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  ?1.0.1?

       ::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 <- Term (AddOp Term)*                            ;
                  MulOp      <- '*' / '/'                                     ;
                  Term       <- Factor (MulOp Factor)*                        ;
                  AddOp      <- '+'/'-'                                       ;
                  Factor     <- '(' Expression ')' / 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 pclass  [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 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                                   1.0.1                         pt::pgen(n)
