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       pt::pgen - Parser Generator


       package require Tcl  8.5

       package require pt::pgen  ?1.0.1?

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



       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


       ::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


              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


       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                   ;

       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

       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.


       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
       [].    Please  also  report  any  ideas  for
       enhancements you may have for either package and/or documentation.


       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


       Parsing and Grammars


       Copyright (c) 2009 Andreas Kupries <>