
 -- procedure: readtable-write-extended-read-macros? READTABLE
 -- procedure: readtable-write-extended-read-macros?-set READTABLE
          NEW-VALUE

     The procedure ‘readtable-write-cdr-read-macros?’ returns the
     content of the ‘write-cdr-read-macros?’ field of READTABLE.  The
     procedure ‘readtable-write-extended-read-macros?’ returns the
     content of the ‘write-extended-read-macros?’ field of READTABLE.

     At all times the printer uses read-macros in its output for datums
     of the form ‘(quote DATUM)’, ‘(quasiquote DATUM)’, ‘(unquote
     DATUM)’, and ‘(unquote-splicing DATUM)’.  That is the following
     read-macro notations will be used respectively: ‘'DATUM’, ‘`DATUM’,
     ‘,DATUM’, and ‘,@DATUM’.  Moreover, normally the read-macros will
     not be used when the form appears in the cdr of a list, for example
     ‘(foo quote bar)’, ‘(foo . (quote bar))’ and ‘(foo . 'bar)’ will
     all be printed as ‘(foo quote bar)’.

     When the content of the ‘write-cdr-read-macros?’ field is not ‘#f’,
     the printer will use read-macros when the forms appear in the cdr
     of a list.  For example ‘(foo quote bar)’ will be printed as ‘(foo
     . 'bar)’.  When the content of the ‘write-extended-read-macros?’
     field is not ‘#f’, the printer will also use extended read-macros,
     for example ‘#'DATUM’ in place of ‘(syntax DATUM)’.

     The procedure ‘readtable-write-cdr-read-macros?-set’ returns a copy
     of READTABLE where only the ‘write-cdr-read-macros?’ field has been
     changed to NEW-VALUE.  The procedure
     ‘readtable-write-extended-read-macros?-set’ returns a copy of
     READTABLE where only the ‘write-extended-read-macros?’ field has
     been changed to NEW-VALUE.

     For example:

          > (output-port-readtable-set!
              (repl-output-port)
              (readtable-write-extended-read-macros?-set
                (output-port-readtable (repl-output-port))
                #t))
          > '(foo (syntax bar))
          (foo #'bar)
          > '(foo syntax bar)
          (foo syntax bar)
          > (output-port-readtable-set!
              (repl-output-port)
              (readtable-write-cdr-read-macros?-set
                (output-port-readtable (repl-output-port))
                #t))
          > '(foo syntax bar)
          (foo . #'bar)

 -- procedure: readtable-max-write-level READTABLE
 -- procedure: readtable-max-write-level-set READTABLE NEW-VALUE

     The procedure ‘readtable-max-write-level’ returns the content of
     the ‘max-write-level’ field of READTABLE.  The printer will display
     an ellipsis for the elements of lists and vectors that are nested
     deeper than this level.

     The procedure ‘readtable-max-write-level-set’ returns a copy of
     READTABLE where only the ‘max-write-level’ field has been changed
     to NEW-VALUE, which must be an nonnegative fixnum.

     For example:

          > (define (wr obj n)
              (call-with-output-string
                (lambda (p)
                  (output-port-readtable-set!
                    p
                    (readtable-max-write-level-set
                      (output-port-readtable p)
                      n))
                  (write obj p))))
          > (wr '(a #(b (c c) #u8(9 9 9) b) a) 3)
          "(a #(b (c c) #u8(9 9 9) b) a)"
          > (wr '(a #(b (c c) #u8(9 9 9) b) a) 2)
          "(a #(b (...) #u8(...) b) a)"
          > (wr '(a #(b (c c) #u8(9 9 9) b) a) 1)
          "(a #(...) a)"
          > (wr '(a #(b (c c) #u8(9 9 9) b) a) 0)
          "(...)"
          > (wr 'hello 0)
          "hello"

 -- procedure: readtable-max-write-length READTABLE
 -- procedure: readtable-max-write-length-set READTABLE NEW-VALUE
     The procedure ‘readtable-max-write-length’ returns the content of
     the ‘max-write-length’ field of READTABLE.  The printer will
     display an ellipsis for the elements of lists and vectors that are
     at an index beyond that length.

     The procedure ‘readtable-max-write-length-set’ returns a copy of
     READTABLE where only the ‘max-write-length’ field has been changed
     to NEW-VALUE, which must be an nonnegative fixnum.

     For example:

          > (define (wr obj n)
              (call-with-output-string
                (lambda (p)
                  (output-port-readtable-set!
                    p
                    (readtable-max-write-length-set
                      (output-port-readtable p)
                      n))
                  (write obj p))))
          > (wr '(a #(b (c c) #u8(9 9 9) b) . a) 4)
          "(a #(b (c c) #u8(9 9 9) b) . a)"
          > (wr '(a #(b (c c) #u8(9 9 9) b) . a) 3)
          "(a #(b (c c) #u8(9 9 9) ...) . a)"
          > (wr '(a #(b (c c) #u8(9 9 9) b) . a) 2)
          "(a #(b (c c) ...) . a)"
          > (wr '(a #(b (c c) #u8(9 9 9) b) . a) 1)
          "(a ...)"
          > (wr '(a #(b (c c) #u8(9 9 9) b) . a) 0)
          "(...)"

 -- procedure: readtable-max-unescaped-char READTABLE
 -- procedure: readtable-max-unescaped-char-set READTABLE NEW-VALUE

     The procedure ‘readtable-max-unescaped-char’ returns the content of
     the ‘max-unescaped-char’ field of READTABLE.  The printer will
     display using an escape sequence any character within symbols,
     strings and character objects greater than ‘max-unescaped-char’.
     When ‘max-unescaped-char’ is ‘#f’, the default value, the printer
     will take into account the output port and use an escape sequence
     for any character that isn’t supported by the port’s character
     encoding.

     The procedure ‘readtable-max-unescaped-char-set’ returns a copy of
     READTABLE where only the ‘max-unescaped-char’ field has been
     changed to NEW-VALUE, which must be a character or ‘#f’.

     For example:

          > (define rt (output-port-readtable (repl-output-port)))
          > (readtable-max-unescaped-char rt)
          #\delete
          > (string (integer->char 233))
          "\351"
          > (define (f c)
              (with-output-to-string
               (list readtable: (readtable-max-unescaped-char-set rt c))
               (lambda () (write (string (integer->char 233))))))
          > (f #\delete)
          "\"\\351\""
          > (string-length (f #\delete))
          6
          > (f #\U0010ffff)
          "\"\351\""
          > (string-length (f #\U0010ffff))
          3
          > (output-port-readtable-set!
             (repl-output-port)
             (readtable-max-unescaped-char-set rt #\U0010ffff))
          > (string (integer->char 233))
          "é"

 -- procedure: readtable-start-syntax READTABLE
 -- procedure: readtable-start-syntax-set READTABLE NEW-VALUE

     The procedure ‘readtable-start-syntax’ returns the content of the
     ‘start-syntax’ field of READTABLE.  The reader uses this field to
     determine in which syntax to start parsing the input.  When the
     content of this field is the symbol ‘six’, the reader starts in the
     infix syntax.  Otherwise the reader starts in the prefix syntax.

     The procedure ‘readtable-start-syntax-set’ returns a copy of
     READTABLE where only the ‘start-syntax’ field has been changed to
     NEW-VALUE.

     For example:

          > (+ 2 3)
          5
          > (input-port-readtable-set!
             (repl-input-port)
             (readtable-start-syntax-set
               (input-port-readtable (repl-input-port))
               'six))
          > 2+3;
          5
          > exit();

15.2 Boolean syntax
===================

Booleans are required to be followed by a delimiter (i.e.  ‘#f64()’ is
not the boolean ‘#f’ followed by the number ‘64’ and the empty list).

15.3 Character syntax
=====================

Characters are required to be followed by a delimiter (i.e.
‘#\spaceballs’ is not the character ‘#\space’ followed by the symbol
‘balls’).  The lexical syntax of characters is extended to allow the
following:

‘#\nul’
     Unicode character 0
‘#\alarm’
     Unicode character 7
‘#\backspace’
     Unicode character 8
‘#\tab’
     Unicode character 9
‘#\newline’
     Unicode character 10 (newline character)
‘#\linefeed’
     Unicode character 10
‘#\vtab’
     Unicode character 11
‘#\page’
     Unicode character 12
‘#\return’
     Unicode character 13
‘#\esc’
     Unicode character 27
‘#\space’
     Unicode character 32 (space character)
‘#\delete’
     Unicode character 127
‘#\xHH’
     character encoded in hexadecimal (>= 1 hexadecimal digit)
‘#\uHHHH’
     character encoded in hexadecimal (exactly 4 hexadecimal digits)
‘#\UHHHHHHHH’
     character encoded in hexadecimal (exactly 8 hexadecimal digits)

15.4 String syntax
==================

The lexical syntax of quoted strings is extended to allow the following
escape codes:

‘\a’
     Unicode character 7
‘\b’
     Unicode character 8
‘\t’
     Unicode character 9
‘\n’
     Unicode character 10 (newline character)
‘\v’
     Unicode character 11
‘\f’
     Unicode character 12
‘\r’
     Unicode character 13
‘\"’
     ‘"’
‘\\’
     ‘\’
‘\|’
     ‘|’
‘\?’
     ‘?’
‘\OOO’
     character encoded in octal (1 to 3 octal digits, first digit must
     be less than 4 when there are 3 octal digits)
‘\xHH’
     character encoded in hexadecimal (>= 1 hexadecimal digit)
‘\uHHHH’
     character encoded in hexadecimal (exactly 4 hexadecimal digits)
‘\UHHHHHHHH’
     character encoded in hexadecimal (exactly 8 hexadecimal digits)
‘\<space>’
     Unicode character 32 (space character)
‘\<newline><whitespace-except-newline>*’
     This sequence expands to nothing (it is useful for splitting a long
     string literal on multiple lines while respecting proper
     indentation of the source code)

   Gambit also supports a “here string” syntax that is similar to shell
“here documents”.  For example:

     > (pretty-print #<<THE-END
     hello
     world
     THE-END
     )
     "hello\nworld"

   The here string starts with the sequence ‘#<<’.  The part of the line
after the ‘#<<’ up to and including the newline character is the key.
The first line afterward that matches the key marks the end of the here
string.  The string contains all the characters between the start key
and the end key, with the exception of the newline character before the
end key.

15.5 Symbol syntax
==================

The lexical syntax of symbols is extended to allow a leading and
trailing vertical bar (e.g.  ‘|a\|b"c:|’).  The symbol’s name
corresponds verbatim to the characters between the vertical bars except
for escaped characters.  The same escape sequences as for strings are
permitted except that ‘"’ does not need to be escaped and ‘|’ needs to
be escaped.

   For example:

     > (symbol->string '|a\|b"c:|)
     "a|b\"c:"

15.6 Keyword syntax
===================

The lexical syntax of keywords is like symbols, but with a colon at the
end (note that this can be changed to a leading colon by setting the
‘keywords-allowed?’ field of the readtable to the symbol ‘prefix’).  A
colon by itself is not a keyword, it is a symbol.  Vertical bars can be
used like symbols but the colon must be outside the vertical bars.  Note
that the string returned by the ‘keyword->string’ procedure does not
include the colon.

   For example:

     > (keyword->string 'foo:)
     "foo"
     > (map keyword? '(|ab()cd:| |ab()cd|: : ||:))
     (#f #t #f #t)

15.7 Box syntax
===============

The lexical syntax of boxes is ‘#&OBJ’ where OBJ is the content of the
box.

   For example:

     > (list '#&"hello" '#&123)
     (#&"hello" #&123)
     > (box (box (+ 10 20)))
     #&#&30

15.8 Number syntax
==================

The lexical syntax of the special inexact real numbers is as follows:

‘+inf.0’
     positive infinity
‘-inf.0’
     negative infinity
‘+nan.0’
     “not a number”
‘-0.’
     negative zero (‘0.’ is the positive zero)

15.9 Homogeneous vector syntax
==============================

Homogeneous vectors are vectors containing raw numbers of the same type
(signed or unsigned exact integers or inexact reals).  There are 10
types of homogeneous vectors: ‘s8vector’ (vector of 8 bit signed
integers), ‘u8vector’ (vector of 8 bit unsigned integers), ‘s16vector’
(vector of 16 bit signed integers), ‘u16vector’ (vector of 16 bit
unsigned integers), ‘s32vector’ (vector of 32 bit signed integers),
‘u32vector’ (vector of 32 bit unsigned integers), ‘s64vector’ (vector of
64 bit signed integers), ‘u64vector’ (vector of 64 bit unsigned
integers), ‘f32vector’ (vector of 32 bit floating point numbers), and
‘f64vector’ (vector of 64 bit floating point numbers).

   The external representation of homogeneous vectors is similar to
normal vectors but with the ‘#(’ prefix replaced respectively with
‘#s8(’, ‘#u8(’, ‘#s16(’, ‘#u16(’, ‘#s32(’, ‘#u32(’, ‘#s64(’, ‘#u64(’,
‘#f32(’, and ‘#f64(’.

   The elements of the integer homogeneous vectors must be exact
integers fitting in the given precision.  The elements of the floating
point homogeneous vectors must be inexact reals.

15.10 Special ‘#!’ syntax
=========================

The lexical syntax of the special ‘#!’ objects is as follows:

‘#!eof’
     end-of-file object
‘#!void’
     void object
‘#!optional’
     optional object
‘#!rest’
     rest object
‘#!key’
     key object

15.11 Multiline comment syntax
==============================

Multiline comments are delimited by the tokens ‘#|’ and ‘|#’.  These
comments can be nested.

15.12 Scheme infix syntax extension
===================================

The reader supports an infix syntax extension which is called SIX
(Scheme Infix eXtension).  This extension is both supported by the
‘read’ procedure and in program source code.

   The backslash character is a delimiter that marks the beginning of a
single datum expressed in the infix syntax (the details are given
below).  One way to think about it is that the backslash character
escapes the prefix syntax temporarily to use the infix syntax.  For
example a three element list could be written as ‘(X \Y Z)’, the
elements X and Z are expressed using the normal prefix syntax and Y is
expressed using the infix syntax.

   When the reader encounters an infix datum, it constructs a syntax
tree for that particular datum.  Each node of this tree is represented
with a list whose first element is a symbol indicating the type of node.
For example, ‘(six.identifier abc)’ is the representation of the infix
identifier ‘abc’ and ‘(six.index (six.identifier abc) (six.identifier
i))’ is the representation of the infix datum ‘abc[i];’.  The reader
will return this representation wrapped with a ‘(six.infix ...)’ form.

15.12.1 SIX grammar
-------------------

The SIX grammar is given below.  On the left hand side are the
production rules.  On the right hand side is the datum that is
constructed by the reader.  The notation $I denotes the datum that is
constructed by the reader for the Ith part of the production rule.

   In this grammar most statements end with a semicolon.  When the
<infix datum> is immediately following the backslash character that
indicates the start of an infix datum, the ending semicolon is optional
(a semicolon is automatically inserted when the datum could be complete
and a whitespace or inappropriate character is encountered such as a
closing parenthesis).  For example ‘(f \2*n (list) \5)’ is equivalent to
‘(f \2*n; (list) \5;)’.

<infix datum> ::=
    <stat>                                  $1
                                            
<stat> ::=
    <if stat>                               $1
  | <for stat>                              $1
  | <while stat>                            $1
  | <do stat>                               $1
  | <switch stat>                           $1
  | <case stat>                             $1
  | <break stat>                            $1
  | <continue stat>                         $1
  | <label stat>                            $1
  | <goto stat>                             $1
  | <return stat>                           $1
  | <import stat>                           $1
  | <from stat>                             $1
  | <expression stat>                       $1
  | <procedure definition>                  $1
  | <variable definition> ‘;’               $1
  | <clause stat>                           $1
  | <compound stat>                         $1
  | ‘;’                                     ‘(six.compound)’
                                            
<if stat> ::=
    ‘if’ ‘(’ <pexpr> ‘)’ <stat>             ‘(six.if $3 $5)’
  | ‘if’ ‘(’ <pexpr> ‘)’ <stat> ‘else’      ‘(six.if $3 $5 $7)’
<stat>                                      
<for stat> ::=
    ‘for’ ‘(’ <stat> <oexpr> ‘;’ <oexpr>    ‘(six.for $3 $4 $6 $8)’
‘)’ <stat>                                  
<while stat> ::=
    ‘while’ ‘(’ <pexpr> ‘)’ <stat>          ‘(six.while $3 $5)’
                                            
<do stat> ::=
    ‘do’ <stat> ‘while’ ‘(’ <pexpr> ‘)’     ‘(six.do-while $2 $5)’
‘;’                                         
<switch stat> ::=
    ‘switch’ ‘(’ <pexpr> ‘)’ <stat>         ‘(six.switch $3 $5)’
                                            
<case stat> ::=
    ‘case’ <expr> ‘:’ <stat>                ‘(six.case $2 $4)’
                                            
<break stat> ::=
    ‘break’ ‘;’                             ‘(six.break)’
                                            
<continue stat> ::=
    ‘continue’ ‘;’                          ‘(six.continue)’
                                            
<label stat> ::=
    <identifier> ‘:’ <stat>                 ‘(six.label $1 $3)’
                                            
<goto stat> ::=
    ‘goto’ <expr> ‘;’                       ‘(six.goto $2)’
                                            
<return stat> ::=
    ‘return’ ‘;’                            ‘(six.return)’
  | ‘return’ <expr> ‘;’                     ‘(six.return $2)’
                                            
<import stat> ::=
    ‘import’ <expr> ‘;’                     ‘(six.import $2)’
                                            
<from stat> ::=
    ‘from’ <expr> ‘import’ <expr> ‘;’       ‘(six.from-import $2 $4)’
    ‘from’ <expr> ‘import’ ‘*’ ‘;’          ‘(six.from-import-* $2)’
                                            
<expression stat> ::=
    <expr> ‘;’                              $1
                                            
<clause stat> ::=
    <expr> ‘.’                              ‘(six.clause $1)’
                                            
<pexpr> ::=
    <procedure definition>                  $1
  | <variable definition>                   $1
  | <expr>                                  $1
                                            
<procedure definition> ::=
    <type> <id-or-prefix> ‘(’               ‘(six.define-procedure $2
<parameters> ‘)’ <body>                     (six.procedure $1 $4 $6))’
  | ‘function’ <id-or-prefix> ‘(’           ‘(six.define-procedure $2
<parameters> ‘)’ <body>                     (six.procedure #f $4 $6))’
                                            
<variable definition> ::=
    <type> <id-or-prefix> <dimensions>      ‘(six.define-variable $2 $1
<iexpr>                                     $3 $4)’
                                            
<iexpr> ::=
    ‘=’ <expr>                              $2
  |                                         ‘#f’
                                            
<dimensions> ::=
  | ‘[’ <expr> ‘]’ <dimensions>             ‘($2 . $4)’
  |                                         ‘()’
                                            
<oexpr> ::=
    <expr>                                  $1
  |                                         ‘#f’
                                            
<expr> ::=
    <expr22>                                $1
                                            
<expr22> ::=
    <expr21> ‘:-’ <expr22>                  ‘(six.x:-y $1 $3)’
  | <expr21>                                $1
                                            
<expr21> ::=
    <expr21> ‘,’ <expr20>                   ‘(|six.x,y| $1 $3)’
  | <expr20>                                $1
                                            
<expr20> ::=
    ‘yield’ <expr20>                        ‘(six.yieldx $2)’
  | <expr19>                                $1
                                            
<expr19> ::=
    <expr18> ‘:=’ <expr19>                  ‘(six.x:=y $1 $3)’
  | <expr18>                                $1
                                            
<expr18> ::=
    <expr17> ‘%=’ <expr18>                  ‘(six.x%=y $1 $3)’
  | <expr17> ‘&=’ <expr18>                  ‘(six.x&=y $1 $3)’
  | <expr17> ‘**=’ <expr18>                 ‘(six.x**=y $1 $3)’
  | <expr17> ‘*=’ <expr18>                  ‘(six.x*=y $1 $3)’
  | <expr17> ‘@=’ <expr18>                  ‘(six.x@=y $1 $3)’
  | <expr17> ‘+=’ <expr18>                  ‘(six.x+=y $1 $3)’
  | <expr17> ‘-=’ <expr18>                  ‘(six.x-=y $1 $3)’
  | <expr17> ‘//=’ <expr18>                 ‘(six.x//=y $1 $3)’
  | <expr17> ‘/=’ <expr18>                  ‘(six.x/=y $1 $3)’
  | <expr17> ‘<<=’ <expr18>                 ‘(six.x<<=y $1 $3)’
  | <expr17> ‘=’ <expr18>                   ‘(six.x=y $1 $3)’
  | <expr17> ‘>>>=’ <expr18>                ‘(six.x>>>=y $1 $3)’
  | <expr17> ‘>>=’ <expr18>                 ‘(six.x>>=y $1 $3)’
  | <expr17> ‘^=’ <expr18>                  ‘(six.x^=y $1 $3)’
  | <expr17> ‘|=’ <expr18>                  ‘(|six.x\|=y| $1 $3)’
  | <expr17>                                $1
                                            
<expr17> ::=
    <expr16> ‘:’ <expr17>                   ‘(six.x:y $1 $3)’
  | <expr16>                                $1
                                            
<expr16> ::=
    <expr15> ‘?’ <expr> ‘:’ <expr16>        ‘(six.x?y:z $1 $3 $5)’
  | <expr15>                                $1
                                            
<expr15> ::=
    <expr15> ‘or’ <expr14>                  ‘(six.xory $1 $3)’
  | <expr14>                                $1
                                            
<expr14> ::=
    <expr14> ‘and’ <expr13>                 ‘(six.xandy $1 $3)’
  | <expr13>                                $1
                                            
<expr13> ::=
    ‘not’ <expr13>                          ‘(six.notx $1)’
  | <expr12>                                $1
                                            
<expr12> ::=
    <expr12> ‘||’ <expr11>                  ‘(|six.x\|\|y| $1 $3)’
  | <expr11>                                $1
                                            
<expr11> ::=
    <expr11> ‘&&’ <expr10>                  ‘(six.x&&y $1 $3)’
  | <expr10>                                $1
                                            
<expr10> ::=
    <expr10> ‘|’ <expr9>                    ‘(|six.x\|y| $1 $3)’
  | <expr9>                                 $1
                                            
<expr9> ::=
    <expr9> ‘^’ <expr8>                     ‘(six.x^y $1 $3)’
  | <expr8>                                 $1
                                            
<expr8> ::=
    <expr8> ‘&’ <expr7>                     ‘(six.x&y $1 $3)’
  | <expr7>                                 $1
                                            
<expr7> ::=
    <expr7> ‘!=’ <expr6>                    ‘(six.x!=y $1 $3)’
  | <expr7> ‘==’ <expr6>                    ‘(six.x==y $1 $3)’
  | <expr7> ‘!==’ <expr6>                   ‘(six.x!==y $1 $3)’
  | <expr7> ‘===’ <expr6>                   ‘(six.x===y $1 $3)’
  | <expr6>                                 $1
                                            
<expr6> ::=
    <expr6> ‘<’ <expr5>                     ‘(six.x<y $1 $3)’
  | <expr6> ‘<=’ <expr5>                    ‘(six.x<=y $1 $3)’
  | <expr6> ‘>’ <expr5>                     ‘(six.x>y $1 $3)’
  | <expr6> ‘>=’ <expr5>                    ‘(six.x>=y $1 $3)’
  | <expr6> ‘in’ <expr5>                    ‘(six.xiny $1 $3)’
  | <expr6> ‘is’ <expr5>                    ‘(six.xisy $1 $3)’
  | <expr6> ‘instanceof’ <expr5>            ‘(six.xinstanceofy $1 $3)’
  | <expr5>                                 $1
                                            
<expr5> ::=
    <expr5> ‘<<’ <expr4>                    ‘(six.x<<y $1 $3)’
  | <expr5> ‘>>’ <expr4>                    ‘(six.x>>y $1 $3)’
  | <expr5> ‘>>>’ <expr4>                   ‘(six.x>>>y $1 $3)’
  | <expr4>                                 $1
                                            
<expr4> ::=
    <expr4> ‘+’ <expr3>                     ‘(six.x+y $1 $3)’
  | <expr4> ‘-’ <expr3>                     ‘(six.x-y $1 $3)’
  | <expr3>                                 $1
                                            
<expr3> ::=
    <expr3> ‘%’ <expr2>                     ‘(six.x%y $1 $3)’
  | <expr3> ‘*’ <expr2>                     ‘(six.x*y $1 $3)’
  | <expr3> ‘@’ <expr2>                     ‘(six.x@y $1 $3)’
  | <expr3> ‘/’ <expr2>                     ‘(six.x/y $1 $3)’
  | <expr3> ‘//’ <expr2>                    ‘(six.x//y $1 $3)’
  | <expr2>                                 $1
                                            
