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  ?0.1.1?

       ::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) 2005-2011 Andreas Kupries <andreas_kupries@users.sourceforge.net>