


grammar::peg::interGrammar operations and 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 charac-
       ter 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 gram-
       mar::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  follow-
       ing  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 pars-
              ing expression grammar to interpret.  This  grammar  has  to  be
              valid, or an error will be thrown.



grammar_peg                    0.1                              1





grammar::peg::interGrammar operations and grammar::peg::interp(n)


       ::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 pre-
              fix nextcmd.

              The  command prefix nextcmd represents the input stream of char-
              acters 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  refer-
              enced  by astvar will always contain the generated abstract syn-
              tax tree, however in the case of an error it will be  only  par-
              tial 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 gram-
       mar_peg    of    the     Tcllib     SF     Trackers     [http://source-
       forge.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, trans-
       ducer, virtual machine

CATEGORY
       Grammars and finite automata

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











grammar_peg                    0.1                              2