<expr2> ::=
    ‘&’ <expr2>                             ‘(six.&x $2)’
  | ‘+’ <expr2>                             ‘(six.+x $2)’
  | ‘-’ <expr2>                             ‘(six.-x $2)’
  | ‘*’ <expr2>                             ‘(six.*x $2)’
  | ‘**’ <expr2>                            ‘(six.**x $2)’
  | ‘!’ <expr2>                             ‘(six.!x $2)’
  | ‘!’                                     ‘(six.!)’
  | ‘++’ <expr2>                            ‘(six.++x $2)’
  | ‘--’ <expr2>                            ‘(six.--x $2)’
  | ‘~’ <expr2>                             ‘(six.~x $2)’
  | <expr2> ‘**’ <expr1>                    ‘(six.x**y $1 $3)’
  | ‘await’ <expr2>                         ‘(six.awaitx $2)’
  | ‘typeof’ <expr2>                        ‘(six.typeofx $2)’
  | <expr1>                                 $1
                                            
<expr1> ::=
    <expr1> ‘++’                            ‘(six.x++ $1)’
  | <expr1> ‘--’                            ‘(six.x-- $1)’
  | <expr1> ‘(’ <arguments> ‘)’             ‘(six.call $1 . $3)’
  | <expr1> ‘[’ <expr> ‘]’                  ‘(six.index $1 $3)’
  | <expr1> ‘->’ <id-or-prefix>             ‘(six.arrow $1 $3)’
  | <expr1> ‘.’ <id-or-prefix>              ‘(six.dot $1 $3)’
  | <expr0>                                 $1
                                            
<expr0> ::=
    <id-or-prefix>                          $1
  | <string>                                ‘(six.literal $1)’
  | <char>                                  ‘(six.literal $1)’
  | <number>                                ‘(six.literal $1)’
  | ‘(’ <expr> ‘)’                          $2
  | ‘(’ <block stat> ‘)’                    $2
  | <datum-starting-with-#-or-backquote>    $1
  | ‘[’ <elements> ‘]’                      $2
  | <type> ‘(’ <parameters> ‘)’ <body>      ‘(six.procedure $1 $3 $5)’
  | ‘function’ ‘(’ <parameters> ‘)’         ‘(six.procedure #f $3 $5)’
<body>
  | ‘new’ <id-or-prefix> ‘(’ <arguments>    ‘(six.new $2 . $4)’
‘)’
  | ‘async’ <expr0>                         ‘(six.asyncx $2)’
                                            
<block stat> ::=
    ‘{’ <stat list> ‘}’                     ‘(six.compound . $2)’
                                            
<body> ::=
    ‘{’ <stat list> ‘}’                     ‘(six.procedure-body . $2)’
                                            
<stat list> ::=
    <stat> <stat list>                      ‘($1 . $2)’
  |                                         ‘()’
                                            
<parameters> ::=
    <nonempty parameters>                   $1
  |                                         ‘()’
                                            
<nonempty parameters> ::=
    <parameter> ‘,’ <nonempty parameters>   ‘($1 . $3)’
  | <parameter>                             ‘($1)’
                                            
<parameter> ::=
    <type> <id-or-prefix>                   ‘($2 $1)’
                                            
<arguments> ::=
    <nonempty arguments>                    $1
  |                                         ‘()’
                                            
<nonempty arguments> ::=
    <expr> ‘,’ <nonempty arguments>         ‘($1 . $3)’
  | <expr>                                  ‘($1)’
                                            
<elements> ::=
    <nonempty elements>                     $1
  |                                         ‘(six.null)’
                                            
<nonempty elements> ::=
    <expr>                                  ‘(six.list $1 (six.null))’
  | <expr> ‘,’ <nonempty elements>          ‘(six.list $1 $3)’
  | <expr> ‘|’ <expr>                       ‘(six.cons $1 $3)’
                                            
<id-or-prefix> ::=
    <identifier>                            ‘(six.identifier $1)’
  | ‘\’ <datum>                             $2
                                            
<type> ::=
    ‘scmobj’                                ‘scmobj’
                                            

15.12.2 SIX semantics
---------------------

The semantics of SIX depends on the definition of the ‘six.XXX’
identifiers (as procedures and macros).  Many of these identifiers are
predefined macros which give SIX a semantics that is close to C’s while
also borrowing some semantics from JavaScript and Python (such as ‘===’
‘in’, ‘**’, ‘//’).  The programmer may override these definitions to
change the semantics either globally or locally.  For example, ‘six.x^y’
is a predefined macro that expands ‘(six.x^y x y)’ into ‘(bitwise-xor x
y)’.  If the programmer prefers the ‘^’ operator to express
exponentiation in a specific procedure, then in that procedure ‘six.x^y’
can be redefined as a macro that expands ‘(six.x^y x y)’ into ‘(expt x
y)’.  Note that the associativity and precedence of operators cannot be
changed as that is a syntactic issue, so this will give a left
associative exponentiation operator with an unusual precedence.

   The following identifiers do not have a predefined semantics (they
are undefined identifiers): ‘six.label’, ‘six.goto’, ‘six.switch’,
‘six.case’, ‘six.break’, ‘six.continue’, ‘six.return’, ‘six.clause’,
‘six.x:-y’, ‘six.@’, ‘six.@=’, ‘six.import’, ‘six.from-import’,
‘six.from-import-*’, and ‘six.!’.

   Here is an example showing some of the predefined syntax and
semantics of SIX:

     > (list (+ 1 2) \3+4 (+ 5 6))
     (3 7 11)
     > \[ 1+2, \(+ 3 4), 5+6 ]
     (3 7 11)
     > (map (lambda (x) \(x*x-1)/log(x+1)) '(1 2 3 4))
     (0 2.730717679880512 5.7707801635558535 9.320024018394177)
     > (map \function (x) { return (x*x-1)/log(x+1); } '(1 2 3 4))
     (0. 2.730717679880512 5.7707801635558535 9.320024018394177)
     > \map(function (x) { return (x*x-1)/log(x+1); }, [1, 2, 3, 4])
     (0. 2.730717679880512 5.7707801635558535 9.320024018394177)
     > \scmobj n=expt(10,5)
     > n
     100000
     > \scmobj t[3][10]=88
     > \t[0][9]=t[2][1]=11
     11
     > t
     #(#(88 88 88 88 88 88 88 88 88 11)
       #(88 88 88 88 88 88 88 88 88 88)
       #(88 11 88 88 88 88 88 88 88 88))
     > \scmobj radix=new parameter(10)
     > \radix(2)
     > \radix()
     2
     > \for (scmobj i=0; i<5; i++) pp(1<<i*8)
     1
     256
     65536
     16777216
     4294967296
     > \function \make-adder(x) { return function (y) { x+y; }; }
     > \map(new adder(100), [1,2,3,4])
     (101 102 103 104)
     > (map (make-adder 100) (list 1 2 3 4))
     (101 102 103 104)

16 C-interface
**************

The Gambit Scheme system offers a mechanism for interfacing Scheme code
and C code called the “C-interface”.  A Scheme program indicates which C
functions it needs to have access to and which Scheme procedures can be
called from C, and the C interface automatically constructs the
corresponding Scheme procedures and C functions.  The conversions needed
to transform data from the Scheme representation to the C representation
(and back), are generated automatically in accordance with the argument
and result types of the C function or Scheme procedure.

   The C-interface places some restrictions on the types of data that
can be exchanged between C and Scheme.  The mapping of data types
between C and Scheme is discussed in the next section.  The remaining
sections of this chapter describe each special form of the C-interface.

16.1 The mapping of types between C and Scheme
==============================================

Scheme and C do not provide the same set of built-in data types so it is
important to understand which Scheme type is compatible with which C
type and how values get mapped from one environment to the other.  To
improve compatibility a new type is added to Scheme, the ‘foreign’
object type, and the following data types are added to C:

‘scheme-object’
     denotes the universal type of Scheme objects (type ‘___SCMOBJ’
     defined in ‘gambit.h’)
‘bool’
     denotes the C++ ‘bool’ type or the C ‘int’ type (type ‘___BOOL’
     defined in ‘gambit.h’)
‘int8’
     8 bit signed integer (type ‘___S8’ defined in ‘gambit.h’)
‘unsigned-int8’
     8 bit unsigned integer (type ‘___U8’ defined in ‘gambit.h’)
‘int16’
     16 bit signed integer (type ‘___S16’ defined in ‘gambit.h’)
‘unsigned-int16’
     16 bit unsigned integer (type ‘___U16’ defined in ‘gambit.h’)
‘int32’
     32 bit signed integer (type ‘___S32’ defined in ‘gambit.h’)
‘unsigned-int32’
     32 bit unsigned integer (type ‘___U32’ defined in ‘gambit.h’)
‘int64’
     64 bit signed integer (type ‘___S64’ defined in ‘gambit.h’)
‘unsigned-int64’
     64 bit unsigned integer (type ‘___U64’ defined in ‘gambit.h’)
‘float32’
     32 bit floating point number (type ‘___F32’ defined in ‘gambit.h’)
‘float64’
     64 bit floating point number (type ‘___F64’ defined in ‘gambit.h’)
‘ISO-8859-1’
     denotes ISO-8859-1 encoded characters (8 bit unsigned integer, type
     ‘___ISO_8859_1’ defined in ‘gambit.h’)
‘UCS-2’
     denotes UCS-2 encoded characters (16 bit unsigned integer, type
     ‘___UCS_2’ defined in ‘gambit.h’)
‘UCS-4’
     denotes UCS-4 encoded characters (32 bit unsigned integer, type
     ‘___UCS_4’ defined in ‘gambit.h’)
‘char-string’
     denotes the C ‘char*’ type when used as a null terminated string
‘nonnull-char-string’
     denotes the nonnull C ‘char*’ type when used as a null terminated
     string
‘nonnull-char-string-list’
     denotes an array of nonnull C ‘char*’ terminated with a null
     pointer
‘ISO-8859-1-string’
     denotes ISO-8859-1 encoded strings (null terminated string of 8 bit
     unsigned integers, i.e.  ‘___ISO_8859_1*’)
‘nonnull-ISO-8859-1-string’
     denotes nonnull ISO-8859-1 encoded strings (null terminated string
     of 8 bit unsigned integers, i.e.  ‘___ISO_8859_1*’)
‘nonnull-ISO-8859-1-stringlist’
     denotes an array of nonnull ISO-8859-1 encoded strings terminated
     with a null pointer
‘UTF-8-string’
     denotes UTF-8 encoded strings (null terminated string of ‘char’,
     i.e.  ‘char*’)
‘nonnull-UTF-8-string’
     denotes nonnull UTF-8 encoded strings (null terminated string of
     ‘char’, i.e.  ‘char*’)
‘nonnull-UTF-8-string-list’
     denotes an array of nonnull UTF-8 encoded strings terminated with a
     null pointer
‘UTF-16-string’
     denotes UTF-16 encoded strings (null terminated string of ‘char’,
     i.e.  ‘char*’)
‘nonnull-UTF-16-string’
     denotes nonnull UTF-16 encoded strings (null terminated string of
     ‘char’, i.e.  ‘char*’)
‘nonnull-UTF-16-string-list’
     denotes an array of nonnull UTF-16 encoded strings terminated with
     a null pointer
‘UCS-2-string’
     denotes UCS-2 encoded strings (null terminated string of 16 bit
     unsigned integers, i.e.  ‘___UCS_2*’)
‘nonnull-UCS-2-string’
     denotes nonnull UCS-2 encoded strings (null terminated string of 16
     bit unsigned integers, i.e.  ‘___UCS_2*’)
‘nonnull-UCS-2-string-list’
     denotes an array of nonnull UCS-2 encoded strings terminated with a
     null pointer
‘UCS-4-string’
     denotes UCS-4 encoded strings (null terminated string of 32 bit
     unsigned integers, i.e.  ‘___UCS_4*’)
‘nonnull-UCS-4-string’
     denotes nonnull UCS-4 encoded strings (null terminated string of 32
     bit unsigned integers, i.e.  ‘___UCS_4*’)
‘nonnull-UCS-4-string-list’
     denotes an array of nonnull UCS-4 encoded strings terminated with a
     null pointer
‘wchar_t-string’
     denotes ‘wchar_t’ encoded strings (null terminated string of
     ‘wchar_t’, i.e.  ‘wchar_t*’)
‘nonnull-wchar_t-string’
     denotes nonnull ‘wchar_t’ encoded strings (null terminated string
     of ‘wchar_t’, i.e.  ‘wchar_t*’)
‘nonnull-wchar_t-string-list’
     denotes an array of nonnull ‘wchar_t’ encoded strings terminated
     with a null pointer

   To specify a particular C type inside the ‘c-lambda’, ‘c-define’ and
‘c-define-type’ forms, the following “Scheme notation” is used:

‘Scheme notation’
     C type
‘void’
     ‘void’
‘bool’
     ‘bool’
‘char’
     ‘char’ (may be signed or unsigned depending on the C compiler)
‘signed-char’
     ‘signed char’
‘unsigned-char’
     ‘unsigned char’
‘ISO-8859-1’
     ‘ISO-8859-1’
‘UCS-2’
     ‘UCS-2’
‘UCS-4’
     ‘UCS-4’
‘wchar_t’
     ‘wchar_t’
‘size_t’
     ‘size_t’ (type ‘___SIZE_T’ defined in ‘gambit.h’)
‘ssize_t’
     ‘ssize_t’ (type ‘___SSIZE_T’ defined in ‘gambit.h’)
‘ptrdiff_t’
     ‘ptrdiff_t’ (type ‘___PTRDIFF_T’ defined in ‘gambit.h’)
‘short’
     ‘short’
‘unsigned-short’
     ‘unsigned short’
‘int’
     ‘int’
‘unsigned-int’
     ‘unsigned int’
‘long’
     ‘long’
‘unsigned-long’
     ‘unsigned long’
‘long-long’
     ‘long long’
‘unsigned-long-long’
     ‘unsigned long long’
‘float’
     ‘float’
‘double’
     ‘double’
‘int8’
     ‘int8’
‘unsigned-int8’
     ‘unsigned-int8’
‘int16’
     ‘int16’
‘unsigned-int16’
     ‘unsigned-int16’
‘int32’
     ‘int32’
‘unsigned-int32’
     ‘unsigned-int32’
‘int64’
     ‘int64’
‘unsigned-int64’
     ‘unsigned-int64’
‘float32’
     ‘float32’
‘float64’
     ‘float64’
‘(struct "C-STRUCT-ID" [TAGS [RELEASE-FUNCTION]])’
     ‘struct C-STRUCT-ID’ (where C-STRUCT-ID is the name of a C
     structure; see below for the meaning of TAGS and RELEASE-FUNCTION)
‘(union "C-UNION-ID" [TAGS [RELEASE-FUNCTION]])’
     ‘union C-UNION-ID’ (where C-UNION-ID is the name of a C union; see
     below for the meaning of TAGS and RELEASE-FUNCTION)
‘(type "C-TYPE-ID" [TAGS [RELEASE-FUNCTION]])’
     ‘C-TYPE-ID’ (where C-TYPE-ID is an identifier naming a C type; see
     below for the meaning of TAGS and RELEASE-FUNCTION)
‘(pointer TYPE [TAGS [RELEASE-FUNCTION]])’
     ‘T*’ (where T is the C equivalent of TYPE which must be the Scheme
     notation of a C type; see below for the meaning of TAGS and
     RELEASE-FUNCTION)
‘(nonnull-pointer TYPE [TAGS [RELEASE-FUNCTION]])’
     same as ‘(pointer TYPE [TAGS [RELEASE-FUNCTION]])’ except the
     ‘NULL’ pointer is not allowed
‘(function (TYPE1...) RESULT-TYPE)’
     function with the given argument types and result type
‘(nonnull-function (TYPE1...) RESULT-TYPE)’
     same as ‘(function (TYPE1...) RESULT-TYPE)’ except the ‘NULL’
     pointer is not allowed
‘char-string’
     ‘char-string’
‘nonnull-char-string’
     ‘nonnull-char-string’
‘nonnull-char-string-list’
     ‘nonnull-char-string-list’
‘ISO-8859-1-string’
     ‘ISO-8859-1-string’
‘nonnull-ISO-8859-1-string’
     ‘nonnull-ISO-8859-1-string’
‘nonnull-ISO-8859-1-string-list’
     ‘nonnull-ISO-8859-1-string-list’
‘UTF-8-string’
     ‘UTF-8-string’
‘nonnull-UTF-8-string’
     ‘nonnull-UTF-8-string’
‘nonnull-UTF-8-string-list’
     ‘nonnull-UTF-8-string-list’
‘UTF-16-string’
     ‘UTF-16-string’
‘nonnull-UTF-16-string’
     ‘nonnull-UTF-16-string’
‘nonnull-UTF-16-string-list’
     ‘nonnull-UTF-16-string-list’
‘UCS-2-string’
     ‘UCS-2-string’
‘nonnull-UCS-2-string’
     ‘nonnull-UCS-2-string’
‘nonnull-UCS-2-string-list’
     ‘nonnull-UCS-2-string-list’
‘UCS-4-string’
     ‘UCS-4-string’
‘nonnull-UCS-4-string’
     ‘nonnull-UCS-4-string’
‘nonnull-UCS-4-string-list’
     ‘nonnull-UCS-4-string-list’
‘wchar_t-string’
     ‘wchar_t-string’
‘nonnull-wchar_t-string’
     ‘nonnull-wchar_t-string’
‘nonnull-wchar_t-string-list’
     ‘nonnull-wchar_t-string-list’
‘scheme-object’
     ‘scheme-object’
‘NAME’
     appropriate translation of NAME (where NAME is a C type defined
     with ‘c-define-type’)
‘"C-TYPE-ID"’
     C-TYPE-ID (this form is equivalent to ‘(type "C-TYPE-ID")’)

   The ‘struct’, ‘union’, ‘type’, ‘pointer’ and ‘nonnull-pointer’ types
are “foreign types” and they are represented on the Scheme side as
“foreign objects”.  A foreign object is internally represented as a
pointer.  This internal pointer is identical to the C pointer being
represented in the case of the ‘pointer’ and ‘nonnull-pointer’ types.

   In the case of the ‘struct’, ‘union’ and ‘type’ types, the internal
pointer points to a copy of the C data type being represented.  When an
instance of one of these types is converted from C to Scheme, a block of
memory is allocated from the C heap and initialized with the instance
and then a foreign object is allocated from the Scheme heap and
initialized with the pointer to this copy.  This approach may appear
overly complex, but it allows the conversion of C++ classes that do not
have a zero parameter constructor or an assignment method (i.e.  when
compiling with a C++ compiler an instance is copied using ‘new TYPE
(INSTANCE)’, which calls the copy-constructor of TYPE if it is a class;
TYPE’s assignment operator is never used).  Conversion from Scheme to C
simply dereferences the internal pointer (no allocation from the C heap
is performed).  Deallocation of the copy on the C heap is under the
control of the release function attached to the foreign object (see
below).

   The optional TAGS field of foreign type specifications is used for
type checking on the Scheme side.  The TAGS field must be ‘#f’, a symbol
or a non-empty list of symbols.  When it is not specified the TAGS field
defaults to a symbol whose name, as returned by ‘symbol->string’, is the
C type declaration for that type.  For example the symbol ‘char**’ is
the default for the type ‘(pointer (pointer char))’.  A TAGS field that
is a single symbol is equivalent to a list containing only that symbol.
The first symbol in the list of tags is the primary tag.  For example
the primary tag of the type ‘(pointer char)’ is ‘char*’ and the primary
tag of the type ‘(pointer char (foo bar))’ is ‘foo’.

   Type compatibility between two foreign types depends on their tags.
An instance of a foreign type T can be used where a foreign type E is
expected if and only if

   • T’s TAGS field is ‘#f’, or

   • E’s TAGS field is ‘#f’, or

   • T’s primary tag is a member of E’s tags.

   For the safest code a TAGS field of ‘#f’ should be used sparingly, as
it completely bypasses type checking.  The external representation of
Scheme foreign objects (used by the ‘write’ procedure) contains the
primary tag (if the TAGS field is not ‘#f’), and the hexadecimal address
denoted by the internal pointer, for example ‘#<char** #2 0x2AAC535C>’.
Note that the hexadecimal address is in C notation, which can be easily
transferred to a C debugger with a “cut-and-paste”.

   A RELEASE-FUNCTION can also be specified within a foreign type
specification.  The RELEASE-FUNCTION must be ‘#f’ or a string naming a C
function with a single parameter of type ‘void*’ (in which the internal
pointer is passed) and with a result of type ‘___SCMOBJ’ (for returning
an error code).  When the RELEASE-FUNCTION is not specified or is ‘#f’ a
default function is constructed by the C-interface.  This default
function does nothing in the case of the ‘pointer’ and ‘nonnull-pointer’
types (deallocation is not the responsibility of the C-interface) and
returns the fixnum ‘___FIX(___NO_ERR)’ to indicate no error.  In the
case of the ‘struct’, ‘union’ and ‘type’ types, the default function
reclaims the copy on the C heap referenced by the internal pointer (when
using a C++ compiler this is done using ‘delete
(TYPE*)INTERNAL-POINTER’, which calls the destructor of TYPE if it is a
class) and returns ‘___FIX(___NO_ERR)’.  In many situations the default
RELEASE-FUNCTION will perform the appropriate cleanup for the foreign
type.  However, in certain cases special operations (such as
decrementing a reference count, removing the object from a table, etc)
must be performed.  For such cases a user supplied RELEASE-FUNCTION is
needed.

   The RELEASE-FUNCTION is invoked at most once for any foreign object.
After the RELEASE-FUNCTION is invoked, the foreign object is considered
“released” and can no longer be used in a foreign type conversion.  When
the garbage collector detects that a foreign object is no longer
reachable by the program, it will invoke the RELEASE-FUNCTION if the
foreign object is not yet released.  When there is a need to release the
foreign object promptly, the program can explicitly call
‘(foreign-release! OBJ)’ which invokes the RELEASE-FUNCTION if the
foreign object is not yet released, and does nothing otherwise.  The
call ‘(foreign-released? OBJ)’ returns a boolean indicating whether the
foreign object OBJ has been released yet or not.  The call
‘(foreign-address OBJ)’ returns the address denoted by the internal
pointer of foreign object OBJ or 0 if it has been released.  The call
‘(foreign? OBJ)’ tests that OBJ is a foreign object.  Finally the call
‘(foreign-tags OBJ)’ returns the list of tags of foreign object OBJ, or
‘#f’.

   The following table gives the C types to which each Scheme type can
be converted:

Scheme type
     Allowed target C types
boolean ‘#f’
     ‘scheme-object’; ‘bool’; ‘pointer’; ‘function’; ‘char-string’;
     ‘ISO-8859-1-string’; ‘UTF-8-string’; ‘UTF-16-string’;
     ‘UCS-2-string’; ‘UCS-4-string’; ‘wchar_t-string’
boolean ‘#t’
     ‘scheme-object’; ‘bool’
character
     ‘scheme-object’; ‘bool’; [‘[un]signed’] ‘char’; ‘ISO-8859-1’;
     ‘UCS-2’; ‘UCS-4’; ‘wchar_t’
exact integer
     ‘scheme-object’; ‘bool’; [‘unsigned-’]
     ‘int8’/‘int16’/‘int32’/‘int64’; [‘unsigned’] ‘short’/‘int’/‘long’;
     ‘size_t’/‘ssize_t’/‘ptrdiff_t’
inexact real
     ‘scheme-object’; ‘bool’; ‘float’; ‘double’; ‘float32’; ‘float64’
string
     ‘scheme-object’; ‘bool’; ‘char-string’; ‘nonnull-char-string’;
     ‘ISO-8859-1-string’; ‘nonnull-ISO-8859-1-string’; ‘UTF-8-string’;
     ‘nonnull-UTF-8-string’; ‘UTF-16-string’; ‘nonnull-UTF-16-string’;
     ‘UCS-2-string’; ‘nonnull-UCS-2-string’; ‘UCS-4-string’;
     ‘nonnull-UCS-4-string’; ‘wchar_t-string’; ‘nonnull-wchar_t-string’
foreign object
     ‘scheme-object’; ‘bool’;
     ‘struct’/‘union’/‘type’/‘pointer’/‘nonnull-pointer’ with the
     appropriate tags
vector
     ‘scheme-object’; ‘bool’
symbol
     ‘scheme-object’; ‘bool’
procedure
     ‘scheme-object’; ‘bool’; ‘function’; ‘nonnull-function’
other objects
     ‘scheme-object’; ‘bool’

   The following table gives the Scheme types to which each C type will
be converted:

C type
     Resulting Scheme type
scheme-object
     the Scheme object encoded
bool
     boolean
[‘[un]signed’] ‘char’; ‘ISO-8859-1’; ‘UCS-2’; ‘UCS-4’; ‘wchar_t’
     character
[‘unsigned-’] ‘int8’/‘int16’/‘int32’/‘int64’; [‘unsigned’] ‘short’/‘int’/‘long’; ‘size_t’/‘ssize_t’/‘ptrdiff_t’
     exact integer
‘float’; ‘double’; ‘float32’; ‘float64’
     inexact real
‘char-string’; ‘ISO-8859-1-string’; ‘UTF-8-string’; ‘UTF-16-string’; ‘UCS-2-string’; ‘UCS-4-string’; ‘wchar_t-string’
     string or ‘#f’ if it is equal to ‘NULL’
‘nonnull-char-string’; ‘nonnull-ISO-8859-1-string’; ‘nonnull-UTF-8-string’; ‘nonnull-UTF-16-string’; ‘nonnull-UCS-2-string’; ‘nonnull-UCS-4-string’; ‘nonnull-wchar_t-string’
     string
‘struct’/‘union’/‘type’/‘pointer’/‘nonnull-pointer’
     foreign object with the appropriate tags or ‘#f’ in the case of a
     ‘pointer’ equal to ‘NULL’
‘function’
     procedure or ‘#f’ if it is equal to ‘NULL’
‘nonnull-function’
     procedure
‘void’
     void object

   All Scheme types are compatible with the C types ‘scheme-object’ and
‘bool’.  Conversion to and from the C type ‘scheme-object’ is the
identity function on the object encoding.  This provides a low-level
mechanism for accessing Scheme’s object representation from C (with the
help of the macros in the ‘gambit.h’ header file).  When a C ‘bool’ type
is expected, an extended Scheme boolean can be passed (‘#f’ is converted
to 0 and all other values are converted to 1).

