grammar::peg::interp(n)  Grammar operations and usage  grammar::peg::interp(n)



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NAME
       grammar::peg::interp - Interpreter for parsing expression grammars

SYNOPSIS
       package require Tcl  8.4

       package require grammar::mengine  ?0.1?

       package require grammar::peg::interp  ?0.1?

       ::grammar::peg::interp::setup peg

       ::grammar::peg::interp::parse nextcmd errorvar astvar

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DESCRIPTION
       This package provides commands for the controlled matching of a
       character stream via a parsing expression grammar and the creation of
       an abstract syntax tree for the stream and partials.

       It is built on top of the virtual machine provided by the package
       grammar::me::tcl and directly interprets the parsing expression grammar
       given to it.  In other words, the grammar is not pre-compiled but used
       as is.

       The grammar to be interpreted is taken from a container object
       following the interface specified by the package
       grammar::peg::container. Only the relevant parts are copied into the
       state of this package.

       It should be noted that the package provides exactly one instance of
       the interpreter, and interpreting a second grammar requires the user to
       either abort or complete a running interpretation, or to put them into
       different Tcl interpreters.

       Also of note is that the implementation assumes a pull-type handling of
       the input. In other words, the interpreter pulls characters from the
       input stream as it needs them. For usage in a push environment, i.e.
       where the environment pushes new characters as they come we have to put
       the engine into its own thread.

THE INTERPRETER API
       The package exports the following API

       ::grammar::peg::interp::setup peg
              This command (re)initializes the interpreter. It returns the
              empty string. This command has to be invoked first, before any
              matching run.

              Its argument peg is the handle of an object containing the
              parsing expression grammar to interpret. This grammar has to be
              valid, or an error will be thrown.

       ::grammar::peg::interp::parse nextcmd errorvar astvar
              This command interprets the loaded grammar and tries to match it
              against the stream of characters represented by the command
              prefix nextcmd.

              The command prefix nextcmd represents the input stream of
              characters and is invoked by the interpreter whenever the a new
              character from the stream is required.  The callback has to
              return either the empty list, or a list of 4 elements containing
              the token, its lexeme attribute, and its location as line number
              and column index, in this order.  The empty list is the signal
              that the end of the input stream has been reached. The lexeme
              attribute is stored in the terminal cache, but otherwise not
              used by the machine.

              The result of the command is a boolean value indicating whether
              the matching process was successful (true), or not (false). In
              the case of a match failure error information will be stored
              into the variable referenced by errorvar. The variable
              referenced by astvar will always contain the generated abstract
              syntax tree, however in the case of an error it will be only
              partial and possibly malformed.

              The abstract syntax tree is represented by a nested list, as
              described in section AST VALUES of document grammar::me_ast.

BUGS, IDEAS, FEEDBACK
       This document, and the package it describes, will undoubtedly contain
       bugs and other problems.  Please report such in the category
       grammar_peg of the Tcllib SF Trackers
       [http://sourceforge.net/tracker/?group_id=12883].  Please also report
       any ideas for enhancements you may have for either package and/or
       documentation.

KEYWORDS
       LL(k), TDPL, context-free languages, expression, grammar, matching,
       parsing, parsing expression, parsing expression grammar, push down
       automaton, recursive descent, state, top-down parsing languages,
       transducer, virtual machine

CATEGORY
       Grammars and finite automata

COPYRIGHT
       Copyright (c) 2005 Andreas Kupries <andreas_kupries@users.sourceforge.net>




grammar_peg                           0.1              grammar::peg::interp(n)
