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



____________________________________________________________________________________________________________

NAME
       pt::pgen - Parser Generator

SYNOPSIS
       package require Tcl  8.5

       package require pt::pgen  ?1?

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

____________________________________________________________________________________________________________

DESCRIPTION
       Are you lost ?  Do you have trouble understanding this document ?  In that case please read the over-view overview
       view provided by the Introduction to Parser Tools. This document is the entrypoint to the whole  sys-tem system
       tem 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  input-format), inputformat),
              format), 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 For-mat. Format.
              mat. 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  run-ning running
       ning 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, transform 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 pars-
       ing 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://source -
       forge.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 pars-ing parsing
       ing languages, transducer

CATEGORY
       Parsing and Grammars

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




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