   The Scheme boolean ‘#f’ can be passed to the C environment where a
‘char-string’, ‘ISO-8859-1-string’, ‘UTF-8-string’, ‘UTF-16-string’,
‘UCS-2-string’, ‘UCS-4-string’, ‘wchar_t-string’, ‘pointer’ or
‘function’ type is expected.  In this case, ‘#f’ is converted to the
‘NULL’ pointer.  C ‘bool’s are extended booleans so any value different
from 0 represents true.  Thus, a C ‘bool’ passed to the Scheme
environment is mapped as follows: 0 to ‘#f’ and all other values to
‘#t’.

   A Scheme character passed to the C environment where any C character
type is expected is converted to the corresponding character in the C
environment.  An error is signaled if the Scheme character does not fit
in the C character.  Any C character type passed to Scheme is converted
to the corresponding Scheme character.  An error is signaled if the C
character does not fit in the Scheme character.

   A Scheme exact integer passed to the C environment where a C integer
type (other than ‘char’) is expected is converted to the corresponding
integral value.  An error is signaled if the value falls outside of the
range representable by that integral type.  C integer values passed to
the Scheme environment are mapped to the same Scheme exact integer.  If
the value is outside the fixnum range, a bignum is created.

   A Scheme inexact real passed to the C environment is converted to the
corresponding ‘float’, ‘double’, ‘float32’ or ‘float64’ value.  C
‘float’, ‘double’, ‘float32’ and ‘float64’ values passed to the Scheme
environment are mapped to the closest Scheme inexact real.

   Scheme’s rational numbers and complex numbers are not compatible with
any C numeric type.

   A Scheme string passed to the C environment where any C string type
is expected is converted to a null terminated string using the
appropriate encoding.  The C string is a fresh copy of the Scheme
string.  If the C string was created for an argument of a ‘c-lambda’,
the C string will be reclaimed when the ‘c-lambda’ returns.  If the C
string was created for returning the result of a ‘c-define’ to C, the
caller is responsible for reclaiming the C string with a call to the
‘___release_string’ function (see below for an example).  Any C string
type passed to the Scheme environment causes the creation of a fresh
Scheme string containing a copy of the C string (unless the C string is
equal to ‘NULL’, in which case it is converted to ‘#f’).

   A foreign type passed to the Scheme environment causes the creation
and initialization of a Scheme foreign object with the appropriate tags
(except for the case of a ‘pointer’ equal to ‘NULL’ which is converted
to ‘#f’).  A Scheme foreign object can be passed where a foreign type is
expected, on the condition that the tags are compatible and the Scheme
foreign object is not yet released.  The value ‘#f’ is also acceptable
for a ‘pointer’ type, and is converted to ‘NULL’.

   Scheme procedures defined with the ‘c-define’ special form can be
passed where the ‘function’ and ‘nonnull-function’ types are expected.
The value ‘#f’ is also acceptable for a ‘function’ type, and is
converted to ‘NULL’.  No other Scheme procedures are acceptable.
Conversion from the ‘function’ and ‘nonnull-function’ types to Scheme
procedures is not currently implemented.

16.2 The ‘c-declare’ special form
=================================

 -- special form: c-declare c-declaration

     Initially, the C file produced by ‘gsc’ contains only an ‘#include’
     of ‘gambit.h’.  This header file provides a number of macro and
     procedure declarations to access the Scheme object representation.
     The special form ‘c-declare’ adds c-declaration (which must be a
     string containing the C declarations) to the C file.  This string
     is copied to the C file on a new line so it can start with
     preprocessor directives.  All types of C declarations are allowed
     (including type declarations, variable declarations, function
     declarations, ‘#include’ directives, ‘#define’s, and so on).  These
     declarations are visible to subsequent ‘c-declare’s,
     ‘c-initialize’s, and ‘c-lambda’s, and ‘c-define’s in the same
     module.  The most common use of this special form is to declare the
     external functions that are referenced in ‘c-lambda’ special forms.
     Such functions must either be declared explicitly or by including a
     header file which contains the appropriate C declarations.

     The ‘c-declare’ special form does not return a value.  This form
     can only appear where a ‘define’ form is acceptable.

     For example:

          (c-declare #<<c-declare-end

          #include <stdio.h>

          extern char *getlogin ();

          #ifdef sparc
          char *host = "sparc";
          #else
          char *host = "unknown";
          #endif

          FILE *tfile;

          c-declare-end
          )

16.3 The ‘c-initialize’ special form
====================================

 -- special form: c-initialize c-code

     Just after the program is loaded and before control is passed to
     the Scheme code, each C file is initialized by calling its
     associated initialization function.  The body of this function is
     normally empty but it can be extended by using the ‘c-initialize’
     form.  Each occurence of the ‘c-initialize’ form adds code to the
     body of the initialization function in the order of appearance in
     the source file.  c-code must be a string containing the C code to
     execute.  This string is copied to the C file on a new line so it
     can start with preprocessor directives.

     The ‘c-initialize’ special form does not return a value.  This form
     can only appear where a ‘define’ form is acceptable.

     For example:

          (c-initialize "tfile = tmpfile ();")

16.4 The ‘c-lambda’ special form
================================

 -- special form: c-lambda (type1...) result-type c-name-or-code

     The ‘c-lambda’ special form makes it possible to create a Scheme
     procedure that will act as a representative of some C function or C
     code sequence.  The first subform is a list containing the type of
     each argument.  The type of the function’s result is given next.
     Finally, the last subform is a string that either contains the name
     of the C function to call or some sequence of C code to execute.
     Variadic C functions are not supported.  The resulting Scheme
     procedure takes exactly the number of arguments specified and
     delivers them in the same order to the C function.  When the Scheme
     procedure is called, the arguments will be converted to their C
     representation and then the C function will be called.  The result
     returned by the C function will be converted to its Scheme
     representation and this value will be returned from the Scheme
     procedure call.  An error will be signaled if some conversion is
     not possible.  The temporary memory allocated from the C heap for
     the conversion of the arguments and result will be reclaimed
     whether there is an error or not.

     When c-name-or-code is not a valid C identifier, it is treated as
     an arbitrary piece of C code.  Within the C code the variables
     ‘___arg1’, ‘___arg2’, etc.  can be referenced to access the
     converted arguments.  Note that the C ‘return’ statement can’t be
     used to return from the procedure.  Instead, the ‘___return’ macro
     must be used.  A procedure whose result-type is not ‘void’ must
     pass the procedure’s result as the single argument to the
     ‘___return’ macro, for example ‘___return(123);’ to return the
     value 123.  When result-type is ‘void’, the ‘___return’ macro must
     be called without a parameter list, for example ‘___return;’.

     The C code is copied to the C file on a new line so it can start
     with preprocessor directives.  Moreover the C code is always placed
     at the head of a compound statement whose lifetime encloses the C
     to Scheme conversion of the procedure’s result.  Consequently,
     temporary storage (strings in particular) declared at the head of
     the C code can be returned with the ‘___return’ macro.

     In the c-name-or-code, the macro ‘___AT_END’ may be defined as the
     piece of C code to execute before control is returned to Scheme but
     after the procedure’s result is converted to its Scheme
     representation.  This is mainly useful to deallocate temporary
     storage contained in the result.

     When passed to the Scheme environment, the C ‘void’ type is
     converted to the void object.

     For example:

          (define fopen
            (c-lambda (nonnull-char-string nonnull-char-string)
                      (pointer "FILE")
             "fopen"))

          (define fgetc
            (c-lambda ((pointer "FILE"))
                      int
             "fgetc"))

          (let ((f (fopen "datafile" "r")))
            (if f (write (fgetc f))))

          (define char-code
            (c-lambda (char) int "___return(___arg1);"))

          (define host
            ((c-lambda () nonnull-char-string "___return(host);")))

          (define stdin
            ((c-lambda () (pointer "FILE") "___return(stdin);")))

          ((c-lambda () void
          #<<c-lambda-end
            printf( "hello\n" );
            printf( "world\n" );
          c-lambda-end
          ))

          (define pack-1-char
            (c-lambda (char)
                      nonnull-char-string
          #<<c-lambda-end
             char *s = (char *)malloc (2);
             if (s != NULL) { s[0] = ___arg1; s[1] = 0; }
             ___return(s);
             #define ___AT_END if (s != NULL) free (s);
          c-lambda-end
          ))

          (define pack-2-chars
            (c-lambda (char char)
                      nonnull-char-string
          #<<c-lambda-end
             char s[3];
             s[0] = ___arg1;
             s[1] = ___arg2;
             s[2] = 0;
             ___return(s);
          c-lambda-end
          ))

16.5 The ‘c-define’ special form
================================

 -- special form: c-define (variable define-formals) (type1...)
          result-type c-name scope body

     The ‘c-define’ special form makes it possible to create a C
     function that will act as a representative of some Scheme
     procedure.  A C function named c-name as well as a Scheme procedure
     bound to the variable variable are defined.  The parameters of the
     Scheme procedure are define-formals and its body is at the end of
     the form.  The type of each argument of the C function, its result
     type and c-name (which must be a string) are specified after the
     parameter specification of the Scheme procedure.  When the C
     function c-name is called from C, its arguments are converted to
     their Scheme representation and passed to the Scheme procedure.
     The result of the Scheme procedure is then converted to its C
     representation and the C function c-name returns it to its caller.

     The scope of the C function can be changed with the scope
     parameter, which must be a string.  This string is placed
     immediately before the declaration of the C function.  So if scope
     is the string ‘"static"’, the scope of c-name is local to the
     module it is in, whereas if scope is the empty string, c-name is
     visible from other modules.

     The ‘c-define’ special form does not return a value.  It can only
     appear at top level.

     For example:

          (c-define (proc x #!optional (y x) #!rest z) (int int char float) int "f" ""
            (write (cons x (cons y z)))
            (newline)
            (+ x y))

          (proc 1 2 #\x 1.5) => 3 and prints (1 2 #\x 1.5)
          (proc 1)           => 2 and prints (1 1)

          ; if f is called from C with the call  f (1, 2, 'x', 1.5)
          ; the value 3 is returned and (1 2 #\x 1.5) is printed.
          ; f has to be called with 4 arguments.

     The ‘c-define’ special form is particularly useful when the driving
     part of an application is written in C and Scheme procedures are
     called directly from C. The Scheme part of the application is in a
     sense a “server” that is providing services to the C part.  The
     Scheme procedures that are to be called from C need to be defined
     using the ‘c-define’ special form.  Before it can be used, the
     Scheme part must be initialized with a call to the function
     ‘___setup’.  Before the program terminates, it must call the
     function ‘___cleanup’ so that the Scheme part may do final cleanup.
     A sample application is given in the file ‘tests/server.scm’.

16.6 The ‘c-define-type’ special form
=====================================

 -- special form: c-define-type name type [c-to-scheme scheme-to-c
          [cleanup]]

     This form associates the type identifier name to the C type type.
     The name must not clash with predefined types (e.g.  ‘char-string’,
     ‘ISO-8859-1’, etc.)  or with types previously defined with
     ‘c-define-type’ in the same file.  The ‘c-define-type’ special form
     does not return a value.  It can only appear at top level.

     If only the two parameters name and type are supplied then after
     this definition, the use of name in a type specification is
     synonymous to type.

     For example:

          (c-define-type FILE "FILE")
          (c-define-type FILE* (pointer FILE))
          (c-define-type time-struct-ptr (pointer (struct "tms")))
          (define fopen (c-lambda (char-string char-string) FILE* "fopen"))
          (define fgetc (c-lambda (FILE*) int "fgetc"))

     Note that identifiers are not case-sensitive in standard Scheme but
     it is good programming practice to use a name with the same case as
     in C.

     If four or more parameters are supplied, then type must be a string
     naming the C type, c-to-scheme and scheme-to-c must be strings
     suffixing the C macros that convert data of that type between C and
     Scheme.  If cleanup is supplied it must be a boolean indicating
     whether it is necessary to perform a cleanup operation (such as
     freeing memory) when data of that type is converted from Scheme to
     C (it defaults to ‘#t’).  The cleanup information is used when the
     C stack is unwound due to a continuation invocation (see *note
     continuations::).  Although it is safe to always specify ‘#t’, it
     is more efficient in time and space to specify ‘#f’ because the
     unwinding mechanism can skip C-interface frames which only contain
     conversions of data types requiring no cleanup.  Two pairs of C
     macros need to be defined for conversions performed by ‘c-lambda’
     forms and two pairs for conversions performed by ‘c-define’ forms:

          ___BEGIN_CFUN_scheme-to-c(___SCMOBJ, type, int)
          ___END_CFUN_scheme-to-c(___SCMOBJ, type, int)

          ___BEGIN_CFUN_c-to-scheme(type, ___SCMOBJ)
          ___END_CFUN_c-to-scheme(type, ___SCMOBJ)

          ___BEGIN_SFUN_c-to-scheme(type, ___SCMOBJ, int)
          ___END_SFUN_c-to-scheme(type, ___SCMOBJ, int)

          ___BEGIN_SFUN_scheme-to-c(___SCMOBJ, type)
          ___END_SFUN_scheme-to-c(___SCMOBJ, type)

     The macros prefixed with ‘___BEGIN’ perform the conversion and
     those prefixed with ‘___END’ perform any cleanup necessary (such as
     freeing memory temporarily allocated for the conversion).  The
     macro ‘___END_CFUN_scheme-to-c’ must free the result of the
     conversion if it is memory allocated, and ‘___END_SFUN_scheme-to-c’
     must not (i.e.  it is the responsibility of the caller to free the
     result).

     The first parameter of these macros is the C variable that contains
     the value to be converted, and the second parameter is the C
     variable in which to store the converted value.  The third
     parameter, when present, is the index (starting at 1) of the
     parameter of the ‘c-lambda’ or ‘c-define’ form that is being
     converted (this is useful for reporting precise error information
     when a conversion is impossible).

     To allow for type checking, the first three ‘___BEGIN’ macros must
     expand to an unterminated compound statement prefixed by an ‘if’,
     conditional on the absence of type check error:

          if ((___err = CONVERSION_OPERATION) == ___FIX(___NO_ERR)) {

     The last ‘___BEGIN’ macro must expand to an unterminated compound
     statement:

          { ___err = CONVERSION_OPERATION;

     If type check errors are impossible then a ‘___BEGIN’ macro can
     simply expand to an unterminated compound statement performing the
     conversion:

          { CONVERSION_OPERATION;

     The ‘___END’ macros must expand to a statement, or to nothing if no
     cleanup is required, followed by a closing brace (to terminate the
     compound statement started at the corresponding ‘___BEGIN’ macro).

     The CONVERSION_OPERATION is typically a function call that returns
     an error code value of type ‘___SCMOBJ’ (the error codes are
     defined in ‘gambit.h’, and the error code ‘___FIX(___UNKNOWN_ERR)’
     is available for generic errors).  CONVERSION_OPERATION can also
     set the variable ‘___errdata’ of type ‘___SCMOBJ’ to a specific
     Scheme string error message.

     Below is a simple example showing how to interface to an ‘EBCDIC’
     character type.  Memory allocation is not needed for conversion and
     type check errors are impossible when converting EBCDIC to Scheme
     characters, but they are possible when converting from Scheme
     characters to EBCDIC since Gambit supports Unicode characters.

          (c-declare #<<c-declare-end

          typedef char EBCDIC; /* EBCDIC encoded characters */

          void put_char (EBCDIC c) { ... } /* EBCDIC I/O functions */
          EBCDIC get_char (void) { ... }

          char EBCDIC_to_ISO_8859_1[256] = { ... }; /* conversion tables */
          char ISO_8859_1_to_EBCDIC[256] = { ... };

          ___SCMOBJ SCMOBJ_to_EBCDIC (___SCMOBJ src, EBCDIC *dst)
          {
            int x = ___INT(src); /* convert from Scheme character to int */
            if (x > 255) return ___FIX(___UNKNOWN_ERR);
            *dst = ISO_8859_1_to_EBCDIC[x];
            return ___FIX(___NO_ERR);
          }

          #define ___BEGIN_CFUN_SCMOBJ_to_EBCDIC(src,dst,i) \
          if ((___err = SCMOBJ_to_EBCDIC (src, &dst)) == ___FIX(___NO_ERR)) {
          #define ___END_CFUN_SCMOBJ_to_EBCDIC(src,dst,i) }

          #define ___BEGIN_CFUN_EBCDIC_to_SCMOBJ(src,dst) \
          { dst = ___CHR(EBCDIC_to_ISO_8859_1[src]);
          #define ___END_CFUN_EBCDIC_to_SCMOBJ(src,dst) }

          #define ___BEGIN_SFUN_EBCDIC_to_SCMOBJ(src,dst,i) \
          { dst = ___CHR(EBCDIC_to_ISO_8859_1[src]);
          #define ___END_SFUN_EBCDIC_to_SCMOBJ(src,dst,i) }

          #define ___BEGIN_SFUN_SCMOBJ_to_EBCDIC(src,dst) \
          { ___err = SCMOBJ_to_EBCDIC (src, &dst);
          #define ___END_SFUN_SCMOBJ_to_EBCDIC(src,dst) }

          c-declare-end
          )

          (c-define-type EBCDIC "EBCDIC" "EBCDIC_to_SCMOBJ" "SCMOBJ_to_EBCDIC" #f)

          (define put-char (c-lambda (EBCDIC) void "put_char"))
          (define get-char (c-lambda () EBCDIC "get_char"))

          (c-define (write-EBCDIC c) (EBCDIC) void "write_EBCDIC" ""
            (write-char c))

          (c-define (read-EBCDIC) () EBCDIC "read_EBCDIC" ""
            (read-char))

     Below is a more complex example that requires memory allocation
     when converting from C to Scheme.  It is an interface to a 2D
     ‘point’ type which is represented in Scheme by a pair of integers.
     The conversion of the ‘x’ and ‘y’ components is done by calls to
     the conversion macros for the ‘int’ type (defined in ‘gambit.h’).
     Note that no cleanup is necessary when converting from Scheme to C
     (i.e.  the last parameter of the ‘c-define-type’ is ‘#f’).

          (c-declare #<<c-declare-end

          typedef struct { int x, y; } point;

          void line_to (point p) { ... }
          point get_mouse (void) { ... }
          point add_points (point p1, point p2) { ... }

          ___SCMOBJ SCMOBJ_to_POINT (___PSD ___SCMOBJ src, point *dst, int arg_num)
          {
            ___SCMOBJ ___err = ___FIX(___NO_ERR);
            if (!___PAIRP(src))
              ___err = ___FIX(___UNKNOWN_ERR);
            else
              {
                ___SCMOBJ car = ___CAR(src);
                ___SCMOBJ cdr = ___CDR(src);
                ___BEGIN_CFUN_SCMOBJ_TO_INT(car,dst->x,arg_num)
                ___BEGIN_CFUN_SCMOBJ_TO_INT(cdr,dst->y,arg_num)
                ___END_CFUN_SCMOBJ_TO_INT(cdr,dst->y,arg_num)
                ___END_CFUN_SCMOBJ_TO_INT(car,dst->x,arg_num)
              }
            return ___err;
          }

          ___SCMOBJ POINT_to_SCMOBJ (___processor_state ___ps, point src, ___SCMOBJ *dst, int arg_num)
          {
            ___SCMOBJ ___err = ___FIX(___NO_ERR);
            ___SCMOBJ x_scmobj;
            ___SCMOBJ y_scmobj;
            ___BEGIN_SFUN_INT_TO_SCMOBJ(src.x,x_scmobj,arg_num)
            ___BEGIN_SFUN_INT_TO_SCMOBJ(src.y,y_scmobj,arg_num)
            *dst = ___EXT(___make_pair) (___ps, x_scmobj, y_scmobj);
            if (___FIXNUMP(*dst))
              ___err = *dst; /* return allocation error */
            ___END_SFUN_INT_TO_SCMOBJ(src.y,y_scmobj,arg_num)
            ___END_SFUN_INT_TO_SCMOBJ(src.x,x_scmobj,arg_num)
            return ___err;
          }

          #define ___BEGIN_CFUN_SCMOBJ_to_POINT(src,dst,i) \
          if ((___err = SCMOBJ_to_POINT (___PSP src, &dst, i)) == ___FIX(___NO_ERR)) {
          #define ___END_CFUN_SCMOBJ_to_POINT(src,dst,i) }

          #define ___BEGIN_CFUN_POINT_to_SCMOBJ(src,dst) \
          if ((___err = POINT_to_SCMOBJ (___ps, src, &dst, ___RETURN_POS)) == ___FIX(___NO_ERR)) {
          #define ___END_CFUN_POINT_to_SCMOBJ(src,dst) \
          ___EXT(___release_scmobj) (dst); }

          #define ___BEGIN_SFUN_POINT_to_SCMOBJ(src,dst,i) \
          if ((___err = POINT_to_SCMOBJ (___ps, src, &dst, i)) == ___FIX(___NO_ERR)) {
          #define ___END_SFUN_POINT_to_SCMOBJ(src,dst,i) \
          ___EXT(___release_scmobj) (dst); }

          #define ___BEGIN_SFUN_SCMOBJ_to_POINT(src,dst) \
          { ___err = SCMOBJ_to_POINT (___PSP src, &dst, ___RETURN_POS);
          #define ___END_SFUN_SCMOBJ_to_POINT(src,dst) }

          c-declare-end
          )

          (c-define-type point "point" "POINT_to_SCMOBJ" "SCMOBJ_to_POINT" #f)

          (define line-to (c-lambda (point) void "line_to"))
          (define get-mouse (c-lambda () point "get_mouse"))
          (define add-points (c-lambda (point point) point "add_points"))

          (c-define (write-point p) (point) void "write_point" ""
            (write p))

          (c-define (read-point) () point "read_point" ""
            (read))

     Note that the pair is allocated using the ‘___make_pair’ runtime
     library function.  The prototype of this function is

          ___SCMOBJ ___make_pair(___processor_state ___ps, ___SCMOBJ car, ___SCMOBJ cdr);

     The fields of the pair are initialized to the ‘car’ and ‘cdr’
     parameters.  The ‘___ps’ parameter indicates how the pair is
     allocated.  A ‘NULL’ ‘___ps’ parameter will allocate the pair
     permanently (i.e.  the pair will only be deallocated when
     ‘___cleanup’ is called).  Otherwise a “still” object is allocated
     and the ‘___ps’ parameter indicates the processor in whose heap the
     object is allocated (this is to support multithreaded execution).
     Still objects are reference counted and initially have a reference
     count equal to 1.  The call to ‘___release_scmobj’ in the macros
     ‘___END_CFUN_POINT_to_SCMOBJ’ and ‘___END_SFUN_POINT_to_SCMOBJ’
     decrement this reference count.  A still object whose reference
     count is zero will be deallocated when a garbage collection is
     performed and there are no references to it from the Scheme world.
     Note that the use of the ‘___PSD’ macro in the parameter list of
     ‘SCMOBJ_to_POINT’ and the ‘___PSP’ macro in the calls of
     ‘SCMOBJ_to_POINT’, are necessary to propagate the current processor
     state to that function.

