Provided by: tcllib_1.21+dfsg-1_all bug

NAME

       pt::pgen - Parser Generator

SYNOPSIS

       package require Tcl  8.5

       package require pt::pgen  ?1.1?

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

_________________________________________________________________________________________________

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

       .nf  +-  Digit  0  0  |  1  |             | +- Term 0 2 --- Number 0 2 -+- Digit 1 1 | 2 |
       |               |    |                              +-    Digit    2    2    |     0     |
       |  Expression  0  4  ---  Factor  0  4  -+-----------------------------  AddOp  3  3 | + |
       | +- Term 4 4 --- Number 4 4 --- Digit 4 4 | 5 .fi

       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  Trackers
       [http://core.tcl.tk/tcllib/reportlist].  Please also report any ideas for enhancements you
       may have for either package and/or documentation.

       When proposing code changes, please provide unified diffs, i.e the output of diff -u.

       Note further that attachments are strongly preferred over inlined patches. Attachments can
       be made by going to the Edit form of the ticket immediately after its creation,  and  then
       using the left-most button in the secondary navigation bar.

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>