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







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








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












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 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.

  










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.








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








Grammars and finite automata








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








  

    0.1
    grammar_peg