     An example that requires memory allocation when converting from C
     to Scheme and Scheme to C is shown below.  It is an interface to a
     “null-terminated array of strings” type which is represented in
     Scheme by a list of strings.  Note that some cleanup is necessary
     when converting from Scheme to C.

          (c-declare #<<c-declare-end

          #include <stdlib.h>
          #include <unistd.h>

          extern char **environ;

          char **get_environ (void) { return environ; }

          void free_strings (char **strings)
          {
            char **ptr = strings;
            while (*ptr != NULL)
              {
                ___EXT(___release_string) (*ptr);
                ptr++;
              }
            free (strings);
          }

          ___SCMOBJ SCMOBJ_to_STRINGS (___PSD ___SCMOBJ src, char ***dst, int arg_num)
          {
            /*
             * Src is a list of Scheme strings.  Dst will be a null terminated
             * array of C strings.
             */

            int i;
            ___SCMOBJ lst = src;
            int len = 4; /* start with a small result array */
            char **result = (char**) malloc (len * sizeof (char*));

            if (result == NULL)
              return ___FIX(___HEAP_OVERFLOW_ERR);

            i = 0;
            result[i] = NULL; /* always keep array null terminated */

            while (___PAIRP(lst))
              {
                ___SCMOBJ scm_str = ___CAR(lst);
                char *c_str;
                ___SCMOBJ ___err;

                if (i >= len-1) /* need to grow the result array? */
                  {
                    char **new_result;
                    int j;

                    len = len * 3 / 2;
                    new_result = (char**) malloc (len * sizeof (char*));
                    if (new_result == NULL)
                      {
                        free_strings (result);
                        return ___FIX(___HEAP_OVERFLOW_ERR);
                      }
                    for (j=i; j>=0; j--)
                      new_result[j] = result[j];
                    free (result);
                    result = new_result;
                  }

                ___err = ___EXT(___SCMOBJ_to_CHARSTRING) (___PSP scm_str, &c_str, arg_num);

                if (___err != ___FIX(___NO_ERR))
                  {
                    free_strings (result);
                    return ___err;
                  }

                result[i++] = c_str;
                result[i] = NULL;
                lst = ___CDR(lst);
              }

            if (!___NULLP(lst))
              {
                free_strings (result);
                return ___FIX(___UNKNOWN_ERR);
              }

            /*
             * Note that the caller is responsible for calling free_strings
             * when it is done with the result.
             */

            *dst = result;
            return ___FIX(___NO_ERR);
          }

          ___SCMOBJ STRINGS_to_SCMOBJ (___processor_state ___ps, char **src, ___SCMOBJ *dst, int arg_num)
          {
            ___SCMOBJ ___err = ___FIX(___NO_ERR);
            ___SCMOBJ result = ___NUL; /* start with the empty list */
            int i = 0;

            while (src[i] != NULL)
              i++;

            /* build the list of strings starting at the tail */

            while (--i >= 0)
              {
                ___SCMOBJ scm_str;
                ___SCMOBJ new_result;

                /*
                 * Invariant: result is either the empty list or a ___STILL pair
                 * with reference count equal to 1.  This is important because
                 * it is possible that ___CHARSTRING_to_SCMOBJ and ___make_pair
                 * will invoke the garbage collector and we don't want the
                 * reference in result to become invalid (which would be the
                 * case if result was a ___MOVABLE pair or if it had a zero
                 * reference count).
                 */

                ___err = ___EXT(___CHARSTRING_to_SCMOBJ) (___ps, src[i], &scm_str, arg_num);

                if (___err != ___FIX(___NO_ERR))
                  {
                    ___EXT(___release_scmobj) (result); /* allow GC to reclaim result */
                    return ___FIX(___UNKNOWN_ERR);
                  }

                /*
                 * Note that scm_str will be a ___STILL object with reference
                 * count equal to 1, so there is no risk that it will be
                 * reclaimed or moved if ___make_pair invokes the garbage
                 * collector.
                 */

                new_result = ___EXT(___make_pair) (___ps, scm_str, result);

                /*
                 * We can zero the reference count of scm_str and result (if
                 * not the empty list) because the pair now references these
                 * objects and the pair is reachable (it can't be reclaimed
                 * or moved by the garbage collector).
                 */

                ___EXT(___release_scmobj) (scm_str);
                ___EXT(___release_scmobj) (result);

                result = new_result;

                if (___FIXNUMP(result))
                  return result; /* allocation failed */
              }

            /*
             * Note that result is either the empty list or a ___STILL pair
             * with a reference count equal to 1.  There will be a call to
             * ___release_scmobj later on (in ___END_CFUN_STRINGS_to_SCMOBJ
             * or ___END_SFUN_STRINGS_to_SCMOBJ) that will allow the garbage
             * collector to reclaim the whole list of strings when the Scheme
             * world no longer references it.
             */

            *dst = result;
            return ___FIX(___NO_ERR);
          }

          #define ___BEGIN_CFUN_SCMOBJ_to_STRINGS(src,dst,i) \
          if ((___err = SCMOBJ_to_STRINGS (___PSP src, &dst, i)) == ___FIX(___NO_ERR)) {
          #define ___END_CFUN_SCMOBJ_to_STRINGS(src,dst,i) \
          free_strings (dst); }

          #define ___BEGIN_CFUN_STRINGS_to_SCMOBJ(src,dst) \
          if ((___err = STRINGS_to_SCMOBJ (___ps, src, &dst, ___RETURN_POS)) == ___FIX(___NO_ERR)) {
          #define ___END_CFUN_STRINGS_to_SCMOBJ(src,dst) \
          ___EXT(___release_scmobj) (dst); }

          #define ___BEGIN_SFUN_STRINGS_to_SCMOBJ(src,dst,i) \
          if ((___err = STRINGS_to_SCMOBJ (___ps, src, &dst, i)) == ___FIX(___NO_ERR)) {
          #define ___END_SFUN_STRINGS_to_SCMOBJ(src,dst,i) \
          ___EXT(___release_scmobj) (dst); }

          #define ___BEGIN_SFUN_SCMOBJ_to_STRINGS(src,dst) \
          { ___err = SCMOBJ_to_STRINGS (___PSP src, &dst, ___RETURN_POS);
          #define ___END_SFUN_SCMOBJ_to_STRINGS(src,dst) }

          c-declare-end
          )

          (c-define-type char** "char**" "STRINGS_to_SCMOBJ" "SCMOBJ_to_STRINGS" #t)

          (define execv (c-lambda (char-string char**) int "execv"))
          (define get-environ (c-lambda () char** "get_environ"))

          (c-define (write-strings x) (char**) void "write_strings" ""
            (write x))

          (c-define (read-strings) () char** "read_strings" ""
            (read))

16.7 Continuations, the C-interface and threads
===============================================

The C-interface allows C to Scheme calls to be nested.  This means that
during a call from C to Scheme another call from C to Scheme can be
performed.  This case occurs in the following program:

     (c-declare #<<c-declare-end

     int p (char *); /* forward declarations */
     int q (void);

     int a (char *x) { return 2 * p (x+1); }
     int b (short y) { return y + q (); }

     c-declare-end
     )

     (define a (c-lambda (char-string) int "a"))
     (define b (c-lambda (short) int "b"))

     (c-define (p z) (char-string) int "p" ""
       (+ (b 10) (string-length z)))

     (c-define (q) () int "q" ""
       123)

     (write (a "hello"))

   In this example, the main Scheme program calls the C function ‘a’
which calls the Scheme procedure ‘p’ which in turn calls the C function
‘b’ which finally calls the Scheme procedure ‘q’.

   Gambit maintains the Scheme continuation separately from the C stack,
thus allowing the Scheme continuation to be unwound independently from
the C stack.  The C stack frame created for the C function ‘f’ is only
removed from the C stack when control returns from ‘f’ or when control
returns to a C function “above” ‘f’.  Special care is required for
programs which escape to Scheme (using first-class continuations) from a
Scheme to C (to Scheme) call because the C stack frame will remain on
the stack.  The C stack may overflow if this happens in a loop with no
intervening return to a C function.  To avoid this problem make sure the
C stack gets cleaned up by executing a normal return from a Scheme to C
call.

   This approach to manage Scheme to C to Scheme calls may cause
problems when used with Scheme threads because context switching is
implemented with continuations.  If a Scheme thread T1 is in the middle
of a Scheme to C to Scheme call and a second thread T2 does a Scheme to
C to Scheme call and there is a Scheme thread context switch back to T1
which completes its call, the C stack frames of T2 will get removed,
preventing T2 (when it gets resumed) to complete its call correctly.
This situation can be avoided by having only one Scheme thread that does
Scheme to C to Scheme calls.  Other Scheme threads are limited to simple
Scheme to C calls that don’t call back to Scheme.

17 System limitations
*********************

   • On some systems floating point overflows will cause the program to
     terminate with a floating point exception.

   • On some systems floating point operations involving ‘+nan.0’
     ‘+inf.0’, ‘-inf.0’, or ‘-0.’ do not return the value required by
     the IEEE 754 floating point standard.

   • The maximum number of arguments that can be passed to a procedure
     by the ‘apply’ procedure is 8192.

18 Copyright and license
************************

The Gambit system release v4.9.4 is Copyright © 1994-2022 by Marc
Feeley, all rights reserved.  The Gambit system release v4.9.4 is
licensed under two licenses: the Apache License, Version 2.0, and the
GNU LESSER GENERAL PUBLIC LICENSE, Version 2.1.  You have the option to
choose which of these two licenses to abide by.  The licenses are copied
below.


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     That's all there is to it!

General index
*************

* Menu:

