[comment {-*- text -*-}]
[manpage_begin wip n 2.3]
[keywords interpreter]
[keywords list]
[keywords word]
[copyright {2007-2010 Andreas Kupries <andreas_kupries@users.sourceforge.net>}]
[moddesc   {Word Interpreter}]
[titledesc {Word Interpreter}]
[category  {Programming tools}]
[require Tcl "8.5 9"]
[require wip [opt 2.3]]
[require snit [opt 1.4]]
[require struct::set]
[description]
[para]

This package provides a micro interpreter for lists of words. Domain
specific languages based on this will have a bit of a Forth feel, with
the input stream segmented into words and any other structuring left
to whatever the language desired. Note that we have here in essence
only the core dispatch loop, and no actual commands whatsoever, making
this definitely only a Forth feel and not an actual Forth.

[para]

The idea is derived from Colin McCormack's [package treeql] processor,
modified to require less boiler plate within the command
implementations, at the expense of, likely, execution speed. In
addition the interface between processor core and commands is more
complex too.

[section {GENERAL BEHAVIOUR}]

Word interpreters have a mappping from the names of the language
commands they shall recognize to the methods in the engine to invoke
for them, and possibly fixed arguments for these methods. This mapping
is largely static, however it is possible to change it during the
execution of a word list (= program).

[para]

At the time a language command is defined the word interpreter will
use [package snit]'s introspection capabilities to determine the
number of arguments expected by the method of the egnine, and together
with the number of fixed arguments supplied in the method prefix of
the mapping it then knows how many arguments the language command is
expecting. This is the command's [term arity]. Variable-argument
methods (i.e. with the last argument named [arg args]) are [emph not]
allowed and will cause the word interpreter to throw an error at
definition time.

[para]

Note that while I said [package snit]'s abilities the engine object
can be written in any way, as long as it understands the method
[method {info args}], which takes a method name and returns the list
of arguments for that method.

[para]

When executing a list of words (aka program) the first word is always
taken as the name of a language command, and the next words as its
arguments, per the [term arity] of the command. Command and argument
words are removed from the list and then associated method of the
engine is executed with the argument words. The process then repeats
using the then-first word of the list.

[para]

Note that the methods implementing the language commands may have full
access to the list of words and are allowed to manipulate as they see
fit.

[list_begin enum]

[enum]
This means, for example, that while we cannot specify
variable-argument methods directly they can consume words after their
fixed arguments before returning to the execution loop. This may be
under the control of their fixed arguments.

[enum]
Another possibility is the use of method [method run_next] and its
variants to execute commands coming after the current command,
changing the order of execution.

[enum]
Execution can be further changed by use of the program accessor
methods which allow a command implementation to modify the remaining
list of words (insert, replace, prepend, append words) without
executing them immediately.

[enum]
At last the basic [method run] methods save and restore an existing
list of words when used, enabling recursive use from within command
implementations.

[list_end]

[section {CLASS API}]

The main command of the package is:

[list_begin definitions]

[call [cmd ::wip] [arg wipName] [arg engine] [arg arg]...]

The command creates a new word interpreter object with an associated
global Tcl command whose name is [arg wipName]. If however the string
[const %AUTO%] was used as object name the package will generate its
own unique name for the object.

[para]

The [arg engine] is the object the word interpreter will dispatch all
recognized commands to, and the [arg arg]uments are a word list which
defines an initial mapping from language words to engine methods.

[para]

The recognized language of this word list is

[list_begin definitions]
[call [cmd def] [arg name]]

Defines [arg name] as command of the language, to be mapped to a
method of the [arg engine] having the same name.

[call [cmd def] [arg name] [arg method_prefix]]

Defines [arg name] as command of the language, to be mapped to the
method of the [arg engine] named in the [arg method_prefix].

[list_end]

[para]

The returned command may be used to invoke various operations on the
object.  It has the following general form:

[list_begin definitions]
[call [cmd wipName] [arg option] [opt [arg "arg arg ..."]]]

[arg Option] and the [arg arg]s determine the exact behavior of the
command.

[list_end]
[list_end]

The package additionally exports the command:

[list_begin definitions]

[call [cmd wip::dsl] [opt [arg suffix]]]

This command is for use within snit types which wish to use one or
more wip interpreters as a component. Use within the type definition
installs most of the boilerplate needed to setup and use a word
interpreter.

[para]

It installs a component named [emph wip], and a method
[method wip_setup] for initializing it. This method has to be called
from within the constructor of the type using the word interpreter.

If further installs a series of procedures which make the object API
of the word interpreter directly available to the type's methods,
without having to specify the component.

[para]

[emph Note] that this does and cannot install the language to
interpret, i.e. the mapping from words to engine methods.

[para]

It is possible to instantiate multiple word interpreter components
within a type by using different suffices as arguments to the command.

In that case the name of the component changes to
'wip_[var \$suffix]', the setup command becomes
'wip_[var \$suffix]_setup' and all the procedures also get the suffix
'_[var \$suffix]'.

