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# wip(n) 2.2 tcllib "Word Interpreter"

## Name

wip - Word Interpreter

## Synopsis

- package require Tcl 8.4
- package require wip ?2.2?
- package require snit ?1.3?
- package require struct::set

- ::wip wipName engine arg...
- def name
- def name method_prefix
- wipName option ?arg arg ...?
- wip::dsl ?suffix?
- wipName def name ?method_prefix?
- wipName defl names
- wipName defd dict
- wipName deflva name...
- wipName defdva (name method_prefix)...
- wipName undefl names
- wipName undefva name...
- wipName unknown cmdprefix
- wipName runl wordlist
- wipName run word...
- wipName run_next
- wipName run_next_while acceptable
- wipName run_next_until rejected
- wipName run_next_if acceptable
- wipName run_next_ifnot rejected
- wipName next
- wipName peek
- wipName peekall
- wipName insertl at wordlist
- wipName replacel wordlist
- wipName pushl wordlist
- wipName addl wordlist
- wipName insert at word...
- wipName replace word...
- wipName push word...
- wipName add word...

## Description

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.

The idea is derived from Colin McCormack's 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.

## 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).

At the time a language command is defined the word interpreter will use 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 arity. Variable-argument methods (i.e. with the last argument named args) are not allowed and will cause the word interpreter to throw an error at definition time.

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

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 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.

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.

1. 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.

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

3. 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.

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

## CLASS API

The main command of the package is:

::wip wipName engine arg...

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

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

  The recognized language of this word list is

  def name

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

  def name method_prefix

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

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

  wipName option ?arg arg ...?

    Option and the args determine the exact behavior of the command.

The package additionally exports the command:

wip::dsl ?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.

  It installs a component named wip, and a 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.

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

  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_$suffix', the setup command becomes 'wip_$suffix_setup' and all the procedures also get the suffix '_$suffix'.

## OBJECT API

The following commands are possible for word interpreter objects:

wipName def name ?method_prefix?

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

wipName defl names

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

wipName defd dict

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

wipName deflva name...

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

wipName defdva (name method_prefix)...

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

wipName undefl names

  Removes the named series of language commands from the mapping.

wipName undefva name...

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

wipName unknown cmdprefix

  Sets the handler for unknown words to 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.

wipName runl wordlist

  Treats the list of words in 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.

  The wordlist is stored in the object for access by the other 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.

wipName run word...

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

wipName 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.

  Exposed for use within command implementations. The methods run and runl use it to execute words until their word list is exhausted.

wipName run_next_while acceptable

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

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

wipName run_next_until rejected

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

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

wipName run_next_if acceptable

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

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

wipName run_next_ifnot rejected

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

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

wipName next

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

wipName peek

  Returns the next word in the programm without removing it

wipName peekall

  Returns the remaining programm in toto.

wipName insertl at wordlist

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

wipName replacel wordlist

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

wipName pushl wordlist

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

      $wip insertl 0 $wordlist

wipName addl wordlist

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

      $wip insertl end $wordlist

wipName insert at word...

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

wipName replace word...

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

wipName push word...

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

wipName add word...

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

## EXAMPLES

No examples yet.

## Bugs, Ideas, Feedback

This document, and the package it describes, will undoubtedly contain bugs and other problems. Please report such in the category wip of the Tcllib Trackers. Please also report any ideas for enhancements you may have for either package and/or documentation.

When proposing code changes, please provide unified diffs, i.e the output of diff -u.

Note further that attachments are strongly preferred over inlined patches. Attachments can be made by going to the Edit form of the ticket immediately after its creation, and then using the left-most button in the secondary navigation bar.

## Keywords

interpreter, list, word

## Category

Programming tools

## Copyright

Copyright © 2007-2010 Andreas Kupries <andreas_kupries@users.sourceforge.net>