* #:                                     Procedures related to debugging.
                                                            (line  2369)
* ##:                                    Procedures related to debugging.
                                                            (line  2369)
* ##import:                              ##namespace and ##import forms.
                                                            (line  4747)
* ##include:                             Legacy Modules.    (line  4615)
* ##namespace:                           ##namespace and ##import forms.
                                                            (line  4745)
* +z:                                    Other compilation options.
                                                            (line  1388)
* ,(b EXPR):                             Debugging commands.
                                                            (line  2129)
* ,(be EXPR):                            Debugging commands.
                                                            (line  2136)
* ,(bed EXPR):                           Debugging commands.
                                                            (line  2139)
* ,(c EXPR):                             Debugging commands.
                                                            (line  2023)
* ,(e EXPR):                             Debugging commands.
                                                            (line  2177)
* ,(ed EXPR):                            Debugging commands.
                                                            (line  2189)
* ,(h SUBJECT):                          Debugging commands.
                                                            (line  1996)
* ,(st EXPR):                            Debugging commands.
                                                            (line  2200)
* ,(v EXPR):                             Debugging commands.
                                                            (line  2204)
* ,+:                                    Debugging commands.
                                                            (line  2083)
* ,++:                                   Debugging commands.
                                                            (line  2089)
* ,-:                                    Debugging commands.
                                                            (line  2086)
* ,--:                                   Debugging commands.
                                                            (line  2093)
* ,?:                                    Debugging commands.
                                                            (line  1993)
* ,b:                                    Debugging commands.
                                                            (line  2114)
* ,be:                                   Debugging commands.
                                                            (line  2123)
* ,bed:                                  Debugging commands.
                                                            (line  2126)
* ,c:                                    Debugging commands.
                                                            (line  2039)
* ,d:                                    Debugging commands.
                                                            (line  2019)
* ,e:                                    Debugging commands.
                                                            (line  2149)
* ,ed:                                   Debugging commands.
                                                            (line  2173)
* ,h:                                    Debugging commands.
                                                            (line  2003)
* ,help:                                 Debugging commands.
                                                            (line  1993)
* ,i:                                    Debugging commands.
                                                            (line  2143)
* ,l:                                    Debugging commands.
                                                            (line  2059)
* ,N:                                    Debugging commands.
                                                            (line  2066)
* ,N+:                                   Debugging commands.
                                                            (line  2071)
* ,N-:                                   Debugging commands.
                                                            (line  2077)
* ,q:                                    Debugging commands.
                                                            (line  2008)
* ,qt:                                   Debugging commands.
                                                            (line  2012)
* ,s:                                    Debugging commands.
                                                            (line  2045)
* ,st:                                   Debugging commands.
                                                            (line  2193)
* ,t:                                    Debugging commands.
                                                            (line  2016)
* ,y:                                    Debugging commands.
                                                            (line  2097)
* -:                                     GSI.               (line   148)
* - <1>:                                 GSI batch mode.    (line   213)
* - <2>:                                 GSC batch mode.    (line   930)
* -:+ARGUMENT:                           Runtime options.   (line  1803)
* -:,:                                   Runtime options.   (line  1894)
* -:-[IO...]:                            Runtime options.   (line  1806)
* -:0[IO...]:                            Runtime options.   (line  1806)
* -:1[IO...]:                            Runtime options.   (line  1806)
* -:2[IO...]:                            Runtime options.   (line  1806)
* -:add-arg=ARGUMENT:                    Runtime options.   (line  1803)
* -:ask-install=WHEN:                    Runtime options.   (line  1884)
* -:d:                                   Runtime options.   (line  1722)
* -:d$[INTF][:PORT]:                     Runtime options.   (line  1786)
* -:d+:                                  Runtime options.   (line  1776)
* -:d-:                                  Runtime options.   (line  1772)
* -:d@[HOST][:PORT]:                     Runtime options.   (line  1780)
* -:da:                                  Runtime options.   (line  1730)
* -:dc:                                  Runtime options.   (line  1769)
* -:dD:                                  Runtime options.   (line  1752)
* -:debug:                               Runtime options.   (line  1722)
* -:debug=$[INTF][:PORT]:                Runtime options.   (line  1786)
* -:debug=+:                             Runtime options.   (line  1776)
* -:debug=-:                             Runtime options.   (line  1772)
* -:debug=@[HOST][:PORT]:                Runtime options.   (line  1780)
* -:debug=a:                             Runtime options.   (line  1730)
* -:debug=c:                             Runtime options.   (line  1769)
* -:debug=D:                             Runtime options.   (line  1752)
* -:debug=LEVEL:                         Runtime options.   (line  1760)
* -:debug=p:                             Runtime options.   (line  1726)
* -:debug=q:                             Runtime options.   (line  1740)
* -:debug=Q:                             Runtime options.   (line  1756)
* -:debug=r:                             Runtime options.   (line  1733)
* -:debug=R:                             Runtime options.   (line  1744)
* -:debug=s:                             Runtime options.   (line  1736)
* -:dLEVEL:                              Runtime options.   (line  1760)
* -:dp:                                  Runtime options.   (line  1726)
* -:dq:                                  Runtime options.   (line  1740)
* -:dQ:                                  Runtime options.   (line  1756)
* -:dr:                                  Runtime options.   (line  1733)
* -:dR:                                  Runtime options.   (line  1744)
* -:ds:                                  Runtime options.   (line  1736)
* -:file-settings=[IO...]:               Runtime options.   (line  1806)
* -:f[IO...]:                            Runtime options.   (line  1806)
* -:gambit:                              Runtime options.   (line  1716)
* -:hSIZE:                               Runtime options.   (line  1701)
* -:io-settings=[IO...]:                 Runtime options.   (line  1806)
* -:i[IO...]:                            Runtime options.   (line  1806)
* -:live-ratio=RATIO:                    Runtime options.   (line  1708)
* -:lRATIO:                              Runtime options.   (line  1708)
* -:max-heap=SIZE:                       Runtime options.   (line  1701)
* -:min-heap=SIZE:                       Runtime options.   (line  1701)
* -:mSIZE:                               Runtime options.   (line  1701)
* -:r5rs:                                Runtime options.   (line  1716)
* -:r7rs:                                Runtime options.   (line  1716)
* -:s:                                   Runtime options.   (line  1716)
* -:S:                                   Runtime options.   (line  1716)
* -:s <1>:                               Miscellaneous extensions.
                                                            (line  3541)
* -:search=[DIR]:                        Runtime options.   (line  1872)
* -:stdio-settings=[IO...]:              Runtime options.   (line  1806)
* -:terminal-settings=[IO...]:           Runtime options.   (line  1806)
* -:t[IO...]:                            Runtime options.   (line  1806)
* -:whitelist=[SOURCE]:                  Runtime options.   (line  1877)
* -:~~NAME=DIRECTORY:                    Runtime options.   (line  1799)
* -c:                                    GSC batch mode.    (line   884)
* -call_shared:                          Other compilation options.
                                                            (line  1388)
* -cc:                                   GSC batch mode.    (line   776)
* -cc-options:                           GSC batch mode.    (line   788)
* -cfg:                                  GSC batch mode.    (line   844)
* -compactness LEVEL:                    GSC batch mode.    (line   953)
* -debug:                                GSC batch mode.    (line   859)
* -debug <1>:                            Miscellaneous extensions.
                                                            (line  3810)
* -debug-environments:                   GSC batch mode.    (line   871)
* -debug-environments <1>:               Miscellaneous extensions.
                                                            (line  3850)
* -debug-location:                       GSC batch mode.    (line   863)
* -debug-location <1>:                   Miscellaneous extensions.
                                                            (line  3824)
* -debug-source:                         GSC batch mode.    (line   867)
* -debug-source <1>:                     Miscellaneous extensions.
                                                            (line  3837)
* -dg:                                   GSC batch mode.    (line   852)
* -dynamic:                              GSC batch mode.    (line   884)
* -D___DYNAMIC:                          Building a loadable library.
                                                            (line  1185)
* -D___LIBRARY:                          Building a shared-library.
                                                            (line  1331)
* -D___PRIMAL:                           Building a shared-library.
                                                            (line  1331)
* -D___SHARED:                           Building a shared-library.
                                                            (line  1331)
* -D___SINGLE_HOST:                      Other compilation options.
                                                            (line  1369)
* -e:                                    GSI.               (line   148)
* -e <1>:                                GSI batch mode.    (line   213)
* -e <2>:                                GSC batch mode.    (line   932)
* -exe:                                  GSC batch mode.    (line   648)
* -exe <1>:                              GSC batch mode.    (line   884)
* -expansion:                            GSC batch mode.    (line   838)
* -f:                                    GSI.               (line   148)
* -f <1>:                                GSC customization. (line   605)
* -flat:                                 GSC batch mode.    (line   906)
* -fpic:                                 Other compilation options.
                                                            (line  1388)
* -fPIC:                                 Other compilation options.
                                                            (line  1388)
* -G:                                    Other compilation options.
                                                            (line  1388)
* -gvm:                                  GSC batch mode.    (line   841)
* -h:                                    GSI.               (line   148)
* -h <1>:                                GSC.               (line   586)
* -help:                                 GSI.               (line   148)
* -help <1>:                             GSC.               (line   586)
* -i:                                    GSI.               (line   148)
* -i <1>:                                GSC.               (line   591)
* -i <2>:                                GSC batch mode.    (line   755)
* -I/usr/local/Gambit/include:           Other compilation options.
                                                            (line  1380)
* -install:                              GSI.               (line   148)
* -install <1>:                          GSI package management mode.
                                                            (line   298)
* -install <2>:                          Installing Modules.
                                                            (line  5606)
* -keep-temp:                            GSC batch mode.    (line   884)
* -Kpic:                                 Other compilation options.
                                                            (line  1388)
* -KPIC:                                 Other compilation options.
                                                            (line  1388)
* -l BASE:                               GSC batch mode.    (line   913)
* -L/usr/local/Gambit/lib:               Other compilation options.
                                                            (line  1380)
* -ld-options:                           GSC batch mode.    (line   799)
* -ld-options-prelude:                   GSC batch mode.    (line   799)
* -link:                                 GSC batch mode.    (line   648)
* -link <1>:                             GSC batch mode.    (line   884)
* -nb-arg-regs N:                        GSC batch mode.    (line   944)
* -nb-gvm-regs N:                        GSC batch mode.    (line   935)
* -nopreload:                            GSC batch mode.    (line   920)
* -O:                                    Other compilation options.
                                                            (line  1369)
* -o OUTPUT:                             GSC batch mode.    (line   880)
* -obj:                                  GSC batch mode.    (line   884)
* -pic:                                  Other compilation options.
                                                            (line  1388)
* -pkg-config:                           GSC batch mode.    (line   813)
* -pkg-config-path:                      GSC batch mode.    (line   813)
* -postlude:                             GSC batch mode.    (line   769)
* -preload:                              GSC batch mode.    (line   920)
* -prelude:                              GSC batch mode.    (line   761)
* -rdynamic:                             Other compilation options.
                                                            (line  1388)
* -report:                               GSC batch mode.    (line   833)
* -shared:                               Other compilation options.
                                                            (line  1388)
* -target:                               GSC batch mode.    (line   758)
* -track-scheme:                         GSC batch mode.    (line   875)
* -uninstall:                            GSI.               (line   148)
* -uninstall <1>:                        GSI package management mode.
                                                            (line   298)
* -uninstall <2>:                        Installing Modules.
                                                            (line  5606)
* -update:                               GSI.               (line   148)
* -upgrade:                              GSI package management mode.
                                                            (line   298)
* -upgrade <1>:                          Installing Modules.
                                                            (line  5606)
* -v:                                    GSI.               (line   148)
* -v <1>:                                GSC.               (line   586)
* -verbose:                              GSC batch mode.    (line   830)
* -warnings:                             GSC batch mode.    (line   827)
* .c:                                    GSC batch mode.    (line   612)
* .js:                                   GSC batch mode.    (line   612)
* .scm:                                  GSC batch mode.    (line   612)
* .six:                                  GSC batch mode.    (line   612)
* .sld:                                  GSC batch mode.    (line   612)
* <:                                     Extensions to numeric procedures.
                                                            (line  5773)
* <=:                                    Extensions to numeric procedures.
                                                            (line  5775)
* =:                                     Extensions to numeric procedures.
                                                            (line  5772)
* >:                                     Extensions to numeric procedures.
                                                            (line  5774)
* >=:                                    Extensions to numeric procedures.
                                                            (line  5776)
* ^C:                                    Runtime options.   (line  1744)
* ^C <1>:                                Debugging model.   (line  1931)
* ^D:                                    Debugging model.   (line  1956)
* ___cleanup:                            c-define.          (line 15719)
* ___setup:                              c-define.          (line 15719)
* |six.x,y|:                             Undocumented extensions.
                                                            (line  4481)
* |six.x\|=y|:                           Undocumented extensions.
                                                            (line  4508)
* |six.x\|y|:                            Undocumented extensions.
                                                            (line  4509)
* |six.x\|\|y|:                          Undocumented extensions.
                                                            (line  4510)
* ~:                                     Handling of file names.
                                                            (line 10407)
* ~username:                             Handling of file names.
                                                            (line 10416)
* ~~:                                    Handling of file names.
                                                            (line 10397)
* abandoned-mutex-exception?:            Exception objects related to threads.
                                                            (line  9431)
* abort:                                 Exception-handling.
                                                            (line  9132)
* absolute path:                         Handling of file names.
                                                            (line 10383)
* absolute path <1>:                     Handling of file names.
                                                            (line 10457)
* acosh:                                 Undocumented extensions.
                                                            (line  4401)
* address-info-family:                   Host information.  (line 11504)
* address-info-protocol:                 Host information.  (line 11526)
* address-info-socket-info:              Host information.  (line 11537)
* address-info-socket-type:              Host information.  (line 11515)
* address-info?:                         Host information.  (line 11491)
* address-infos:                         Host information.  (line 11424)
* all-bits-set?:                         Bitwise-operations on exact integers.
                                                            (line  6099)
* allocation-limit:                      Miscellaneous extensions.
                                                            (line  3756)
* any-bits-set?:                         Bitwise-operations on exact integers.
                                                            (line  6084)
* append-reverse:                        Undocumented extensions.
                                                            (line  4383)
* append-reverse!:                       Undocumented extensions.
                                                            (line  4384)
* apropos:                               Procedures related to debugging.
                                                            (line  2312)
* arithmetic-shift:                      Bitwise-operations on exact integers.
                                                            (line  5845)
* asinh:                                 Undocumented extensions.
                                                            (line  4403)
* atanh:                                 Undocumented extensions.
                                                            (line  4405)
* bit-count:                             Bitwise-operations on exact integers.
                                                            (line  6027)
* bit-set?:                              Bitwise-operations on exact integers.
                                                            (line  6072)
* bits:                                  Undocumented extensions.
                                                            (line  4415)
* bits->list:                            Undocumented extensions.
                                                            (line  4416)
* bits->vector:                          Undocumented extensions.
                                                            (line  4418)
* bitwise-and:                           Bitwise-operations on exact integers.
                                                            (line  5876)
* bitwise-andc1:                         Bitwise-operations on exact integers.
                                                            (line  5892)
* bitwise-andc2:                         Bitwise-operations on exact integers.
                                                            (line  5904)
* bitwise-eqv:                           Bitwise-operations on exact integers.
                                                            (line  5916)
* bitwise-ior:                           Bitwise-operations on exact integers.
                                                            (line  5933)
* bitwise-merge:                         Bitwise-operations on exact integers.
                                                            (line  5860)
* bitwise-nand:                          Bitwise-operations on exact integers.
                                                            (line  5950)
* bitwise-nor:                           Bitwise-operations on exact integers.
                                                            (line  5962)
* bitwise-not:                           Bitwise-operations on exact integers.
                                                            (line  5974)
* bitwise-orc1:                          Bitwise-operations on exact integers.
                                                            (line  5986)
* bitwise-orc2:                          Bitwise-operations on exact integers.
                                                            (line  5998)
* bitwise-xor:                           Bitwise-operations on exact integers.
                                                            (line  6010)
* block:                                 Miscellaneous extensions.
                                                            (line  3720)
* box:                                   Miscellaneous extensions.
                                                            (line  3366)
* box?:                                  Miscellaneous extensions.
                                                            (line  3367)
* boxes:                                 Miscellaneous extensions.
                                                            (line  3370)
* break:                                 Procedures related to debugging.
                                                            (line  2556)
* c-declare:                             c-declare.         (line 15510)
* c-declare <1>:                         c-declare.         (line 15510)
* c-define:                              c-define.          (line 15679)
* c-define <1>:                          c-define.          (line 15679)
* c-define-type:                         c-define-type.     (line 15733)
* c-define-type <1>:                     c-define-type.     (line 15733)
* c-initialize:                          c-initialize.      (line 15553)
* c-initialize <1>:                      c-initialize.      (line 15553)
* c-lambda:                              c-lambda.          (line 15575)
* c-lambda <1>:                          c-lambda.          (line 15575)
* call-with-current-continuation:        Extensions to standard procedures.
                                                            (line  2930)
* call-with-input-file:                  Filesystem devices.
                                                            (line 12589)
* call-with-input-process:               Process devices.   (line 12732)
* call-with-input-string:                String-ports.      (line 13727)
* call-with-input-u8vector:              U8vector-ports.    (line 13784)
* call-with-input-vector:                Vector-ports.      (line 13577)
* call-with-output-file:                 Filesystem devices.
                                                            (line 12590)
* call-with-output-process:              Process devices.   (line 12733)
* call-with-output-string:               String-ports.      (line 13728)
* call-with-output-u8vector:             U8vector-ports.    (line 13785)
* call-with-output-vector:               Vector-ports.      (line 13578)
* call/cc:                               Extensions to standard procedures.
                                                            (line  2931)
* car+cdr:                               Undocumented extensions.
                                                            (line  4331)
* case-lambda:                           Undocumented extensions.
                                                            (line  4320)
* central installation directory:        Handling of file names.
                                                            (line 10383)
* cfun-conversion-exception-arguments:   Exception objects related to C-interface.
                                                            (line  9605)
* cfun-conversion-exception-code:        Exception objects related to C-interface.
                                                            (line  9606)
* cfun-conversion-exception-message:     Exception objects related to C-interface.
                                                            (line  9607)
* cfun-conversion-exception-procedure:   Exception objects related to C-interface.
                                                            (line  9604)
* cfun-conversion-exception?:            Exception objects related to C-interface.
                                                            (line  9603)
* char->integer:                         Extensions to character procedures.
                                                            (line  6656)
* char-ci<=?:                            Extensions to character procedures.
                                                            (line  6685)
* char-ci<?:                             Extensions to character procedures.
                                                            (line  6683)
* char-ci=?:                             Extensions to character procedures.
                                                            (line  6682)
* char-ci>=?:                            Extensions to character procedures.
                                                            (line  6686)
* char-ci>?:                             Extensions to character procedures.
                                                            (line  6684)
* char<=?:                               Extensions to character procedures.
                                                            (line  6680)
* char<?:                                Extensions to character procedures.
                                                            (line  6678)
* char=?:                                Extensions to character procedures.
                                                            (line  6677)
* char>=?:                               Extensions to character procedures.
                                                            (line  6681)
* char>?:                                Extensions to character procedures.
                                                            (line  6679)
* circular-list:                         Undocumented extensions.
                                                            (line  4371)
* circular-list?:                        Undocumented extensions.
                                                            (line  4350)
* clear-bit-field:                       Bitwise-operations on exact integers.
                                                            (line  6130)
* close-input-port:                      Object-port operations.
                                                            (line 12029)
* close-output-port:                     Object-port operations.
                                                            (line 12030)
* close-port:                            Object-port operations.
                                                            (line 12031)
* command-args:                          Undocumented extensions.
                                                            (line  4217)
* command-line:                          Scheme scripts.    (line   437)
* command-line <1>:                      Command line arguments.
                                                            (line 10769)
* command-name:                          Undocumented extensions.
                                                            (line  4216)
* compilation-target:                    Miscellaneous extensions.
                                                            (line  3567)
* compile-file:                          Procedures specific to compiler.
                                                            (line  1440)
* compile-file-to-target:                Procedures specific to compiler.
                                                            (line  1400)
* compiler:                              GSC.               (line   568)
* compiler options:                      GSC batch mode.    (line   656)
* concatenate:                           Undocumented extensions.
                                                            (line  4358)
* concatenate!:                          Undocumented extensions.
                                                            (line  4359)
* cond-expand:                           Miscellaneous extensions.
                                                            (line  3628)
* condition-variable-broadcast!:         Procedures.        (line  8727)
* condition-variable-name:               Procedures.        (line  8647)
* condition-variable-signal!:            Procedures.        (line  8672)
* condition-variable-specific:           Procedures.        (line  8655)
* condition-variable-specific-set!:      Procedures.        (line  8656)
* condition-variable?:                   Procedures.        (line  8622)
* configure-command-string:              Undocumented extensions.
                                                            (line  4277)
* conjugate:                             Undocumented extensions.
                                                            (line  4413)
* cons*:                                 Undocumented extensions.
                                                            (line  4373)
* console-port:                          Undocumented extensions.
                                                            (line  4241)
* constant-fold:                         Miscellaneous extensions.
                                                            (line  3771)
* continuation-capture:                  Miscellaneous extensions.
                                                            (line  3965)
* continuation-graft:                    Miscellaneous extensions.
                                                            (line  3966)
* continuation-return:                   Miscellaneous extensions.
                                                            (line  3967)
* continuation?:                         Miscellaneous extensions.
                                                            (line  3964)
* continuations:                         continuations.     (line 16191)
* copy-bit-field:                        Bitwise-operations on exact integers.
                                                            (line  6132)
* copy-file:                             Filesystem operations.
                                                            (line 10653)
* cosh:                                  Undocumented extensions.
                                                            (line  4407)
* cpu-time:                              Measuring time.    (line 10871)
* create-directory:                      Filesystem operations.
                                                            (line 10563)
* create-fifo:                           Filesystem operations.
                                                            (line 10596)
* create-link:                           Filesystem operations.
                                                            (line 10626)
* create-symbolic-link:                  Filesystem operations.
                                                            (line 10634)
* create-temporary-directory:            Filesystem operations.
                                                            (line 10564)
* current exception-handler:             Exception-handling.
                                                            (line  9046)
* current working directory:             Handling of file names.
                                                            (line 10383)
* current working directory <1>:         Handling of file names.
                                                            (line 10428)
* current-directory:                     Handling of file names.
                                                            (line 10422)
* current-error-port:                    Other procedures related to I/O.
                                                            (line 13820)
* current-exception-handler:             Exception-handling.
                                                            (line  9044)
* current-input-port:                    Other procedures related to I/O.
                                                            (line 13818)
* current-jiffy:                         Undocumented extensions.
                                                            (line  4206)
* current-output-port:                   Other procedures related to I/O.
                                                            (line 13819)
* current-processor:                     Undocumented extensions.
                                                            (line  4199)
* current-readtable:                     Other procedures related to I/O.
                                                            (line 13821)
* current-second:                        Undocumented extensions.
                                                            (line  4204)
* current-thread:                        Procedures.        (line  7919)
* current-time:                          Measuring time.    (line 10835)
* current-user-interrupt-handler:        Undocumented extensions.
                                                            (line  4243)
* datum->syntax:                         Undocumented extensions.
                                                            (line  4324)
* datum-parsing-exception-kind:          Exception objects related to the reader.
                                                            (line  9781)
* datum-parsing-exception-parameters:    Exception objects related to the reader.
                                                            (line  9782)
* datum-parsing-exception-readenv:       Exception objects related to the reader.
                                                            (line  9783)
* datum-parsing-exception?:              Exception objects related to the reader.
                                                            (line  9780)
* dead-end:                              Undocumented extensions.
                                                            (line  4303)
* deadlock-exception?:                   Exception objects related to threads.
                                                            (line  9410)
* debug:                                 GSC batch mode.    (line   859)
* debug <1>:                             Miscellaneous extensions.
                                                            (line  3810)
* debug-environments:                    GSC batch mode.    (line   871)
* debug-environments <1>:                Miscellaneous extensions.
                                                            (line  3850)
* debug-location:                        GSC batch mode.    (line   863)
* debug-location <1>:                    Miscellaneous extensions.
                                                            (line  3824)
* debug-source:                          GSC batch mode.    (line   867)
* debug-source <1>:                      Miscellaneous extensions.
                                                            (line  3837)
* declare:                               Miscellaneous extensions.
                                                            (line  3706)
* default-random-source:                 Pseudo random numbers.
                                                            (line  6419)
* default-user-interrupt-handler:        Undocumented extensions.
                                                            (line  4244)
* defer-user-interrupts:                 Undocumented extensions.
                                                            (line  4245)
* define:                                Extensions to standard special forms.
                                                            (line  2940)
* define-cond-expand-feature:            Miscellaneous extensions.
                                                            (line  3689)
* define-library:                        R7RS Compatible Modules.
                                                            (line  5348)
* define-macro:                          Miscellaneous extensions.
                                                            (line  3507)
* define-module-alias:                   Undocumented extensions.
                                                            (line  4316)
* define-record-type:                    Undocumented extensions.
                                                            (line  4307)
* define-structure:                      Records.           (line  7646)
* define-syntax:                         Miscellaneous extensions.
                                                            (line  3540)
* define-syntax <1>:                     Miscellaneous extensions.
                                                            (line  3541)
* define-type:                           Undocumented extensions.
                                                            (line  4308)
* define-type-of-thread:                 Undocumented extensions.
                                                            (line  4169)
* define-values:                         Undocumented extensions.
                                                            (line  4314)
* delete-directory:                      Filesystem operations.
                                                            (line 10665)
* delete-file:                           Filesystem operations.
                                                            (line 10660)
* delete-file-or-directory:              Filesystem operations.
                                                            (line 10670)
* deserialization:                       Homogeneous numeric vectors.
                                                            (line  6966)
* deserialization <1>:                   Readtables.        (line 14050)
* directory-files:                       Filesystem operations.
                                                            (line 10678)
* display-continuation-backtrace:        Miscellaneous extensions.
                                                            (line  4048)
* display-continuation-dynamic-environment: Miscellaneous extensions.
                                                            (line  4046)
* display-continuation-environment:      Miscellaneous extensions.
                                                            (line  4045)
* display-dynamic-environment?:          Procedures related to debugging.
                                                            (line  2664)
* display-environment-set!:              Procedures related to debugging.
                                                            (line  2644)
* display-exception:                     Miscellaneous extensions.
                                                            (line  4042)
* display-exception-in-context:          Miscellaneous extensions.
                                                            (line  4043)
* display-procedure-environment:         Miscellaneous extensions.
                                                            (line  4044)
* divide-by-zero-exception-arguments:    Exception objects related to type checking.
                                                            (line 10088)
* divide-by-zero-exception-procedure:    Exception objects related to type checking.
                                                            (line 10087)
* divide-by-zero-exception?:             Exception objects related to type checking.
                                                            (line 10086)
* dotted-list?:                          Undocumented extensions.
                                                            (line  4352)
* drop:                                  Undocumented extensions.
                                                            (line  4389)
* eighth:                                Undocumented extensions.
                                                            (line  4340)
* Emacs:                                 Emacs interface.   (line  2836)
* eq?-hash:                              Hashing.           (line  7133)
* equal?-hash:                           Hashing.           (line  7157)
* eqv?-hash:                             Hashing.           (line  7145)
* err-code->string:                      Undocumented extensions.
                                                            (line  4249)
* error:                                 Other exception objects.
                                                            (line 10337)
* error-exception-message:               Other exception objects.
                                                            (line 10335)
* error-exception-parameters:            Other exception objects.
                                                            (line 10336)
* error-exception?:                      Other exception objects.
                                                            (line 10334)
* eval:                                  Miscellaneous extensions.
                                                            (line  3489)
* executable-path:                       Undocumented extensions.
                                                            (line  4214)
* exit:                                  Process termination.
                                                            (line 10750)
* expression-parsing-exception-kind:     Exception objects related to evaluation and compilation.
                                                            (line  9857)
* expression-parsing-exception-parameters: Exception objects related to evaluation and compilation.
                                                            (line  9858)
* expression-parsing-exception-source:   Exception objects related to evaluation and compilation.
                                                            (line  9859)
* expression-parsing-exception?:         Exception objects related to evaluation and compilation.
                                                            (line  9856)
* extended-bindings:                     Miscellaneous extensions.
                                                            (line  3779)
* extract-bit-field:                     Bitwise-operations on exact integers.
                                                            (line  6128)
* f32vector:                             Homogeneous numeric vectors.
                                                            (line  6910)
* f32vector->list:                       Homogeneous numeric vectors.
                                                            (line  6915)
* f32vector-append:                      Homogeneous numeric vectors.
                                                            (line  6923)
* f32vector-concatenate:                 Homogeneous numeric vectors.
                                                            (line  6919)
* f32vector-copy:                        Homogeneous numeric vectors.
                                                            (line  6920)
* f32vector-copy!:                       Homogeneous numeric vectors.
                                                            (line  6921)
* f32vector-fill!:                       Homogeneous numeric vectors.
                                                            (line  6917)
* f32vector-length:                      Homogeneous numeric vectors.
                                                            (line  6911)
* f32vector-ref:                         Homogeneous numeric vectors.
                                                            (line  6912)
* f32vector-set:                         Homogeneous numeric vectors.
                                                            (line  6913)
* f32vector-set!:                        Homogeneous numeric vectors.
                                                            (line  6914)
* f32vector-shrink!:                     Homogeneous numeric vectors.
                                                            (line  6927)
* f32vector?:                            Homogeneous numeric vectors.
                                                            (line  6908)
* f64vector:                             Homogeneous numeric vectors.
                                                            (line  6931)
* f64vector->list:                       Homogeneous numeric vectors.
                                                            (line  6936)
* f64vector-append:                      Homogeneous numeric vectors.
                                                            (line  6944)
* f64vector-concatenate:                 Homogeneous numeric vectors.
                                                            (line  6940)
* f64vector-copy:                        Homogeneous numeric vectors.
                                                            (line  6941)
* f64vector-copy!:                       Homogeneous numeric vectors.
                                                            (line  6942)
* f64vector-fill!:                       Homogeneous numeric vectors.
                                                            (line  6938)
* f64vector-length:                      Homogeneous numeric vectors.
                                                            (line  6932)
* f64vector-ref:                         Homogeneous numeric vectors.
                                                            (line  6933)
* f64vector-set:                         Homogeneous numeric vectors.
                                                            (line  6934)
* f64vector-set!:                        Homogeneous numeric vectors.
                                                            (line  6935)
* f64vector-shrink!:                     Homogeneous numeric vectors.
                                                            (line  6948)
* f64vector?:                            Homogeneous numeric vectors.
                                                            (line  6929)
* FFI:                                   C-interface.       (line 15024)
* fifth:                                 Undocumented extensions.
                                                            (line  4337)
* file names:                            Handling of file names.
                                                            (line 10383)
* file-attributes:                       File information.  (line 11144)
* file-creation-time:                    File information.  (line 11145)
* file-device:                           File information.  (line 11134)
* file-exists-exception-arguments:       Exception objects related to the host environment.
                                                            (line  9292)
* file-exists-exception-procedure:       Exception objects related to the host environment.
                                                            (line  9291)
* file-exists-exception?:                Exception objects related to the host environment.
                                                            (line  9290)
* file-exists?:                          File information.  (line 10931)
* file-group:                            File information.  (line 11139)
* file-info:                             File information.  (line 10945)
* file-info-attributes:                  File information.  (line 11113)
* file-info-creation-time:               File information.  (line 11123)
* file-info-device:                      File information.  (line 11019)
* file-info-group:                       File information.  (line 11065)
* file-info-inode:                       File information.  (line 11028)
* file-info-last-access-time:            File information.  (line 11083)
* file-info-last-change-time:            File information.  (line 11103)
* file-info-last-modification-time:      File information.  (line 11093)
* file-info-mode:                        File information.  (line 11037)
* file-info-number-of-links:             File information.  (line 11046)
* file-info-owner:                       File information.  (line 11056)
* file-info-size:                        File information.  (line 11074)
* file-info-type:                        File information.  (line 10990)
* file-info?:                            File information.  (line 10978)
* file-inode:                            File information.  (line 11135)
* file-last-access-and-modification-times-set!: File information.
                                                            (line 11152)
* file-last-access-time:                 File information.  (line 11141)
* file-last-change-time:                 File information.  (line 11143)
* file-last-modification-time:           File information.  (line 11142)
* file-mode:                             File information.  (line 11136)
* file-number-of-links:                  File information.  (line 11137)
* file-owner:                            File information.  (line 11138)
* file-size:                             File information.  (line 11140)
* file-type:                             File information.  (line 11133)
* FILE.c:                                GSC batch mode.    (line   612)
* FILE.js:                               GSC batch mode.    (line   612)
* FILE.scm:                              GSC batch mode.    (line   612)
* FILE.six:                              GSC batch mode.    (line   612)
* FILE.sld:                              GSC batch mode.    (line   612)
* filter:                                Undocumented extensions.
                                                            (line  4354)
* finite?:                               Undocumented extensions.
                                                            (line  4397)
* first:                                 Undocumented extensions.
                                                            (line  4333)
* first-set-bit:                         Bitwise-operations on exact integers.
                                                            (line  6114)
* fixnum:                                Miscellaneous extensions.
                                                            (line  3916)
* fixnum->flonum:                        Flonum specific operations.
                                                            (line  6305)
* fixnum-overflow-exception-arguments:   Fixnum specific operations.
                                                            (line  6272)
* fixnum-overflow-exception-procedure:   Fixnum specific operations.
                                                            (line  6271)
* fixnum-overflow-exception?:            Fixnum specific operations.
                                                            (line  6270)
* fixnum?:                               Fixnum specific operations.
                                                            (line  6172)
* fl*:                                   Flonum specific operations.
                                                            (line  6307)
* fl+:                                   Flonum specific operations.
                                                            (line  6309)
* fl+*:                                  Flonum specific operations.
                                                            (line  6384)
* fl-:                                   Flonum specific operations.
                                                            (line  6311)
* fl/:                                   Flonum specific operations.
                                                            (line  6313)
* fl<:                                   Flonum specific operations.
                                                            (line  6315)
* fl<=:                                  Flonum specific operations.
                                                            (line  6317)
* fl=:                                   Flonum specific operations.
                                                            (line  6319)
* fl>:                                   Flonum specific operations.
                                                            (line  6321)
* fl>=:                                  Flonum specific operations.
                                                            (line  6323)
* flabs:                                 Flonum specific operations.
                                                            (line  6325)
* flacos:                                Flonum specific operations.
                                                            (line  6327)
* flacosh:                               Flonum specific operations.
                                                            (line  6386)
* flasin:                                Flonum specific operations.
                                                            (line  6329)
* flasinh:                               Flonum specific operations.
                                                            (line  6388)
* flatan:                                Flonum specific operations.
                                                            (line  6331)
* flatan <1>:                            Flonum specific operations.
                                                            (line  6332)
* flatanh:                               Flonum specific operations.
                                                            (line  6390)
* flceiling:                             Flonum specific operations.
                                                            (line  6334)
* flcos:                                 Flonum specific operations.
                                                            (line  6336)
* flcosh:                                Flonum specific operations.
                                                            (line  6392)
* fldenominator:                         Flonum specific operations.
                                                            (line  6338)
* fleven?:                               Flonum specific operations.
                                                            (line  6340)
* flexp:                                 Flonum specific operations.
                                                            (line  6342)
* flexpm1:                               Flonum specific operations.
                                                            (line  6394)
* flexpt:                                Flonum specific operations.
                                                            (line  6344)
* flfinite?:                             Flonum specific operations.
                                                            (line  6348)
* flfloor:                               Flonum specific operations.
                                                            (line  6350)
* flhypot:                               Flonum specific operations.
                                                            (line  6346)
* flilogb:                               Flonum specific operations.
                                                            (line  6396)
* flinfinite?:                           Flonum specific operations.
                                                            (line  6352)
* flinteger?:                            Flonum specific operations.
                                                            (line  6354)
* fllog:                                 Flonum specific operations.
                                                            (line  6356)
* fllog1p:                               Flonum specific operations.
                                                            (line  6398)
* flmax:                                 Flonum specific operations.
                                                            (line  6358)
* flmin:                                 Flonum specific operations.
                                                            (line  6360)
* flnan?:                                Flonum specific operations.
                                                            (line  6362)
* flnegative?:                           Flonum specific operations.
                                                            (line  6364)
* flnumerator:                           Flonum specific operations.
                                                            (line  6366)
* floating point overflow:               System limitations.
                                                            (line 16247)
* flodd?:                                Flonum specific operations.
