pt::pgen(n)							   Parser Tools 						       pt::pgen(n)

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NAME

       pt::pgen - Parser Generator

SYNOPSIS

       package require Tcl  8.5

       package require pt::pgen  ?1?

       ::pt::pgen inputformat text resultformat ?options...?

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DESCRIPTION

       Are  you  lost  ?  Do you have trouble understanding this document ?  In that case please read the overview provided by the Introduction to
       Parser Tools. This document is the entrypoint to the whole system the current package is a part of.

       This package provides a command implementing a parser generator taking parsing expression grammars as input.

       It is the implementation of method generate of pt, the Parser Tools Application.

       As such the intended audience of this document are people wishing to modify and/or extend this part of pt's functionality. Users of  pt	on
       the other hand are hereby refered to the applications' manpage, i.e. Parser Tools Application.

       It resides in the User Package Layer of Parser Tools.

       IMAGE: arch_user_pkg

API

       ::pt::pgen inputformat text resultformat ?options...?
	      This  command takes the parsing expression grammar in text (in the format specified by inputformat), and returns the same grammar in
	      the format resultformat as the result of the command.

	      The two known input formats are peg and json.  Introductions to them, including their formal specifications, can be found in the PEG
	      Language Tutorial and The JSON Grammar Exchange Format. The packages used to parse these formats are

	      peg    pt::peg::from::peg

	      json   pt::peg::from::json

       On the output side the known formats, and the packages used to generate them are

	      c      pt::peg::to::cparam

	      container
		     pt::peg::to::container

	      critcl pt::peg::to::cparam + pt::cparam::configuration::critcl

	      json   pt::peg::to::json

	      oo     pt::peg::to::tclparam + pt::tclparam::configuration::tcloo

	      peg    pt::peg::to::peg

	      snit   pt::peg::to::tclparam + pt::tclparam::configuration::snit
       The options supported by each of these formats are documented with their respective packages.

EXAMPLE

       In  this  section  we are working a complete example, starting with a PEG grammar and ending with running the parser generated from it over
       some input, following the outline shown in the figure below:

       IMAGE: flow

       Our grammar, assumed to the stored in the file "calculator.peg" is

       PEG calculator (Expression)
	   Digit      <- '0'/'1'/'2'/'3'/'4'/'5'/'6'/'7'/'8'/'9'   ;
	   Sign       <- '-' / '+'			 ;
	   Number     <- Sign? Digit+			      ;
	   Expression <- '(' Expression ')' / (Factor (MulOp Factor)*)	;
	   MulOp      <- '*' / '/'			 ;
	   Factor     <- Term (AddOp Term)*		      ;
	   AddOp      <- '+'/'-'			 ;
	   Term       <- Number 		    ;
       END;

       From this we create a snit-based parser using the script "gen"

       package require Tcl 8.5
       package require fileutil
       package require pt::pgen

       lassign $argv name
       set grammar [fileutil::cat $name.peg]
       set plass [pt::pgen
		     peg $gr  snit  -class $name      -file  $name.peg	-name  $name]
       fileutil::writeFile $name.tcl $pclass
       exit 0

       calling it like
	tclsh8.5 gen calculator
       which leaves us with the parser package and class written to the file "calculator.tcl".	 Assuming  that  this  package	is  then  properly
       installed in a place where Tcl can find it we can now use this class via a script like

	   package require calculator

	   lassign $argv input
	   set channel [open $input r]

	   set parser [calculator]
	   set ast [$parser parse $channel]
	   $parser destroy
	   close $channel

	   ... now process the returned abstract syntax tree ...

       where the abstract syntax tree stored in the variable will look like

       set ast {Expression 0 4
	   {Factor 0 4
	       {Term 0 2
		   {Number 0 2
		       {Digit 0 0}
		       {Digit 1 1}
		       {Digit 2 2}
		   }
	       }
	       {AddOp 3 3}
	       {Term 4 4
		   {Number 4 4
		       {Digit 4 4}
		   }
	       }
	   }
       }

       assuming that the input file and channel contained the text
	120+5
       A more graphical representation of the tree would be

       IMAGE: expr_ast

       Regardless, at this point it is the user's responsibility to work with the tree to reach whatever goal she desires. I.e. analyze it, trans-
       form it, etc. The package pt::ast should be of help here, providing commands to walk such ASTs structures in various ways.

       One important thing to note is that the parsers used here return a data structure representing the structure of the input per  the  grammar
       underlying the parser. There are no callbacks during the parsing process, i.e. no parsing actions, as most other parsers will have.

       Going  back  to	the  last  snippet of code, the execution of the parser for some input, note how the parser instance follows the specified
       Parser API.

BUGS, IDEAS, FEEDBACK
       This document, and the package it describes, will undoubtedly contain bugs and other problems.  Please report such in the  category  pt	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

       EBNF, LL(k), PEG, TDPL, context-free languages, expression, grammar, matching, parser, parsing expression, parsing expression grammar, push
       down automaton, recursive descent, state, top-down parsing languages, transducer

CATEGORY

       Parsing and Grammars

COPYRIGHT

       Copyright (c) 2009 Andreas Kupries <andreas_kupries@users.sourceforge.net>

pt									 1							       pt::pgen(n)