Pegex::Miscellany(3)  User Contributed Perl Documentation Pegex::Miscellany(3)


Miscellany
       This document contains things about Pegex that were written but seemed
       out of place in their original documents. Still they are possibly
       useful so live here for now.

Pegex Overview
       In the diagram below, there is a simple language called Foo. The
       diagram shows how Pegex can take a text grammar defining Foo and
       generate a parser that can parse Foo sources into data (abstract syntax
       trees).

                                     Parsing a language called "Foo"
                                        with the Pegex toolset.

                                       .-----------------------.
           .--------------------.      |    Pegex::Compiler    |
           |    Foo Language    |      |-----------------------|    Serialize
           |--------------------|----->| Pegex::Grammar::Pegex |---------.
           | Pegex grammar text |      | Pegex::Receiver       |         |
           '--------------------'      '-----------------------'         v
           ......................                  |                 .------.
           |                    |                  | compile()       | YAML |
           |foo: verb noun      |                  v                 '------'
           |verb: /Hello/       |       .--------------------.       .------.
           |noun: /world/       |       | Foo grammar tree   |       | JSON |
           |                    |       '--------------------'       '------'
           ......................                  |                 .------.
                                                   |                 | Perl |
                                                   v                 '------'
                                        .---------------------.      .--------.
                                        | Pegex::Grammar::Foo |      | Python |
                                        |---------------------|      '--------'
                                        | Pegex::Parser       |      .-----.
                                        | Pegex::AST::Foo     |      | etc |
            .-----------------.         '---------------------'      '-----'
            |  Foo Language   |                    |
            |-----------------|------------------->| parse()
            | Foo source text |                    v
            '-----------------'        .----------------------.
            ...................        | Parsed Foo Data Tree |
            |Hello world      |        '----------------------'
            ...................        ........................
                                       |- verb: Hello         |
                                       |- noun: world         |
                                       ........................

FYI
       Pegex is self-hosting. This means that the Pegex grammar language
       syntax is defined by a Pegex grammar! This is important because (just
       like any Pegex based language) it makes it easier to port to new
       programming languages. You can find the Pegex grammar for Pegex
       grammars here: <http://github.com/ingydotnet/pegex-pgx/>.

       Pegex was originally inspired by Perl 6 Rules. It also takes ideas from
       Damian Conway's Perl 5 module, Regexp::Grammars. Pegex tries to take
       the best ideas from these great works, and make them work in as many
       languages as possible. That's Acmeism.

Self Compilation Tricks
       You can have some fun using Pegex to compile itself. First get the
       Pegex grammar repo:

           git clone git://github.com/ingydotnet/pegex-pgx.git
           cd pegex-pgx

       Then parse and dump the Pegex grammar with Pegex:

           perl -MXXX -MPegex -e 'XXX pegex("pegex.pgx")->parse("pegex.pgx")'

       For a different view of the data tree, try:

           perl -MXXX -MPegex -e 'XXX pegex("pegex.pgx", receiver => "Pegex::Tree")->parse("pegex.pgx")'

       Finally to emulate the Pegex compiler do this:

           perl -MXXX -MPegex -e 'XXX pegex("pegex.pgx", receiver => "Pegex::Pegex::AST")->parse("pegex.pgx")'

       This specifies a "receiving" class that can shape the results into
       something useful. Indeed, this is the exact guts of
       Pegex::Grammar::Pegex.

A Real World EXAMPLE
       TestML is a new Acmeist unit test language. It is perfect for software
       that needs to run equivalently in more than one language. In fact,
       Pegex itself is tested with TestML!!

       TestML has a language specification grammar:
       <http://www.testml.org/specification/language/>

       The Perl6 implementation of TestML uses this grammar in:
       <https://github.com/ingydotnet/testml-pm6/blob/master/lib/TestML/Parser/Grammar.pm>

       All other implementations of TestML use this Pegex grammar:
       <https://github.com/ingydotnet/testml-pgx/blob/master/testml.pgx>

       In Perl 5, Pegex::Compiler is used to compile the grammar into this
       simple data structure (shown in YAML):
       <https://github.com/ingydotnet/testml-pgx/blob/master/testml.pgx.yaml>

       The grammar can also be precompiled to JSON:
       <https://github.com/ingydotnet/testml-pgx/blob/master/testml.pgx.json>

       Pegex::Compiler further compiles this into a Perl 5 only grammar tree,
       which becomes this module:
       <https://github.com/ingydotnet/testml-pm/blob/master/lib/TestML/Compiler/Pegex/Grammar.pm>

       TestML::Parser::Grammar is a subclass of Pegex::Grammar. It can be used
       to parse TestML files. TestML::Parser calls the parse() method of the
       grammar with a TestML::AST object that receives callbacks when various
       rules match, and uses the information to build a TestML::Document
       object.

       Thus TestML is an Acmeist language written in Pegex. It can be easily
       ported to every language where Pegex exists. In fact, it must be ported
       to those languages in order to test the new Pegex implementation!

perl v5.40.0                      2024-09-01              Pegex::Miscellany(3)