                                                            (line  6368)
* flonum:                                Miscellaneous extensions.
                                                            (line  3916)
* flonum?:                               Flonum specific operations.
                                                            (line  6303)
* flpositive?:                           Flonum specific operations.
                                                            (line  6370)
* flround:                               Flonum specific operations.
                                                            (line  6372)
* flscalbn:                              Flonum specific operations.
                                                            (line  6400)
* flsin:                                 Flonum specific operations.
                                                            (line  6374)
* flsinh:                                Flonum specific operations.
                                                            (line  6402)
* flsqrt:                                Flonum specific operations.
                                                            (line  6376)
* flsquare:                              Flonum specific operations.
                                                            (line  6404)
* fltan:                                 Flonum specific operations.
                                                            (line  6378)
* fltanh:                                Flonum specific operations.
                                                            (line  6406)
* fltruncate:                            Flonum specific operations.
                                                            (line  6380)
* flzero?:                               Flonum specific operations.
                                                            (line  6382)
* fold:                                  Undocumented extensions.
                                                            (line  4366)
* fold-right:                            Undocumented extensions.
                                                            (line  4367)
* force-output:                          Object-port operations.
                                                            (line 11991)
* foreign function interface:            C-interface.       (line 15024)
* foreign-address:                       Undocumented extensions.
                                                            (line  4253)
* foreign-release!:                      Undocumented extensions.
                                                            (line  4254)
* foreign-released?:                     Undocumented extensions.
                                                            (line  4255)
* foreign-tags:                          Undocumented extensions.
                                                            (line  4252)
* foreign?:                              Undocumented extensions.
                                                            (line  4251)
* fourth:                                Undocumented extensions.
                                                            (line  4336)
* future:                                Undocumented extensions.
                                                            (line  4281)
* fx*:                                   Fixnum specific operations.
                                                            (line  6174)
* fx+:                                   Fixnum specific operations.
                                                            (line  6176)
* fx-:                                   Fixnum specific operations.
                                                            (line  6178)
* fx<:                                   Fixnum specific operations.
                                                            (line  6180)
* fx<=:                                  Fixnum specific operations.
                                                            (line  6182)
* fx=:                                   Fixnum specific operations.
                                                            (line  6184)
* fx>:                                   Fixnum specific operations.
                                                            (line  6186)
* fx>=:                                  Fixnum specific operations.
                                                            (line  6188)
* fxabs:                                 Fixnum specific operations.
                                                            (line  6190)
* fxand:                                 Fixnum specific operations.
                                                            (line  6192)
* fxandc1:                               Fixnum specific operations.
                                                            (line  6194)
* fxandc2:                               Fixnum specific operations.
                                                            (line  6196)
* fxarithmetic-shift:                    Fixnum specific operations.
                                                            (line  6198)
* fxarithmetic-shift-left:               Fixnum specific operations.
                                                            (line  6200)
* fxarithmetic-shift-right:              Fixnum specific operations.
                                                            (line  6202)
* fxbit-count:                           Fixnum specific operations.
                                                            (line  6204)
* fxbit-set?:                            Fixnum specific operations.
                                                            (line  6206)
* fxeqv:                                 Fixnum specific operations.
                                                            (line  6208)
* fxeven?:                               Fixnum specific operations.
                                                            (line  6210)
* fxfirst-set-bit:                       Fixnum specific operations.
                                                            (line  6212)
* fxif:                                  Fixnum specific operations.
                                                            (line  6214)
* fxior:                                 Fixnum specific operations.
                                                            (line  6216)
* fxlength:                              Fixnum specific operations.
                                                            (line  6218)
* fxmax:                                 Fixnum specific operations.
                                                            (line  6220)
* fxmin:                                 Fixnum specific operations.
                                                            (line  6222)
* fxmodulo:                              Fixnum specific operations.
                                                            (line  6224)
* fxnand:                                Fixnum specific operations.
                                                            (line  6228)
* fxnegative?:                           Fixnum specific operations.
                                                            (line  6226)
* fxnor:                                 Fixnum specific operations.
                                                            (line  6230)
* fxnot:                                 Fixnum specific operations.
                                                            (line  6232)
* fxodd?:                                Fixnum specific operations.
                                                            (line  6234)
* fxorc1:                                Fixnum specific operations.
                                                            (line  6236)
* fxorc2:                                Fixnum specific operations.
                                                            (line  6238)
* fxpositive?:                           Fixnum specific operations.
                                                            (line  6240)
* fxquotient:                            Fixnum specific operations.
                                                            (line  6242)
* fxremainder:                           Fixnum specific operations.
                                                            (line  6244)
* fxsquare:                              Fixnum specific operations.
                                                            (line  6266)
* fxwrap*:                               Fixnum specific operations.
                                                            (line  6246)
* fxwrap+:                               Fixnum specific operations.
                                                            (line  6248)
* fxwrap-:                               Fixnum specific operations.
                                                            (line  6250)
* fxwrapabs:                             Fixnum specific operations.
                                                            (line  6252)
* fxwraparithmetic-shift:                Fixnum specific operations.
                                                            (line  6254)
* fxwraparithmetic-shift-left:           Fixnum specific operations.
                                                            (line  6256)
* fxwraplogical-shift-right:             Fixnum specific operations.
                                                            (line  6258)
* fxwrapquotient:                        Fixnum specific operations.
                                                            (line  6260)
* fxwrapsquare:                          Fixnum specific operations.
                                                            (line  6268)
* fxxor:                                 Fixnum specific operations.
                                                            (line  6262)
* fxzero?:                               Fixnum specific operations.
                                                            (line  6264)
* Gambit:                                Top.               (line    28)
* Gambit <1>:                            The Gambit system. (line    33)
* gambit-scheme:                         Miscellaneous extensions.
                                                            (line  3715)
* gambit.el:                             Emacs interface.   (line  2836)
* GAMBOPT, environment variable:         Runtime options.   (line  1894)
* GC:                                    Procedures related to debugging.
                                                            (line  2701)
* gc-report-set!:                        Procedures related to debugging.
                                                            (line  2700)
* generate-proper-tail-calls:            Procedures related to debugging.
                                                            (line  2602)
* generative-lambda:                     Miscellaneous extensions.
                                                            (line  3877)
* generative-lambda <1>:                 Miscellaneous extensions.
                                                            (line  3877)
* generic:                               Miscellaneous extensions.
                                                            (line  3916)
* gensym:                                Miscellaneous extensions.
                                                            (line  3419)
* get-environment-variable:              Undocumented extensions.
                                                            (line  4210)
* get-environment-variables:             Undocumented extensions.
                                                            (line  4212)
* get-output-string:                     String-ports.      (line 13733)
* get-output-u8vector:                   U8vector-ports.    (line 13790)
* get-output-vector:                     Vector-ports.      (line 13702)
* getenv:                                Environment variables.
                                                            (line 10790)
* group-info:                            Group information. (line 11189)
* group-info-gid:                        Group information. (line 11233)
* group-info-members:                    Group information. (line 11243)
* group-info-name:                       Group information. (line 11223)
* group-info?:                           Group information. (line 11211)
* gsc:                                   The Gambit system. (line    33)
* gsc <1>:                               GSC batch mode.    (line   612)
* gsc <2>:                               Procedures specific to compiler.
                                                            (line  1402)
* gsc <3>:                               Procedures specific to compiler.
                                                            (line  1444)
* gsc <4>:                               Procedures specific to compiler.
                                                            (line  1519)
* gsc <5>:                               Procedures specific to compiler.
                                                            (line  1570)
* gsc <6>:                               Runtime options.   (line  1637)
* gsc-script:                            Scheme scripts.    (line   421)
* gsi:                                   The Gambit system. (line    33)
* gsi <1>:                               GSI.               (line   148)
* gsi <2>:                               Runtime options.   (line  1637)
* gsi-script:                            Scheme scripts.    (line   417)
* hashing:                               Hashing and weak references.
                                                            (line  7013)
* heap-overflow-exception?:              Exception objects related to memory management.
                                                            (line  9171)
* help:                                  Procedures related to debugging.
                                                            (line  2276)
* help-browser:                          Procedures related to debugging.
                                                            (line  2277)
* home directory:                        Handling of file names.
                                                            (line 10383)
* Homogeneous numeric vectors:           Homogeneous numeric vectors.
                                                            (line  6720)
* homogeneous vectors:                   Homogeneous vector syntax.
                                                            (line 14579)
* host-info:                             Host information.  (line 11361)
* host-info-addresses:                   Host information.  (line 11413)
* host-info-aliases:                     Host information.  (line 11403)
* host-info-name:                        Host information.  (line 11393)
* host-info?:                            Host information.  (line 11381)
* host-name:                             Host information.  (line 11352)
* identity:                              Miscellaneous extensions.
                                                            (line  3480)
* ieee-scheme:                           Miscellaneous extensions.
                                                            (line  3715)
* import:                                Miscellaneous extensions.
                                                            (line  3508)
* import <1>:                            ##namespace and ##import forms.
                                                            (line  4746)
* inactive-thread-exception-arguments:   Undocumented extensions.
                                                            (line  4183)
* inactive-thread-exception-procedure:   Undocumented extensions.
                                                            (line  4182)
* inactive-thread-exception?:            Undocumented extensions.
                                                            (line  4181)
* include:                               Miscellaneous extensions.
                                                            (line  3508)
* include <1>:                           Legacy Modules.    (line  4614)
* infinite?:                             Undocumented extensions.
                                                            (line  4398)
* initial current working directory:     Handling of file names.
                                                            (line 10424)
* initial-current-directory:             Handling of file names.
                                                            (line 10421)
* initialized-thread-exception-arguments: Undocumented extensions.
                                                            (line  4175)
* initialized-thread-exception-procedure: Undocumented extensions.
                                                            (line  4174)
* initialized-thread-exception?:         Undocumented extensions.
                                                            (line  4173)
* inline:                                Miscellaneous extensions.
                                                            (line  3727)
* inline-primitives:                     Miscellaneous extensions.
                                                            (line  3730)
* inlining-limit:                        Miscellaneous extensions.
                                                            (line  3734)
* input-port-byte-position:              Filesystem devices.
                                                            (line 12667)
* input-port-bytes-buffered:             Undocumented extensions.
                                                            (line  4228)
* input-port-char-position:              Undocumented extensions.
                                                            (line  4296)
* input-port-characters-buffered:        Undocumented extensions.
                                                            (line  4230)
* input-port-column:                     Character-port operations.
                                                            (line 12138)
* input-port-line:                       Character-port operations.
                                                            (line 12137)
* input-port-readtable:                  Character-port operations.
                                                            (line 12292)
* input-port-readtable-set!:             Character-port operations.
                                                            (line 12300)
* input-port-timeout-set!:               Object-port operations.
                                                            (line 12056)
* input-port?:                           Object-port operations.
                                                            (line 11900)
* installation directories:              Handling of file names.
                                                            (line 10383)
* integer->char:                         Extensions to character procedures.
                                                            (line  6657)
* integer-length:                        Bitwise-operations on exact integers.
                                                            (line  6049)
* integer-nth-root:                      Integer square root and nth root.
                                                            (line  5820)
* integer-sqrt:                          Integer square root and nth root.
                                                            (line  5810)
* interpreter:                           GSI.               (line   142)
* interpreter <1>:                       GSC.               (line   568)
* interrupts-enabled:                    Miscellaneous extensions.
                                                            (line  3797)
* invalid-hash-number-exception-arguments: Undocumented extensions.
                                                            (line  4259)
* invalid-hash-number-exception-procedure: Undocumented extensions.
                                                            (line  4258)
* invalid-hash-number-exception?:        Undocumented extensions.
                                                            (line  4257)
* invalid-utf8-encoding-exception-arguments: Undocumented extensions.
                                                            (line  4192)
* invalid-utf8-encoding-exception-procedure: Undocumented extensions.
                                                            (line  4191)
* invalid-utf8-encoding-exception?:      Undocumented extensions.
                                                            (line  4190)
* iota:                                  Undocumented extensions.
                                                            (line  4369)
* jiffies-per-second:                    Undocumented extensions.
                                                            (line  4208)
* join-timeout-exception-arguments:      Exception objects related to threads.
                                                            (line  9459)
* join-timeout-exception-procedure:      Exception objects related to threads.
                                                            (line  9458)
* join-timeout-exception?:               Exception objects related to threads.
                                                            (line  9457)
* keyword->string:                       Miscellaneous extensions.
                                                            (line  3395)
* keyword-expected-exception-arguments:  Exception objects related to procedure call.
                                                            (line 10303)
* keyword-expected-exception-procedure:  Exception objects related to procedure call.
                                                            (line 10302)
* keyword-expected-exception?:           Exception objects related to procedure call.
                                                            (line 10301)
* keyword-hash:                          Hashing.           (line  7097)
* keyword?:                              Miscellaneous extensions.
                                                            (line  3394)
* keywords:                              Miscellaneous extensions.
                                                            (line  3397)
* lambda:                                Extensions to standard special forms.
                                                            (line  2939)
* lambda-lift:                           Miscellaneous extensions.
                                                            (line  3768)
* last:                                  Undocumented extensions.
                                                            (line  4391)
* last-pair:                             Undocumented extensions.
                                                            (line  4392)
* LAST_.c:                               GSC batch mode.    (line   884)
* LAST_.js:                              GSC batch mode.    (line   884)
* length+:                               Undocumented extensions.
                                                            (line  4329)
* length-mismatch-exception-arg-id:      Exception objects related to type checking.
                                                            (line 10118)
* length-mismatch-exception-arguments:   Exception objects related to type checking.
                                                            (line 10117)
* length-mismatch-exception-procedure:   Exception objects related to type checking.
                                                            (line 10116)
* length-mismatch-exception?:            Exception objects related to type checking.
                                                            (line 10115)
* limitations:                           System limitations.
                                                            (line 16247)
* link-flat:                             Procedures specific to compiler.
                                                            (line  1568)
* link-incremental:                      Procedures specific to compiler.
                                                            (line  1517)
* list->bits:                            Undocumented extensions.
                                                            (line  4417)
* list->f32vector:                       Homogeneous numeric vectors.
                                                            (line  6916)
* list->f64vector:                       Homogeneous numeric vectors.
                                                            (line  6937)
* list->s16vector:                       Homogeneous numeric vectors.
                                                            (line  6790)
* list->s32vector:                       Homogeneous numeric vectors.
                                                            (line  6832)
* list->s64vector:                       Homogeneous numeric vectors.
                                                            (line  6874)
* list->s8vector:                        Homogeneous numeric vectors.
                                                            (line  6748)
* list->table:                           Tables.            (line  7534)
* list->u16vector:                       Homogeneous numeric vectors.
                                                            (line  6811)
* list->u32vector:                       Homogeneous numeric vectors.
                                                            (line  6853)
* list->u64vector:                       Homogeneous numeric vectors.
                                                            (line  6895)
* list->u8vector:                        Homogeneous numeric vectors.
                                                            (line  6769)
* list-copy:                             Undocumented extensions.
                                                            (line  4375)
* list-set:                              Undocumented extensions.
                                                            (line  4363)
* list-set!:                             Undocumented extensions.
                                                            (line  4364)
* list-sort:                             Undocumented extensions.
                                                            (line  4394)
* list-sort!:                            Undocumented extensions.
                                                            (line  4395)
* list-tabulate:                         Undocumented extensions.
                                                            (line  4377)
* list=:                                 Undocumented extensions.
                                                            (line  4361)
* load:                                  Procedures specific to compiler.
                                                            (line  1444)
* load <1>:                              Legacy Modules.    (line  4543)
* mailbox-receive-timeout-exception-arguments: Procedures.  (line  8355)
* mailbox-receive-timeout-exception-procedure: Procedures.  (line  8354)
* mailbox-receive-timeout-exception?:    Procedures.        (line  8353)
* main:                                  Undocumented extensions.
                                                            (line  4301)
* make-condition-variable:               Procedures.        (line  8634)
* make-f32vector:                        Homogeneous numeric vectors.
                                                            (line  6909)
* make-f64vector:                        Homogeneous numeric vectors.
                                                            (line  6930)
* make-list:                             Undocumented extensions.
                                                            (line  4379)
* make-mutex:                            Procedures.        (line  8397)
* make-parameter:                        Dynamic environment.
                                                            (line  8846)
* make-random-source:                    Pseudo random numbers.
                                                            (line  6480)
* make-root-thread:                      Procedures.        (line  7941)
* make-s16vector:                        Homogeneous numeric vectors.
                                                            (line  6783)
* make-s32vector:                        Homogeneous numeric vectors.
                                                            (line  6825)
* make-s64vector:                        Homogeneous numeric vectors.
                                                            (line  6867)
* make-s8vector:                         Homogeneous numeric vectors.
                                                            (line  6741)
* make-table:                            Tables.            (line  7321)
* make-thread:                           Procedures.        (line  7940)
* make-thread-group:                     Undocumented extensions.
                                                            (line  4134)
* make-tls-context:                      Network devices.   (line 13183)
* make-u16vector:                        Homogeneous numeric vectors.
                                                            (line  6804)
* make-u32vector:                        Homogeneous numeric vectors.
                                                            (line  6846)
* make-u64vector:                        Homogeneous numeric vectors.
                                                            (line  6888)
* make-u8vector:                         Homogeneous numeric vectors.
                                                            (line  6762)
* make-will:                             Wills.             (line  7201)
* module-not-found-exception-arguments:  Undocumented extensions.
                                                            (line  4196)
* module-not-found-exception-procedure:  Undocumented extensions.
                                                            (line  4195)
* module-not-found-exception?:           Undocumented extensions.
                                                            (line  4194)
* module-search-order-add!:              Installing Modules.
                                                            (line  5517)
* module-search-order-reset!:            Installing Modules.
                                                            (line  5516)
* module-whitelist-add!:                 Installing Modules.
                                                            (line  5549)
* module-whitelist-reset!:               Installing Modules.
                                                            (line  5548)
* mostly-fixnum:                         Miscellaneous extensions.
                                                            (line  3922)
* mostly-fixnum-flonum:                  Miscellaneous extensions.
                                                            (line  3922)
* mostly-flonum:                         Miscellaneous extensions.
                                                            (line  3922)
* mostly-flonum-fixnum:                  Miscellaneous extensions.
                                                            (line  3922)
* mostly-generic:                        Miscellaneous extensions.
                                                            (line  3922)
* multiple-c-return-exception?:          Exception objects related to C-interface.
                                                            (line  9724)
* mutex-lock!:                           Procedures.        (line  8479)
* mutex-name:                            Procedures.        (line  8412)
* mutex-specific:                        Procedures.        (line  8419)
* mutex-specific-set!:                   Procedures.        (line  8420)
* mutex-state:                           Procedures.        (line  8452)
* mutex-unlock!:                         Procedures.        (line  8572)
* mutex?:                                Procedures.        (line  8385)
* namespace:                             ##namespace and ##import forms.
                                                            (line  4744)
* nan?:                                  Undocumented extensions.
                                                            (line  4399)
* network-info:                          Network information.
                                                            (line 11710)
* network-info-aliases:                  Network information.
                                                            (line 11756)
* network-info-name:                     Network information.
                                                            (line 11746)
* network-info-number:                   Network information.
                                                            (line 11766)
* network-info?:                         Network information.
                                                            (line 11734)
* newline:                               Object-port operations.
                                                            (line 11969)
* ninth:                                 Undocumented extensions.
                                                            (line  4341)
* no-such-file-or-directory-exception-arguments: Exception objects related to the host environment.
                                                            (line  9260)
* no-such-file-or-directory-exception-procedure: Exception objects related to the host environment.
                                                            (line  9259)
* no-such-file-or-directory-exception?:  Exception objects related to the host environment.
                                                            (line  9258)
* noncontinuable-exception-reason:       Exception-handling.
                                                            (line  9134)
* noncontinuable-exception?:             Exception-handling.
                                                            (line  9133)
* nonempty-input-port-character-buffer-exception-arguments: Undocumented extensions.
                                                            (line  4233)
* nonempty-input-port-character-buffer-exception-procedure: Undocumented extensions.
                                                            (line  4235)
* nonempty-input-port-character-buffer-exception?: Undocumented extensions.
                                                            (line  4232)
* nonprocedure-operator-exception-arguments: Exception objects related to procedure call.
                                                            (line 10209)
* nonprocedure-operator-exception-code:  Exception objects related to procedure call.
                                                            (line 10210)
* nonprocedure-operator-exception-operator: Exception objects related to procedure call.
                                                            (line 10208)
* nonprocedure-operator-exception-rte:   Exception objects related to procedure call.
                                                            (line 10211)
* nonprocedure-operator-exception?:      Exception objects related to procedure call.
                                                            (line 10207)
* normalized path:                       Handling of file names.
                                                            (line 10485)
* not-in-compilation-context-exception-arguments: Exception objects related to evaluation and compilation.
                                                            (line  9971)
* not-in-compilation-context-exception-procedure: Exception objects related to evaluation and compilation.
                                                            (line  9970)
* not-in-compilation-context-exception?: Exception objects related to evaluation and compilation.
                                                            (line  9969)
* not-pair?:                             Undocumented extensions.
                                                            (line  4344)
* null-list?:                            Undocumented extensions.
                                                            (line  4346)
* number-of-arguments-limit-exception-arguments: Exception objects related to procedure call.
                                                            (line 10175)
* number-of-arguments-limit-exception-procedure: Exception objects related to procedure call.
                                                            (line 10174)
* number-of-arguments-limit-exception?:  Exception objects related to procedure call.
                                                            (line 10173)
* object file:                           Procedures specific to compiler.
                                                            (line  1444)
* object->serial-number:                 Hashing.           (line  7016)
* object->string:                        String-ports.      (line 13760)
* object->u8vector:                      Homogeneous numeric vectors.
                                                            (line  6964)
* open-directory:                        Directory-ports.   (line 13511)
* open-dummy:                            Undocumented extensions.
                                                            (line  4222)
* open-event-queue:                      Undocumented extensions.
                                                            (line  4299)
* open-file:                             Filesystem devices.
                                                            (line 12586)
* open-input-file:                       Filesystem devices.
                                                            (line 12587)
* open-input-process:                    Process devices.   (line 12730)
* open-input-string:                     String-ports.      (line 13725)
* open-input-u8vector:                   U8vector-ports.    (line 13782)
* open-input-vector:                     Vector-ports.      (line 13575)
* open-output-bytevector:                Undocumented extensions.
                                                            (line  4523)
* open-output-file:                      Filesystem devices.
                                                            (line 12588)
* open-output-process:                   Process devices.   (line 12731)
* open-output-string:                    String-ports.      (line 13726)
* open-output-u8vector:                  U8vector-ports.    (line 13783)
* open-output-vector:                    Vector-ports.      (line 13576)
* open-process:                          Process devices.   (line 12729)
* open-string:                           String-ports.      (line 13724)
* open-string-pipe:                      String-ports.      (line 13731)
* open-tcp-client:                       Network devices.   (line 12930)
* open-tcp-server:                       Network devices.   (line 13040)
* open-u8vector:                         U8vector-ports.    (line 13781)
* open-u8vector-pipe:                    U8vector-ports.    (line 13788)
* open-udp:                              Network devices.   (line 13294)
* open-vector:                           Vector-ports.      (line 13574)
* open-vector-pipe:                      Vector-ports.      (line 13657)
* optimize-dead-definitions:             Miscellaneous extensions.
                                                            (line  3902)
* optimize-dead-local-variables:         Miscellaneous extensions.
                                                            (line  3889)
* options, compiler:                     GSC batch mode.    (line   656)
* options, runtime:                      Runtime options.   (line  1637)
* os-exception-arguments:                Exception objects related to the host environment.
                                                            (line  9220)
* os-exception-code:                     Exception objects related to the host environment.
                                                            (line  9221)
* os-exception-message:                  Exception objects related to the host environment.
                                                            (line  9222)
* os-exception-procedure:                Exception objects related to the host environment.
                                                            (line  9219)
* os-exception?:                         Exception objects related to the host environment.
                                                            (line  9218)
* output-port-byte-position:             Filesystem devices.
                                                            (line 12668)
* output-port-char-position:             Undocumented extensions.
                                                            (line  4297)
* output-port-column:                    Character-port operations.
                                                            (line 12140)
* output-port-line:                      Character-port operations.
                                                            (line 12139)
* output-port-readtable:                 Character-port operations.
                                                            (line 12293)
* output-port-readtable-set!:            Character-port operations.
                                                            (line 12301)
* output-port-timeout-set!:              Object-port operations.
                                                            (line 12057)
* output-port-width:                     Character-port operations.
                                                            (line 12163)
* output-port?:                          Object-port operations.
                                                            (line 11901)
* overflow, floating point:              System limitations.
                                                            (line 16247)
* parameterize:                          Dynamic environment.
                                                            (line  8933)
* path-directory:                        Handling of file names.
                                                            (line 10515)
* path-expand:                           Handling of file names.
                                                            (line 10455)
* path-extension:                        Handling of file names.
                                                            (line 10513)
* path-normalize:                        Handling of file names.
                                                            (line 10483)
* path-strip-directory:                  Handling of file names.
                                                            (line 10516)
* path-strip-extension:                  Handling of file names.
                                                            (line 10514)
* path-strip-trailing-directory-separator: Handling of file names.
                                                            (line 10517)
* path-strip-volume:                     Handling of file names.
                                                            (line 10519)
* path-volume:                           Handling of file names.
                                                            (line 10518)
* peek-char:                             Character-port operations.
                                                            (line 12193)
* peek-u8:                               Byte-port operations.
                                                            (line 12496)
* permission-denied-exception-arguments: Exception objects related to the host environment.
                                                            (line  9326)
* permission-denied-exception-procedure: Exception objects related to the host environment.
                                                            (line  9325)
* permission-denied-exception?:          Exception objects related to the host environment.
                                                            (line  9324)
* poll-on-return:                        Miscellaneous extensions.
                                                            (line  3803)
* poll-point:                            Undocumented extensions.
                                                            (line  4305)
* port-io-exception-handler-set!:        Undocumented extensions.
                                                            (line  4226)
* port-settings-set!:                    Undocumented extensions.
                                                            (line  4224)
* port?:                                 Object-port operations.
                                                            (line 11902)
* pp:                                    Procedures related to debugging.
                                                            (line  2684)
* pretty-print:                          Procedures related to debugging.
                                                            (line  2671)
* primordial-exception-handler:          Undocumented extensions.
                                                            (line  4247)
* print:                                 Other procedures related to I/O.
                                                            (line 13827)
* println:                               Other procedures related to I/O.
                                                            (line 13828)
* process-pid:                           Process devices.   (line 12861)
* process-status:                        Process devices.   (line 12872)
* process-times:                         Measuring time.    (line 10870)
* processor-id:                          Undocumented extensions.
                                                            (line  4200)
* processor?:                            Undocumented extensions.
                                                            (line  4198)
* proper tail-calls:                     Procedures related to debugging.
                                                            (line  2604)
* proper tail-calls <1>:                 Miscellaneous extensions.
                                                            (line  3864)
* proper-list?:                          Undocumented extensions.
                                                            (line  4348)
* proper-tail-calls:                     Miscellaneous extensions.
                                                            (line  3864)
* protocol-info:                         Protocol information.
                                                            (line 11647)
* protocol-info-aliases:                 Protocol information.
                                                            (line 11687)
* protocol-info-name:                    Protocol information.
                                                            (line 11677)
* protocol-info-number:                  Protocol information.
                                                            (line 11697)
* protocol-info?:                        Protocol information.
                                                            (line 11665)
* r4rs-scheme:                           Miscellaneous extensions.
                                                            (line  3715)
* r5rs-scheme:                           Miscellaneous extensions.
                                                            (line  3715)
* r7rs-guard:                            Undocumented extensions.
                                                            (line  4318)
* raise:                                 Exception-handling.
                                                            (line  9115)
* random-f64vector:                      Pseudo random numbers.
                                                            (line  6467)
* random-integer:                        Pseudo random numbers.
                                                            (line  6425)
* random-real:                           Pseudo random numbers.
                                                            (line  6441)
* random-source-make-f64vectors:         Pseudo random numbers.
                                                            (line  6611)
* random-source-make-integers:           Pseudo random numbers.
                                                            (line  6558)
* random-source-make-reals:              Pseudo random numbers.
                                                            (line  6575)
* random-source-make-u8vectors:          Pseudo random numbers.
                                                            (line  6594)
* random-source-pseudo-randomize!:       Pseudo random numbers.
                                                            (line  6525)
* random-source-randomize!:              Pseudo random numbers.
                                                            (line  6524)
* random-source-state-ref:               Pseudo random numbers.
                                                            (line  6504)
* random-source-state-set!:              Pseudo random numbers.
                                                            (line  6505)
* random-source?:                        Pseudo random numbers.
                                                            (line  6492)
* random-u8vector:                       Pseudo random numbers.
                                                            (line  6454)
* range-exception-arg-id:                Exception objects related to type checking.
                                                            (line 10053)
* range-exception-arguments:             Exception objects related to type checking.
                                                            (line 10052)
* range-exception-procedure:             Exception objects related to type checking.
                                                            (line 10051)
* range-exception?:                      Exception objects related to type checking.
                                                            (line 10050)
* read:                                  Object-port operations.
                                                            (line 11924)
* read-all:                              Object-port operations.
                                                            (line 11938)
* read-char:                             Character-port operations.
                                                            (line 12174)
* read-file-string:                      Other procedures related to I/O.
                                                            (line 13855)
* read-file-string-list:                 Other procedures related to I/O.
                                                            (line 13856)
* read-file-u8vector:                    Other procedures related to I/O.
                                                            (line 13857)
* read-line:                             Character-port operations.
                                                            (line 12221)
* read-substring:                        Character-port operations.
                                                            (line 12256)
* read-subu8vector:                      Byte-port operations.
                                                            (line 12538)
* read-u8:                               Byte-port operations.
                                                            (line 12495)
* readtable-case-conversion?:            Readtables.        (line 13958)
* readtable-case-conversion?-set:        Readtables.        (line 13959)
* readtable-comment-handler:             Undocumented extensions.
                                                            (line  4520)
* readtable-comment-handler-set:         Undocumented extensions.
                                                            (line  4521)
* readtable-eval-allowed?:               Readtables.        (line 14199)
* readtable-eval-allowed?-set:           Readtables.        (line 14200)
* readtable-keywords-allowed?:           Readtables.        (line 14005)
* readtable-keywords-allowed?-set:       Readtables.        (line 14006)
* readtable-max-unescaped-char:          Readtables.        (line 14343)
* readtable-max-unescaped-char-set:      Readtables.        (line 14344)
* readtable-max-write-length:            Readtables.        (line 14310)
* readtable-max-write-length-set:        Readtables.        (line 14311)
* readtable-max-write-level:             Readtables.        (line 14276)
* readtable-max-write-level-set:         Readtables.        (line 14277)
* readtable-sharing-allowed?:            Readtables.        (line 14048)
* readtable-sharing-allowed?-set:        Readtables.        (line 14049)
* readtable-start-syntax:                Readtables.        (line 14384)
* readtable-start-syntax-set:            Readtables.        (line 14385)
* readtable-write-cdr-read-macros?:      Readtables.        (line 14223)
* readtable-write-cdr-read-macros?-set:  Readtables.        (line 14224)
* readtable-write-extended-read-macros?: Readtables.        (line 14225)
* readtable-write-extended-read-macros?-set: Readtables.    (line 14226)
* readtable?:                            Readtables.        (line 13946)
* real-time:                             Measuring time.    (line 10872)
* receive:                               Undocumented extensions.
                                                            (line  4312)
* relative path:                         Handling of file names.
                                                            (line 10383)
* relative path <1>:                     Handling of file names.
                                                            (line 10457)
* remove:                                Undocumented extensions.
                                                            (line  4355)
* remq:                                  Undocumented extensions.
                                                            (line  4356)
* rename-file:                           Filesystem operations.
                                                            (line 10642)
* repl-display-environment?:             Procedures related to debugging.
                                                            (line  2655)
* repl-error-port:                       Undocumented extensions.
                                                            (line  4240)
* repl-input-port:                       Undocumented extensions.
                                                            (line  4238)
* repl-output-port:                      Undocumented extensions.
                                                            (line  4239)
* repl-result-history-max-length-set!:   Procedures related to debugging.
                                                            (line  2367)
* repl-result-history-ref:               Procedures related to debugging.
                                                            (line  2366)
* replace-bit-field:                     Bitwise-operations on exact integers.
                                                            (line  6131)
* reverse!:                              Undocumented extensions.
                                                            (line  4381)
* rpc-remote-error-exception-arguments:  Undocumented extensions.
                                                            (line  4187)
* rpc-remote-error-exception-message:    Undocumented extensions.
                                                            (line  4188)
* rpc-remote-error-exception-procedure:  Undocumented extensions.
                                                            (line  4186)
* rpc-remote-error-exception?:           Undocumented extensions.
                                                            (line  4185)
* run-time-bindings:                     Miscellaneous extensions.
                                                            (line  3784)
* runtime options:                       Runtime options.   (line  1637)
* s16vector:                             Homogeneous numeric vectors.
                                                            (line  6784)
* s16vector->list:                       Homogeneous numeric vectors.
                                                            (line  6789)
* s16vector-append:                      Homogeneous numeric vectors.
                                                            (line  6797)
* s16vector-concatenate:                 Homogeneous numeric vectors.
                                                            (line  6793)
* s16vector-copy:                        Homogeneous numeric vectors.
                                                            (line  6794)
* s16vector-copy!:                       Homogeneous numeric vectors.
                                                            (line  6795)
* s16vector-fill!:                       Homogeneous numeric vectors.
                                                            (line  6791)
* s16vector-length:                      Homogeneous numeric vectors.
                                                            (line  6785)
* s16vector-ref:                         Homogeneous numeric vectors.
                                                            (line  6786)
* s16vector-set:                         Homogeneous numeric vectors.
                                                            (line  6787)
* s16vector-set!:                        Homogeneous numeric vectors.
                                                            (line  6788)
* s16vector-shrink!:                     Homogeneous numeric vectors.
                                                            (line  6801)
* s16vector?:                            Homogeneous numeric vectors.
                                                            (line  6782)
* s32vector:                             Homogeneous numeric vectors.
                                                            (line  6826)
* s32vector->list:                       Homogeneous numeric vectors.
                                                            (line  6831)
* s32vector-append:                      Homogeneous numeric vectors.
                                                            (line  6839)
* s32vector-concatenate:                 Homogeneous numeric vectors.
                                                            (line  6835)
* s32vector-copy:                        Homogeneous numeric vectors.
                                                            (line  6836)
* s32vector-copy!:                       Homogeneous numeric vectors.
                                                            (line  6837)
* s32vector-fill!:                       Homogeneous numeric vectors.
                                                            (line  6833)
* s32vector-length:                      Homogeneous numeric vectors.
                                                            (line  6827)
* s32vector-ref:                         Homogeneous numeric vectors.
                                                            (line  6828)
* s32vector-set:                         Homogeneous numeric vectors.
                                                            (line  6829)
* s32vector-set!:                        Homogeneous numeric vectors.
                                                            (line  6830)
* s32vector-shrink!:                     Homogeneous numeric vectors.
                                                            (line  6843)
* s32vector?:                            Homogeneous numeric vectors.
                                                            (line  6824)
* s64vector:                             Homogeneous numeric vectors.
                                                            (line  6868)
* s64vector->list:                       Homogeneous numeric vectors.
                                                            (line  6873)
* s64vector-append:                      Homogeneous numeric vectors.
                                                            (line  6881)
* s64vector-concatenate:                 Homogeneous numeric vectors.
                                                            (line  6877)
* s64vector-copy:                        Homogeneous numeric vectors.
                                                            (line  6878)
* s64vector-copy!:                       Homogeneous numeric vectors.
                                                            (line  6879)