[list_end]

[section {OBJECT API}]

The following commands are possible for word interpreter objects:

[list_begin definitions]

[call [arg wipName] [method def] [arg name] [opt [arg method_prefix]]]

Defines a language command [arg name] and maps it to the method named
in the engine's [arg method_prefix]. If the [arg method_prefix] name
is not specified it is simply the name of the language command.

[call [arg wipName] [method defl] [arg names]]

Defines a series of language commands, specified through the list of
[arg names], all of which are mapped to engine methods of the same
name.

[call [arg wipName] [method defd] [arg dict]]

Defines a series of language commands, specified through the
dictionary [arg dict] of names and method prefixes.

[call [arg wipName] [method deflva] [arg name]...]

As method [method defl], however the list of names is specified
through multiple arguments.

[call [arg wipName] [method defdva] ([arg name] [arg method_prefix])...]

As method [method defd], however the dictionary of names and method
prefixes is specified through multiple arguments.

[call [arg wipName] [method undefl] [arg names]]

Removes the named series of language commands from the mapping.

[call [arg wipName] [method undefva] [arg name]...]

As method [method undefl], however the list of names is specified
through multiple arguments.

[call [arg wipName] [method unknown] [arg cmdprefix]]

Sets the handler for unknown words to [arg cmdprefix]. This command
prefix takes one argument, the current word, and either throws some
error, or returns the result of executing the word, as defined by the
handler. The default handler simply throws an error.

[call [arg wipName] [method runl] [arg wordlist]]

Treats the list of words in [arg wordlist] as a program and executes
the contained command one by one. The result of the command executed
last is returned as the result of this command.

[para]

The [arg wordlist] is stored in the object for access by the other
[term run]-methods, and the general program accessor methods (see
below). A previously stored wordlist is saved during the execution of
this method and restored before it returns. This enables the recursive
execution of word lists within word lists.

[call [arg wipName] [method run]  [arg word]...]

As method [method runl], however the list of words to execute is
specified through multiple arguments.

[call [arg wipName] [method run_next]]

Low-level method. Determines the next word in the list of words, and
its arguments, and then executes it. The result of the executed word
is the result of this method.

[para]

Exposed for use within command implementations.

The methods [method run] and [method runl] use it to execute words
until their word list is exhausted.

[call [arg wipName] [method run_next_while] [arg acceptable]]

Low-level method. Invokes the method [method run_next] as long as the
next word is in the set of [arg acceptable] words, and the program is
not empty. The result of the command executed last is returned as the
result of this command.

[para]

Exposed for use within command implementations to change the order of
execution.

[call [arg wipName] [method run_next_until] [arg rejected]]

Low-level method. Invokes the method [method run_next] until the next
word is in the set of [arg rejected] words, and the program is not
empty. The result of the command executed last is returned as the
result of this command.

[para]

Exposed for use within command implementations to change the order of
execution.

[call [arg wipName] [method run_next_if] [arg acceptable]]

Low-level method. Invokes the method [method run_next] if the next
word is in the set of [arg acceptable] words, and the program is not
empty. The result of the command executed last is returned as the
result of this command.

[para]

Exposed for use within command implementations to change the order of
execution.

[call [arg wipName] [method run_next_ifnot] [arg rejected]]

Low-level method. Invokes the method [method run_next] if the next
word is not in the set of [arg rejected] words, and the program is not
empty. The result of the command executed last is returned as the
result of this command.

[para]

Exposed for use within command implementations to change the order of
execution.

[call [arg wipName] [method next]]

Returns the next word in the programm. The word is also removed.

[call [arg wipName] [method peek]]

Returns the next word in the programm without removing it

[call [arg wipName] [method peekall]]

Returns the remaining programm in toto.

[call [arg wipName] [method insertl] [arg at] [arg wordlist]]

Basic programm accessor method. Inserts the specified [arg wordlist]
into the program, just before the word at position [arg at]. Positions
are counted from [const zero].

[call [arg wipName] [method replacel] [arg wordlist]]

Basic programm accessor method. Replaces the whole stored program with
the specified [arg wordlist].

[call [arg wipName] [method pushl] [arg wordlist]]

Program accessor method. The specified [arg wordlist] is added to the
front of the remaining program. Equivalent to

[para]
[example {$wip insertl 0 $wordlist}]

[call [arg wipName] [method addl] [arg wordlist]]

Program accessor method. The specified [arg wordlist] is appended at
the end of the remaining program. Equivalent to

[para]
[example {$wip insertl end $wordlist}]

[call [arg wipName] [method insert]  [arg at] [arg word]...]

Like method [method insertl], except the words are specified through
multiple arguments.

[call [arg wipName] [method replace] [arg word]...]

Like method [method setl], except the words are specified through
multiple arguments.

[call [arg wipName] [method push] [arg word]...]

Like method [method pushl], except the words are specified through
multiple arguments.

[call [arg wipName] [method add] [arg word]...]

Like method [method addl], except the words are specified through
multiple arguments.

[list_end]

[section EXAMPLES]

No examples yet.

[vset CATEGORY wip]
[include ../common-text/feedback.inc]
[manpage_end]
