=head1 The Pegex API

Pegex can be used in many ways: inside scripts, from the command line or as the foundation of a modular parsing framework. This document details the various ways to use Pegex.

At the most abstract level, Pegex works like this:

    $result = $parser->new($grammar, $receiver)->parse($input);

Which is to say, abstractly: a Pegex parser, under the direction of a Pegex grammar, parses an input stream, and reports matches to a Pegex receiver, which produces a result.

The parser, grammar, receiver and even the input, are Pegex objects. These 4 objects are involved in every Pegex parse operation, so let's review them briefly:

=over

=item L<Pegex::Parser>

The Pegex parsing engine. This engine applies the logic of the grammar to an input text. A B<parser> object contains a B<grammar> object and a B<receiver> object. Its primary method is called C<parse>. The default parser engine is non-backtracking, recursive descent. However there are parser subclasses for various alternative types of parsing.

=item L<Pegex::Grammar>

A Pegex grammar starts as a text file/string composed in the B<Pegex> syntax. Before it can be used in by a Parser it must be compiled. After compilation, it is turned into a data tree consisting of rules and regexes. In modules that are based on a Pegex grammar, the grammar will be compiled into a class file.  Pegex itself, uses a Pegex grammar class called L<Pegex::Pegex::Grammar> to parse various Pegex grammars.

=item L<Pegex::Receiver>

A parser on it's own has no idea what to do with the text it matches. A Pegex B<receiver> is a class that contains methods corresponding to the rules in a grammar. As a rule in the grammar matches, its corresponding receiver method (if one exists) is called with the data that has been matched. It is the receiver's job to take action on the data, often building it into some new structure. Pegex will use L<Pegex::Tree::Wrap> as the default receiver; it produces a reasonably readable tree of the matched / captured data.

=item L<Pegex::Input>

Pegex abstracts its input streams into an object interface as well. Any operation that can take an input string, can also take an input object. Pegex will turn regular strings into these objects. This is probably the API concept you will encounter the least, but it is covered here for completeness.

=back

All of these object classes can be subclassed to acheive various results. Normally, you will write your own Pegex grammar and a Pegex receiver to achieve a task.

=head2 Starting Simple - The `pegex` Function

The L<Pegex> module exports a function called C<pegex> that you can use for smaller tasks. Here is an example:

    use Pegex;
    use YAML;

    $grammar = "
    expr: num PLUS num
    num: /( DIGIT+ )/
    ";

    print Dump pegex($grammar)->parse('2+2');

This program would produce:

    expr:
    - num: 2
    - num: 2

Let's review what's happening here. The L<Pegex> module is exporting a C<pegex> function. This function takes a Pegex grammar string as input. Internally this function compiles the grammar string into a grammar object. Then it creates a parser object containing the grammar object and returns it.

The parse method is called on the input string: C<'2+2'>. The string matches, and a nice data structure is returned.

So how was the data structure created? By the receiver object, of course! But we didn't specify one, did we? Nope. It used the default receiver, L<Pegex::Tree::Wrap>. We could have said:

    print Dump pegex($grammar, 'Pegex::Tree::Wrap')->parse('2+2');

This receiver basically generates a mapping, where rule names of matches are the keys, and the leaf values are the regex captures.

The more basic receiver called L<Pegex::Tree> generates a tree of sequences that contain just the data (without the rule names). This code:

    print Dump pegex($grammar, 'Pegex::Tree')->parse('2+2');

would produce:

    - 2
    - 2

If we wrote our own receiver class called C<Calculator> like this:

    package Calculator;
    use base 'Pegex::Tree';

    sub got_expr {
        my ($receiver, $data) = @_;
        my ($a, $b) = @$data;
        return $a + $b;
    }

Then, this:

    print pegex(grammar, 'Calculator')->parse('2+2');

would print:

    4

=head2 More Explicit Usage

Continuing with the example above, let's see how to do it a little more formally.

    use Pegex::Parser;
    use Pegex::Grammar;
    use Pegex::Tree;
    use Pegex::Input;

    $grammar_text = "
    expr: num PLUS num
    num: /( DIGIT+ )/
    ";

    $grammar = Pegex::Grammar->new(text => $grammar_text);
    $receiver = Pegex::Tree->new();
    $parser = Pegex::Parser->new(
        grammar => $grammar,
        receiver => $receiver,
    );
    $input = Pegex::Input->new(string => '2+2');

    print Dump parser->parse($input);

This code does the same thing as the first example, but this time we've made all the objects ourselves.

=head2 Precompiled Grammars

If you ship a Pegex grammar as part of a CPAN distribution, you'll want it to be precompiled into a module. Pegex makes that easy.

Say the grammar_text about is stored in a file called C<share/expr.pgx>. If you create a module called C<lib/MyThing/Grammar.pm> with content like this:

    package MyThing::Grammar;
    use base 'Pegex::Grammar';
    use constant file => './share/expr.pgx';
    sub make_tree {
    }
    1;

Then run this command line:

    perl -Ilib -MMyThing::Grammar=compile

It will rewrite your module to look something like this:

    package MyThing::Grammar;
    use base 'Pegex::Grammar';
    use constant file => './share/expr.pgx';
    sub make_tree {
      { '+toprule' => 'expr',
        'PLUS' => { '.rgx' => qr/\G\+/ },
        'expr' => {
          '.all' => [
            { '.ref' => 'num' },
            { '.ref' => 'PLUS' },
            { '.ref' => 'num' }
          ]
        },
        'num' => { '.rgx' => qr/\G([0-9]+)/ }
      }
    }
    1;

This command found the file where your grammar is, compiled it, and used L<Data::Dumper> to output it back into your module's C<make_tree> method.

This is what a compiled Pegex grammar looks like. As soon as this module is loaded, the grammar is ready to be used by Pegex.

If you find yourself needing to compile your grammar module a lot during development, just set this environment variable like so:

    export PERL_PEGEX_AUTO_COMPILE=MyThing::Grammar

Now, every time the grammar module is loaded it will check to see if it needs to be recompiled, and do it on the fly.

If you have more than one grammar to recompile, just list all the names separated by commas.

=head1 See Also

=over

=item * L<Pegex::Parser>

=item * L<Pegex::Grammar>

=item * L<Pegex::Receiver>

=item * L<Pegex::Tree>

=item * L<Pegex::Tree::Wrap>

=item * L<Pegex::Input>

=back