* s64vector-fill!:                       Homogeneous numeric vectors.
                                                            (line  6875)
* s64vector-length:                      Homogeneous numeric vectors.
                                                            (line  6869)
* s64vector-ref:                         Homogeneous numeric vectors.
                                                            (line  6870)
* s64vector-set:                         Homogeneous numeric vectors.
                                                            (line  6871)
* s64vector-set!:                        Homogeneous numeric vectors.
                                                            (line  6872)
* s64vector-shrink!:                     Homogeneous numeric vectors.
                                                            (line  6885)
* s64vector?:                            Homogeneous numeric vectors.
                                                            (line  6866)
* s8vector:                              Homogeneous numeric vectors.
                                                            (line  6742)
* s8vector->list:                        Homogeneous numeric vectors.
                                                            (line  6747)
* s8vector-append:                       Homogeneous numeric vectors.
                                                            (line  6755)
* s8vector-concatenate:                  Homogeneous numeric vectors.
                                                            (line  6751)
* s8vector-copy:                         Homogeneous numeric vectors.
                                                            (line  6752)
* s8vector-copy!:                        Homogeneous numeric vectors.
                                                            (line  6753)
* s8vector-fill!:                        Homogeneous numeric vectors.
                                                            (line  6749)
* s8vector-length:                       Homogeneous numeric vectors.
                                                            (line  6743)
* s8vector-ref:                          Homogeneous numeric vectors.
                                                            (line  6744)
* s8vector-set:                          Homogeneous numeric vectors.
                                                            (line  6745)
* s8vector-set!:                         Homogeneous numeric vectors.
                                                            (line  6746)
* s8vector-shrink!:                      Homogeneous numeric vectors.
                                                            (line  6759)
* s8vector?:                             Homogeneous numeric vectors.
                                                            (line  6740)
* safe:                                  Miscellaneous extensions.
                                                            (line  3790)
* scheduler-exception-reason:            Exception objects related to threads.
                                                            (line  9396)
* scheduler-exception?:                  Exception objects related to threads.
                                                            (line  9395)
* Scheme:                                The Gambit system. (line    33)
* Scheme, implementation of:             Top.               (line    28)
* scheme-ieee-1178-1990:                 Scheme scripts.    (line   408)
* scheme-r4rs:                           Scheme scripts.    (line   400)
* scheme-r5rs:                           Scheme scripts.    (line   404)
* scheme-srfi-0:                         Scheme scripts.    (line   412)
* script-directory:                      Undocumented extensions.
                                                            (line  4220)
* script-file:                           Undocumented extensions.
                                                            (line  4219)
* second:                                Undocumented extensions.
                                                            (line  4334)
* seconds->time:                         Measuring time.    (line 10838)
* separate:                              Miscellaneous extensions.
                                                            (line  3720)
* serial-number->object:                 Hashing.           (line  7017)
* serialization:                         Homogeneous numeric vectors.
                                                            (line  6966)
* serialization <1>:                     Readtables.        (line 14050)
* service-info:                          Service information.
                                                            (line 11566)
* service-info-aliases:                  Service information.
                                                            (line 11614)
* service-info-name:                     Service information.
                                                            (line 11604)
* service-info-port-number:              Service information.
                                                            (line 11624)
* service-info-protocol:                 Service information.
                                                            (line 11634)
* service-info?:                         Service information.
                                                            (line 11592)
* set-box!:                              Miscellaneous extensions.
                                                            (line  3369)
* setenv:                                Environment variables.
                                                            (line 10791)
* seventh:                               Undocumented extensions.
                                                            (line  4339)
* sfun-conversion-exception-arguments:   Exception objects related to C-interface.
                                                            (line  9668)
* sfun-conversion-exception-code:        Exception objects related to C-interface.
                                                            (line  9669)
* sfun-conversion-exception-message:     Exception objects related to C-interface.
                                                            (line  9670)
* sfun-conversion-exception-procedure:   Exception objects related to C-interface.
                                                            (line  9667)
* sfun-conversion-exception?:            Exception objects related to C-interface.
                                                            (line  9666)
* shell-command:                         Shell command execution.
                                                            (line 10718)
* sinh:                                  Undocumented extensions.
                                                            (line  4409)
* six-script:                            Scheme scripts.    (line   425)
* six.!:                                 Undocumented extensions.
                                                            (line  4422)
* six.!x:                                Undocumented extensions.
                                                            (line  4423)
* six.&x:                                Undocumented extensions.
                                                            (line  4424)
* six.**x:                               Undocumented extensions.
                                                            (line  4425)
* six.*x:                                Undocumented extensions.
                                                            (line  4426)
* six.++x:                               Undocumented extensions.
                                                            (line  4427)
* six.+x:                                Undocumented extensions.
                                                            (line  4428)
* six.--x:                               Undocumented extensions.
                                                            (line  4429)
* six.-x:                                Undocumented extensions.
                                                            (line  4430)
* six.arrow:                             Undocumented extensions.
                                                            (line  4431)
* six.asyncx:                            Undocumented extensions.
                                                            (line  4432)
* six.awaitx:                            Undocumented extensions.
                                                            (line  4433)
* six.break:                             Undocumented extensions.
                                                            (line  4434)
* six.call:                              Undocumented extensions.
                                                            (line  4435)
* six.case:                              Undocumented extensions.
                                                            (line  4436)
* six.clause:                            Undocumented extensions.
                                                            (line  4437)
* six.compound:                          Undocumented extensions.
                                                            (line  4438)
* six.cons:                              Undocumented extensions.
                                                            (line  4439)
* six.continue:                          Undocumented extensions.
                                                            (line  4440)
* six.define-procedure:                  Undocumented extensions.
                                                            (line  4441)
* six.define-variable:                   Undocumented extensions.
                                                            (line  4442)
* six.do-while:                          Undocumented extensions.
                                                            (line  4443)
* six.dot:                               Undocumented extensions.
                                                            (line  4444)
* six.for:                               Undocumented extensions.
                                                            (line  4445)
* six.from-import:                       Undocumented extensions.
                                                            (line  4447)
* six.from-import-*:                     Undocumented extensions.
                                                            (line  4448)
* six.goto:                              Undocumented extensions.
                                                            (line  4446)
* six.identifier:                        Undocumented extensions.
                                                            (line  4449)
* six.if:                                Undocumented extensions.
                                                            (line  4450)
* six.import:                            Undocumented extensions.
                                                            (line  4451)
* six.index:                             Undocumented extensions.
                                                            (line  4452)
* six.infix:                             Undocumented extensions.
                                                            (line  4421)
* six.label:                             Undocumented extensions.
                                                            (line  4453)
* six.list:                              Undocumented extensions.
                                                            (line  4454)
* six.literal:                           Undocumented extensions.
                                                            (line  4455)
* six.make-array:                        Undocumented extensions.
                                                            (line  4456)
* six.new:                               Undocumented extensions.
                                                            (line  4457)
* six.notx:                              Undocumented extensions.
                                                            (line  4515)
* six.null:                              Undocumented extensions.
                                                            (line  4458)
* six.procedure:                         Undocumented extensions.
                                                            (line  4459)
* six.procedure-body:                    Undocumented extensions.
                                                            (line  4460)
* six.return:                            Undocumented extensions.
                                                            (line  4461)
* six.switch:                            Undocumented extensions.
                                                            (line  4462)
* six.typeofx:                           Undocumented extensions.
                                                            (line  4463)
* six.while:                             Undocumented extensions.
                                                            (line  4464)
* six.x!==y:                             Undocumented extensions.
                                                            (line  4465)
* six.x!=y:                              Undocumented extensions.
                                                            (line  4466)
* six.x%=y:                              Undocumented extensions.
                                                            (line  4467)
* six.x%y:                               Undocumented extensions.
                                                            (line  4468)
* six.x&&y:                              Undocumented extensions.
                                                            (line  4469)
* six.x&=y:                              Undocumented extensions.
                                                            (line  4470)
* six.x&y:                               Undocumented extensions.
                                                            (line  4471)
* six.x**=y:                             Undocumented extensions.
                                                            (line  4472)
* six.x**y:                              Undocumented extensions.
                                                            (line  4473)
* six.x*=y:                              Undocumented extensions.
                                                            (line  4474)
* six.x*y:                               Undocumented extensions.
                                                            (line  4475)
* six.x++:                               Undocumented extensions.
                                                            (line  4478)
* six.x+=y:                              Undocumented extensions.
                                                            (line  4479)
* six.x+y:                               Undocumented extensions.
                                                            (line  4480)
* six.x--:                               Undocumented extensions.
                                                            (line  4482)
* six.x-=y:                              Undocumented extensions.
                                                            (line  4483)
* six.x-y:                               Undocumented extensions.
                                                            (line  4484)
* six.x//=y:                             Undocumented extensions.
                                                            (line  4485)
* six.x//y:                              Undocumented extensions.
                                                            (line  4486)
* six.x/=y:                              Undocumented extensions.
                                                            (line  4487)
* six.x/y:                               Undocumented extensions.
                                                            (line  4488)
* six.x:-y:                              Undocumented extensions.
                                                            (line  4489)
* six.x:=y:                              Undocumented extensions.
                                                            (line  4490)
* six.x:y:                               Undocumented extensions.
                                                            (line  4491)
* six.x<<=y:                             Undocumented extensions.
                                                            (line  4492)
* six.x<<y:                              Undocumented extensions.
                                                            (line  4493)
* six.x<=y:                              Undocumented extensions.
                                                            (line  4494)
* six.x<y:                               Undocumented extensions.
                                                            (line  4495)
* six.x===y:                             Undocumented extensions.
                                                            (line  4496)
* six.x==y:                              Undocumented extensions.
                                                            (line  4497)
* six.x=y:                               Undocumented extensions.
                                                            (line  4498)
* six.x>=y:                              Undocumented extensions.
                                                            (line  4499)
* six.x>>=y:                             Undocumented extensions.
                                                            (line  4502)
* six.x>>>=y:                            Undocumented extensions.
                                                            (line  4500)
* six.x>>>y:                             Undocumented extensions.
                                                            (line  4501)
* six.x>>y:                              Undocumented extensions.
                                                            (line  4503)
* six.x>y:                               Undocumented extensions.
                                                            (line  4504)
* six.x?y:z:                             Undocumented extensions.
                                                            (line  4505)
* six.x@=y:                              Undocumented extensions.
                                                            (line  4476)
* six.x@y:                               Undocumented extensions.
                                                            (line  4477)
* six.xandy:                             Undocumented extensions.
                                                            (line  4511)
* six.xinstanceofy:                      Undocumented extensions.
                                                            (line  4512)
* six.xiny:                              Undocumented extensions.
                                                            (line  4513)
* six.xisy:                              Undocumented extensions.
                                                            (line  4514)
* six.xory:                              Undocumented extensions.
                                                            (line  4516)
* six.x^=y:                              Undocumented extensions.
                                                            (line  4506)
* six.x^y:                               Undocumented extensions.
                                                            (line  4507)
* six.yieldx:                            Undocumented extensions.
                                                            (line  4518)
* six.~x:                                Undocumented extensions.
                                                            (line  4517)
* sixth:                                 Undocumented extensions.
                                                            (line  4338)
* socket-info-address:                   Undocumented extensions.
                                                            (line  4268)
* socket-info-family:                    Undocumented extensions.
                                                            (line  4269)
* socket-info-port-number:               Undocumented extensions.
                                                            (line  4270)
* socket-info?:                          Undocumented extensions.
                                                            (line  4267)
* stack-overflow-exception?:             Exception objects related to memory management.
                                                            (line  9193)
* standard-bindings:                     Miscellaneous extensions.
                                                            (line  3774)
* started-thread-exception-arguments:    Exception objects related to threads.
                                                            (line  9494)
* started-thread-exception-procedure:    Exception objects related to threads.
                                                            (line  9493)
* started-thread-exception?:             Exception objects related to threads.
                                                            (line  9492)
* step:                                  Procedures related to debugging.
                                                            (line  2480)
* step-level-set!:                       Procedures related to debugging.
                                                            (line  2481)
* string->keyword:                       Miscellaneous extensions.
                                                            (line  3396)
* string->uninterned-keyword:            Miscellaneous extensions.
                                                            (line  3459)
* string->uninterned-symbol:             Miscellaneous extensions.
                                                            (line  3436)
* string-ci<=?:                          Extensions to string procedures.
                                                            (line  6705)
* string-ci<?:                           Extensions to string procedures.
                                                            (line  6703)
* string-ci=?:                           Extensions to string procedures.
                                                            (line  6702)
* string-ci=?-hash:                      Hashing.           (line  7121)
* string-ci>=?:                          Extensions to string procedures.
                                                            (line  6706)
* string-ci>?:                           Extensions to string procedures.
                                                            (line  6704)
* string-concatenate:                    Miscellaneous extensions.
                                                            (line  3305)
* string-set:                            Miscellaneous extensions.
                                                            (line  3291)
* string-shrink!:                        Miscellaneous extensions.
                                                            (line  3352)
* string<=?:                             Extensions to string procedures.
                                                            (line  6700)
* string<?:                              Extensions to string procedures.
                                                            (line  6698)
* string=?:                              Extensions to string procedures.
                                                            (line  6697)
* string=?-hash:                         Hashing.           (line  7109)
* string>=?:                             Extensions to string procedures.
                                                            (line  6701)
* string>?:                              Extensions to string procedures.
                                                            (line  6699)
* subf32vector:                          Homogeneous numeric vectors.
                                                            (line  6924)
* subf32vector-fill!:                    Homogeneous numeric vectors.
                                                            (line  6918)
* subf32vector-move!:                    Homogeneous numeric vectors.
                                                            (line  6925)
* subf64vector:                          Homogeneous numeric vectors.
                                                            (line  6945)
* subf64vector-fill!:                    Homogeneous numeric vectors.
                                                            (line  6939)
* subf64vector-move!:                    Homogeneous numeric vectors.
                                                            (line  6946)
* subs16vector:                          Homogeneous numeric vectors.
                                                            (line  6798)
* subs16vector-fill!:                    Homogeneous numeric vectors.
                                                            (line  6792)
* subs16vector-move!:                    Homogeneous numeric vectors.
                                                            (line  6799)
* subs32vector:                          Homogeneous numeric vectors.
                                                            (line  6840)
* subs32vector-fill!:                    Homogeneous numeric vectors.
                                                            (line  6834)
* subs32vector-move!:                    Homogeneous numeric vectors.
                                                            (line  6841)
* subs64vector:                          Homogeneous numeric vectors.
                                                            (line  6882)
* subs64vector-fill!:                    Homogeneous numeric vectors.
                                                            (line  6876)
* subs64vector-move!:                    Homogeneous numeric vectors.
                                                            (line  6883)
* subs8vector:                           Homogeneous numeric vectors.
                                                            (line  6756)
* subs8vector-fill!:                     Homogeneous numeric vectors.
                                                            (line  6750)
* subs8vector-move!:                     Homogeneous numeric vectors.
                                                            (line  6757)
* substring-fill!:                       Miscellaneous extensions.
                                                            (line  3321)
* substring-move!:                       Miscellaneous extensions.
                                                            (line  3335)
* subu16vector:                          Homogeneous numeric vectors.
                                                            (line  6819)
* subu16vector-fill!:                    Homogeneous numeric vectors.
                                                            (line  6813)
* subu16vector-move!:                    Homogeneous numeric vectors.
                                                            (line  6820)
* subu32vector:                          Homogeneous numeric vectors.
                                                            (line  6861)
* subu32vector-fill!:                    Homogeneous numeric vectors.
                                                            (line  6855)
* subu32vector-move!:                    Homogeneous numeric vectors.
                                                            (line  6862)
* subu64vector:                          Homogeneous numeric vectors.
                                                            (line  6903)
* subu64vector-fill!:                    Homogeneous numeric vectors.
                                                            (line  6897)
* subu64vector-move!:                    Homogeneous numeric vectors.
                                                            (line  6904)
* subu8vector:                           Homogeneous numeric vectors.
                                                            (line  6777)
* subu8vector-fill!:                     Homogeneous numeric vectors.
                                                            (line  6771)
* subu8vector-move!:                     Homogeneous numeric vectors.
                                                            (line  6778)
* subvector:                             Miscellaneous extensions.
                                                            (line  3105)
* subvector-fill!:                       Miscellaneous extensions.
                                                            (line  3190)
* subvector-move!:                       Miscellaneous extensions.
                                                            (line  3204)
* symbol-hash:                           Hashing.           (line  7085)
* syntax:                                Undocumented extensions.
                                                            (line  4322)
* syntax->datum:                         Undocumented extensions.
                                                            (line  4325)
* syntax->list:                          Undocumented extensions.
                                                            (line  4326)
* syntax->vector:                        Undocumented extensions.
                                                            (line  4327)
* syntax-case:                           Miscellaneous extensions.
                                                            (line  3541)
* syntax-case <1>:                       Undocumented extensions.
                                                            (line  4321)
* syntax-rules:                          Miscellaneous extensions.
                                                            (line  3541)
* system-stamp:                          Undocumented extensions.
                                                            (line  4279)
* system-type:                           Undocumented extensions.
                                                            (line  4275)
* system-type-string:                    Undocumented extensions.
                                                            (line  4276)
* system-version:                        Undocumented extensions.
                                                            (line  4272)
* system-version-string:                 Undocumented extensions.
                                                            (line  4273)
* table->list:                           Tables.            (line  7518)
* table-copy:                            Tables.            (line  7585)
* table-for-each:                        Tables.            (line  7490)
* table-length:                          Tables.            (line  7391)
* table-merge:                           Tables.            (line  7623)
* table-merge!:                          Tables.            (line  7602)
* table-ref:                             Tables.            (line  7416)
* table-search:                          Tables.            (line  7459)
* table-set!:                            Tables.            (line  7439)
* table?:                                Tables.            (line  7379)
* tables:                                Hashing and weak references.
                                                            (line  7013)
* tail-calls:                            Procedures related to debugging.
                                                            (line  2604)
* tail-calls <1>:                        Miscellaneous extensions.
                                                            (line  3864)
* take:                                  Undocumented extensions.
                                                            (line  4388)
* tanh:                                  Undocumented extensions.
                                                            (line  4411)
* tcp-client-local-socket-info:          Undocumented extensions.
                                                            (line  4261)
* tcp-client-peer-socket-info:           Undocumented extensions.
                                                            (line  4262)
* tcp-client-self-socket-info:           Undocumented extensions.
                                                            (line  4263)
* tcp-server-socket-info:                Undocumented extensions.
                                                            (line  4265)
* tcp-service-register!:                 Network devices.   (line 13149)
* tcp-service-unregister!:               Network devices.   (line 13151)
* tenth:                                 Undocumented extensions.
                                                            (line  4342)
* terminated-thread-exception-arguments: Exception objects related to threads.
                                                            (line  9526)
* terminated-thread-exception-procedure: Exception objects related to threads.
                                                            (line  9525)
* terminated-thread-exception?:          Exception objects related to threads.
                                                            (line  9524)
* test-bit-field?:                       Bitwise-operations on exact integers.
                                                            (line  6129)
* third:                                 Undocumented extensions.
                                                            (line  4335)
* this-source-file:                      Undocumented extensions.
                                                            (line  4310)
* thread:                                Procedures.        (line  7990)
* thread-base-priority:                  Procedures.        (line  8025)
* thread-base-priority-set!:             Procedures.        (line  8026)
* thread-group->thread-group-list:       Undocumented extensions.
                                                            (line  4141)
* thread-group->thread-group-vector:     Undocumented extensions.
                                                            (line  4142)
* thread-group->thread-list:             Undocumented extensions.
                                                            (line  4143)
* thread-group->thread-vector:           Undocumented extensions.
                                                            (line  4144)
* thread-group-name:                     Undocumented extensions.
                                                            (line  4136)
* thread-group-parent:                   Undocumented extensions.
                                                            (line  4137)
* thread-group-resume!:                  Undocumented extensions.
                                                            (line  4138)
* thread-group-specific:                 Undocumented extensions.
                                                            (line  4145)
* thread-group-specific-set!:            Undocumented extensions.
                                                            (line  4146)
* thread-group-suspend!:                 Undocumented extensions.
                                                            (line  4139)
* thread-group-terminate!:               Undocumented extensions.
                                                            (line  4140)
* thread-group?:                         Undocumented extensions.
                                                            (line  4135)
* thread-init!:                          Undocumented extensions.
                                                            (line  4171)
* thread-interrupt!:                     Undocumented extensions.
                                                            (line  4162)
* thread-join!:                          Procedures.        (line  8225)
* thread-mailbox-extract-and-rewind:     Procedures.        (line  8296)
* thread-mailbox-next:                   Procedures.        (line  8294)
* thread-mailbox-rewind:                 Procedures.        (line  8295)
* thread-name:                           Procedures.        (line  8003)
* thread-priority-boost:                 Procedures.        (line  8041)
* thread-priority-boost-set!:            Procedures.        (line  8042)
* thread-quantum:                        Procedures.        (line  8057)
* thread-quantum-set!:                   Procedures.        (line  8058)
* thread-receive:                        Procedures.        (line  8293)
* thread-resume!:                        Undocumented extensions.
                                                            (line  4165)
* thread-send:                           Procedures.        (line  8276)
* thread-sleep!:                         Procedures.        (line  8115)
* thread-specific:                       Procedures.        (line  8010)
* thread-specific-set!:                  Procedures.        (line  8011)
* thread-start!:                         Procedures.        (line  8077)
* thread-state:                          Undocumented extensions.
                                                            (line  4148)
* thread-state-abnormally-terminated-reason: Undocumented extensions.
                                                            (line  4159)
* thread-state-abnormally-terminated?:   Undocumented extensions.
                                                            (line  4158)
* thread-state-initialized?:             Undocumented extensions.
                                                            (line  4150)
* thread-state-normally-terminated-result: Undocumented extensions.
                                                            (line  4157)
* thread-state-normally-terminated?:     Undocumented extensions.
                                                            (line  4156)
* thread-state-running-processor:        Undocumented extensions.
                                                            (line  4152)
* thread-state-running?:                 Undocumented extensions.
                                                            (line  4151)
* thread-state-uninitialized?:           Undocumented extensions.
                                                            (line  4149)
* thread-state-waiting-for:              Undocumented extensions.
                                                            (line  4154)
* thread-state-waiting-timeout:          Undocumented extensions.
                                                            (line  4155)
* thread-state-waiting?:                 Undocumented extensions.
                                                            (line  4153)
* thread-suspend!:                       Undocumented extensions.
                                                            (line  4164)
* thread-terminate!:                     Procedures.        (line  8139)
* thread-thread-group:                   Undocumented extensions.
                                                            (line  4167)
* thread-yield!:                         Procedures.        (line  8096)
* thread?:                               Procedures.        (line  7928)
* threads:                               Threads.           (line  7691)
* time:                                  Measuring time.    (line 10899)
* time->seconds:                         Measuring time.    (line 10837)
* time?:                                 Measuring time.    (line 10836)
* timeout->time:                         Undocumented extensions.
                                                            (line  4202)
* top:                                   Undocumented extensions.
                                                            (line  4160)
* touch:                                 Undocumented extensions.
                                                            (line  4282)
* trace:                                 Procedures related to debugging.
                                                            (line  2407)
* transcript-off:                        Extensions to standard procedures.
                                                            (line  2926)
* transcript-on:                         Extensions to standard procedures.
                                                            (line  2925)
* tty-history:                           Undocumented extensions.
                                                            (line  4285)
* tty-history-max-length-set!:           Undocumented extensions.
                                                            (line  4287)
* tty-history-set!:                      Undocumented extensions.
                                                            (line  4286)
* tty-mode-set!:                         Undocumented extensions.
                                                            (line  4290)
* tty-paren-balance-duration-set!:       Undocumented extensions.
                                                            (line  4288)
* tty-text-attributes-set!:              Undocumented extensions.
                                                            (line  4289)
* tty-type-set!:                         Undocumented extensions.
                                                            (line  4291)
* tty?:                                  Undocumented extensions.
                                                            (line  4284)
* type-exception-arg-id:                 Exception objects related to type checking.
                                                            (line 10006)
* type-exception-arguments:              Exception objects related to type checking.
                                                            (line 10005)
* type-exception-procedure:              Exception objects related to type checking.
                                                            (line 10004)
* type-exception-type-id:                Exception objects related to type checking.
                                                            (line 10007)
* type-exception?:                       Exception objects related to type checking.
                                                            (line 10003)
* u16vector:                             Homogeneous numeric vectors.
                                                            (line  6805)
* u16vector->list:                       Homogeneous numeric vectors.
                                                            (line  6810)
* u16vector-append:                      Homogeneous numeric vectors.
                                                            (line  6818)
* u16vector-concatenate:                 Homogeneous numeric vectors.
                                                            (line  6814)
* u16vector-copy:                        Homogeneous numeric vectors.
                                                            (line  6815)
* u16vector-copy!:                       Homogeneous numeric vectors.
                                                            (line  6816)
* u16vector-fill!:                       Homogeneous numeric vectors.
                                                            (line  6812)
* u16vector-length:                      Homogeneous numeric vectors.
                                                            (line  6806)
* u16vector-ref:                         Homogeneous numeric vectors.
                                                            (line  6807)
* u16vector-set:                         Homogeneous numeric vectors.
                                                            (line  6808)
* u16vector-set!:                        Homogeneous numeric vectors.
                                                            (line  6809)
* u16vector-shrink!:                     Homogeneous numeric vectors.
                                                            (line  6822)
* u16vector?:                            Homogeneous numeric vectors.
                                                            (line  6803)
* u32vector:                             Homogeneous numeric vectors.
                                                            (line  6847)
* u32vector->list:                       Homogeneous numeric vectors.
                                                            (line  6852)
* u32vector-append:                      Homogeneous numeric vectors.
                                                            (line  6860)
* u32vector-concatenate:                 Homogeneous numeric vectors.
                                                            (line  6856)
* u32vector-copy:                        Homogeneous numeric vectors.
                                                            (line  6857)
* u32vector-copy!:                       Homogeneous numeric vectors.
                                                            (line  6858)
* u32vector-fill!:                       Homogeneous numeric vectors.
                                                            (line  6854)
* u32vector-length:                      Homogeneous numeric vectors.
                                                            (line  6848)
* u32vector-ref:                         Homogeneous numeric vectors.
                                                            (line  6849)
* u32vector-set:                         Homogeneous numeric vectors.
                                                            (line  6850)
* u32vector-set!:                        Homogeneous numeric vectors.
                                                            (line  6851)
* u32vector-shrink!:                     Homogeneous numeric vectors.
                                                            (line  6864)
* u32vector?:                            Homogeneous numeric vectors.
                                                            (line  6845)
* u64vector:                             Homogeneous numeric vectors.
                                                            (line  6889)
* u64vector->list:                       Homogeneous numeric vectors.
                                                            (line  6894)
* u64vector-append:                      Homogeneous numeric vectors.
                                                            (line  6902)
* u64vector-concatenate:                 Homogeneous numeric vectors.
                                                            (line  6898)
* u64vector-copy:                        Homogeneous numeric vectors.
                                                            (line  6899)
* u64vector-copy!:                       Homogeneous numeric vectors.
                                                            (line  6900)
* u64vector-fill!:                       Homogeneous numeric vectors.
                                                            (line  6896)
* u64vector-length:                      Homogeneous numeric vectors.
                                                            (line  6890)
* u64vector-ref:                         Homogeneous numeric vectors.
                                                            (line  6891)
* u64vector-set:                         Homogeneous numeric vectors.
                                                            (line  6892)
* u64vector-set!:                        Homogeneous numeric vectors.
                                                            (line  6893)
* u64vector-shrink!:                     Homogeneous numeric vectors.
                                                            (line  6906)
* u64vector?:                            Homogeneous numeric vectors.
                                                            (line  6887)
* u8vector:                              Homogeneous numeric vectors.
                                                            (line  6763)
* u8vector->list:                        Homogeneous numeric vectors.
                                                            (line  6768)
* u8vector->object:                      Homogeneous numeric vectors.
                                                            (line  6965)
* u8vector-append:                       Homogeneous numeric vectors.
                                                            (line  6776)
* u8vector-concatenate:                  Homogeneous numeric vectors.
                                                            (line  6772)
* u8vector-copy:                         Homogeneous numeric vectors.
                                                            (line  6773)
* u8vector-copy!:                        Homogeneous numeric vectors.
                                                            (line  6774)
* u8vector-fill!:                        Homogeneous numeric vectors.
                                                            (line  6770)
* u8vector-length:                       Homogeneous numeric vectors.
                                                            (line  6764)
* u8vector-ref:                          Homogeneous numeric vectors.
                                                            (line  6765)
* u8vector-set:                          Homogeneous numeric vectors.
                                                            (line  6766)
* u8vector-set!:                         Homogeneous numeric vectors.
                                                            (line  6767)
* u8vector-shrink!:                      Homogeneous numeric vectors.
                                                            (line  6780)
* u8vector?:                             Homogeneous numeric vectors.
                                                            (line  6761)
* udp-destination-set!:                  Network devices.   (line 13377)
* udp-local-socket-info:                 Network devices.   (line 13484)
* udp-read-subu8vector:                  Network devices.   (line 13386)
* udp-read-u8vector:                     Network devices.   (line 13384)
* udp-source-socket-info:                Network devices.   (line 13485)
* udp-write-subu8vector:                 Network devices.   (line 13387)
* udp-write-u8vector:                    Network devices.   (line 13385)
* unbound-global-exception-code:         Exception objects related to evaluation and compilation.
                                                            (line  9945)
* unbound-global-exception-rte:          Exception objects related to evaluation and compilation.
                                                            (line  9946)
* unbound-global-exception-variable:     Exception objects related to evaluation and compilation.
                                                            (line  9944)
* unbound-global-exception?:             Exception objects related to evaluation and compilation.
                                                            (line  9943)
* unbound-key-exception-arguments:       Tables.            (line  7557)
* unbound-key-exception-procedure:       Tables.            (line  7556)
* unbound-key-exception?:                Tables.            (line  7555)
* unbound-os-environment-variable-exception-arguments: Exception objects related to the host environment.
                                                            (line  9363)
* unbound-os-environment-variable-exception-procedure: Exception objects related to the host environment.
                                                            (line  9362)
* unbound-os-environment-variable-exception?: Exception objects related to the host environment.
                                                            (line  9361)
* unbound-serial-number-exception-arguments: Hashing.       (line  7057)
* unbound-serial-number-exception-procedure: Hashing.       (line  7056)
* unbound-serial-number-exception?:      Hashing.           (line  7055)
* unbox:                                 Miscellaneous extensions.
                                                            (line  3368)
* unbreak:                               Procedures related to debugging.
                                                            (line  2557)
* uncaught-exception-arguments:          Exception objects related to threads.
                                                            (line  9561)
* uncaught-exception-procedure:          Exception objects related to threads.
                                                            (line  9560)
* uncaught-exception-reason:             Exception objects related to threads.
                                                            (line  9562)
* uncaught-exception?:                   Exception objects related to threads.
                                                            (line  9559)
* uninitialized-thread-exception-arguments: Undocumented extensions.
                                                            (line  4179)
* uninitialized-thread-exception-procedure: Undocumented extensions.
                                                            (line  4178)
* uninitialized-thread-exception?:       Undocumented extensions.
                                                            (line  4177)
* uninterned-keyword?:                   Miscellaneous extensions.
                                                            (line  3460)
* uninterned-symbol?:                    Miscellaneous extensions.
                                                            (line  3437)
* unknown-keyword-argument-exception-arguments: Exception objects related to procedure call.
                                                            (line 10272)
* unknown-keyword-argument-exception-procedure: Exception objects related to procedure call.
                                                            (line 10271)
* unknown-keyword-argument-exception?:   Exception objects related to procedure call.
                                                            (line 10270)
* unterminated-process-exception-arguments: Process devices.
                                                            (line 12896)
* unterminated-process-exception-procedure: Process devices.
                                                            (line 12895)
* unterminated-process-exception?:       Process devices.   (line 12894)
* untrace:                               Procedures related to debugging.
                                                            (line  2408)
* user-info:                             User information.  (line 11265)
* user-info-gid:                         User information.  (line 11320)
* user-info-home:                        User information.  (line 11330)
* user-info-name:                        User information.  (line 11301)
* user-info-shell:                       User information.  (line 11340)
* user-info-uid:                         User information.  (line 11311)
* user-info?:                            User information.  (line 11289)
* user-name:                             User information.  (line 11256)
* vector->bits:                          Undocumented extensions.
                                                            (line  4419)
* vector-append:                         Miscellaneous extensions.
                                                            (line  3162)
* vector-cas!:                           Miscellaneous extensions.
                                                            (line  3235)
* vector-concatenate:                    Miscellaneous extensions.
                                                            (line  3174)
* vector-copy:                           Miscellaneous extensions.
                                                            (line  3117)
* vector-copy!:                          Miscellaneous extensions.
                                                            (line  3137)
* vector-inc!:                           Miscellaneous extensions.
                                                            (line  3257)
* vector-set:                            Miscellaneous extensions.
                                                            (line  3277)
* vector-shrink!:                        Miscellaneous extensions.
                                                            (line  3221)
* void:                                  Miscellaneous extensions.
                                                            (line  3484)
* weak references:                       Hashing and weak references.
                                                            (line  7013)
* will-execute!:                         Wills.             (line  7204)
* will-testator:                         Wills.             (line  7203)
* will?:                                 Wills.             (line  7202)
* with-exception-catcher:                Exception-handling.
                                                            (line  9086)
* with-exception-handler:                Exception-handling.
                                                            (line  9061)
* with-input-from-file:                  Filesystem devices.
                                                            (line 12591)
* with-input-from-port:                  Undocumented extensions.
                                                            (line  4293)
* with-input-from-process:               Process devices.   (line 12734)
* with-input-from-string:                String-ports.      (line 13729)
* with-input-from-u8vector:              U8vector-ports.    (line 13786)
* with-input-from-vector:                Vector-ports.      (line 13579)
* with-output-to-file:                   Filesystem devices.
                                                            (line 12592)
* with-output-to-port:                   Undocumented extensions.
                                                            (line  4294)
* with-output-to-process:                Process devices.   (line 12735)
* with-output-to-string:                 String-ports.      (line 13730)
* with-output-to-u8vector:               U8vector-ports.    (line 13787)
* with-output-to-vector:                 Vector-ports.      (line 13580)
* write:                                 Object-port operations.
                                                            (line 11958)
* write-char:                            Character-port operations.
                                                            (line 12209)
* write-file-string:                     Other procedures related to I/O.
                                                            (line 13889)
* write-file-string-list:                Other procedures related to I/O.
                                                            (line 13890)
* write-file-u8vector:                   Other procedures related to I/O.
                                                            (line 13891)
* write-substring:                       Character-port operations.
                                                            (line 12257)
* write-subu8vector:                     Byte-port operations.
                                                            (line 12539)
* write-u8:                              Byte-port operations.
                                                            (line 12526)
* wrong-number-of-arguments-exception-arguments: Exception objects related to procedure call.
                                                            (line 10145)
* wrong-number-of-arguments-exception-procedure: Exception objects related to procedure call.
                                                            (line 10144)
* wrong-number-of-arguments-exception?:  Exception objects related to procedure call.
                                                            (line 10143)
* wrong-number-of-values-exception-code: Exception objects related to procedure call.
                                                            (line 10242)
* wrong-number-of-values-exception-rte:  Exception objects related to procedure call.
                                                            (line 10243)
* wrong-number-of-values-exception-vals: Exception objects related to procedure call.
                                                            (line 10241)
* wrong-number-of-values-exception?:     Exception objects related to procedure call.
                                                            (line 10240)
* wrong-processor-c-return-exception?:   Exception objects related to C-interface.
                                                            (line  9767)
* xcons:                                 Undocumented extensions.
                                                            (line  4386)

Gambit
1 The Gambit system
  1.1 Accessing the system files
2 The Gambit Scheme interpreter
  2.1 Interactive mode
  2.2 Batch mode
  2.3 Module management mode
  2.4 Customization
  2.5 Process exit status
  2.6 Scheme scripts
    2.6.1 Scripts under UNIX and macOS
    2.6.2 Scripts under Microsoft Windows
    2.6.3 Compiling scripts
3 The Gambit Scheme compiler
  3.1 Interactive mode
  3.2 Customization
  3.3 Batch mode
  3.4 Link files
    3.4.1 Building an executable program
    3.4.2 Building a loadable library
    3.4.3 Building a shared-library
    3.4.4 Other compilation options
  3.5 Procedures specific to compiler
4 Runtime options
5 Debugging
  5.1 Debugging model
  5.2 Debugging commands
  5.3 Debugging example
  5.4 Procedures related to debugging
  5.5 Console line-editing
  5.6 Emacs interface
  5.7 GUIDE
6 Scheme extensions
  6.1 Extensions to standard procedures
  6.2 Extensions to standard special forms
  6.3 Miscellaneous extensions
  6.4 Undocumented extensions
7 Modules
  7.1 Legacy Modules
  7.2 Primitive Modules
    7.2.1 ‘##demand-module’ and ‘##supply-module’ forms
    7.2.2 ‘##namespace’ and ‘##import’ forms
    7.2.3 Macros
  7.3 Primitive Procedures
    7.3.1 Type specifiers
      7.3.1.1 Basic types (other than numbers)
      7.3.1.2 Numbers
      7.3.1.3 Time types
      7.3.1.4 Ports
      7.3.1.5 List and vector variants of above
      7.3.1.6 Gambit types
      7.3.1.7 Others
  7.4 R7RS Compatible Modules
    7.4.1 Identifying libraries
    7.4.2 The ‘define-library’ form
    7.4.3 ‘(export <EXPORT SPEC> ...)’
    7.4.4 ‘(import <IMPORT SET> ...)’
    7.4.5 ‘(begin <COMMAND OR DEFINITION> ...)’, ‘(include <FILENAME> ...)’, and ‘(include-ci <FILENAME> ...)’
    7.4.6 ‘(include-library-declarations <FILENAME> ...)’
    7.4.7 ‘(cond-expand <COND EXPAND FEATURES> ...)’
    7.4.8 Extensions to the R7RS library declarations
  7.5 Installing Modules
  7.6 Compiling Modules
8 Built-in data types
  8.1 Numbers
    8.1.1 Extensions to numeric procedures
    8.1.2 IEEE floating point arithmetic
    8.1.3 Integer square root and nth root
    8.1.4 Bitwise-operations on exact integers
    8.1.5 Fixnum specific operations
    8.1.6 Flonum specific operations
    8.1.7 Pseudo random numbers
  8.2 Booleans
  8.3 Pairs and lists
  8.4 Symbols and keywords
  8.5 Characters and strings
  8.6 Extensions to character procedures
  8.7 Extensions to string procedures
  8.8 Vectors
  8.9 Homogeneous numeric vectors
  8.10 Hashing and weak references
    8.10.1 Hashing
    8.10.2 Weak references
      8.10.2.1 Wills
    8.10.3 Tables
9 Records
10 Threads
  10.1 Introduction
  10.2 Thread objects
  10.3 Mutex objects
  10.4 Condition variable objects
  10.5 Fairness
  10.6 Memory coherency
  10.7 Timeouts
  10.8 Primordial thread
  10.9 Procedures
11 Dynamic environment
12 Exceptions
  12.1 Exception-handling
  12.2 Exception objects related to memory management
  12.3 Exception objects related to the host environment
  12.4 Exception objects related to threads
  12.5 Exception objects related to C-interface
  12.6 Exception objects related to the reader
  12.7 Exception objects related to evaluation and compilation
  12.8 Exception objects related to type checking
  12.9 Exception objects related to procedure call
  12.10 Other exception objects
13 Host environment
  13.1 Handling of file names
  13.2 Filesystem operations
  13.3 Shell command execution
  13.4 Process termination
  13.5 Command line arguments
  13.6 Environment variables
  13.7 Measuring time
  13.8 File information
  13.9 Group information
  13.10 User information
  13.11 Host information
  13.12 Service information
  13.13 Protocol information
  13.14 Network information
14 I/O and ports
  14.1 Unidirectional and bidirectional ports
  14.2 Port classes
  14.3 Port settings
  14.4 Object-ports
    14.4.1 Object-port settings
    14.4.2 Object-port operations
  14.5 Character-ports
    14.5.1 Character-port settings
    14.5.2 Character-port operations
  14.6 Byte-ports
    14.6.1 Byte-port settings
    14.6.2 Byte-port operations
  14.7 Device-ports
    14.7.1 Filesystem devices
    14.7.2 Process devices
    14.7.3 Network devices
  14.8 Directory-ports
  14.9 Vector-ports
  14.10 String-ports
  14.11 U8vector-ports
  14.12 Other procedures related to I/O
15 Lexical syntax and readtables
  15.1 Readtables
  15.2 Boolean syntax
  15.3 Character syntax
  15.4 String syntax
  15.5 Symbol syntax
  15.6 Keyword syntax
  15.7 Box syntax
  15.8 Number syntax
  15.9 Homogeneous vector syntax
  15.10 Special ‘#!’ syntax
  15.11 Multiline comment syntax
  15.12 Scheme infix syntax extension
    15.12.1 SIX grammar
    15.12.2 SIX semantics
16 C-interface
  16.1 The mapping of types between C and Scheme
  16.2 The ‘c-declare’ special form
  16.3 The ‘c-initialize’ special form
  16.4 The ‘c-lambda’ special form
  16.5 The ‘c-define’ special form
  16.6 The ‘c-define-type’ special form
  16.7 Continuations, the C-interface and threads
17 System limitations
18 Copyright and license
General index
