Using Marpa with a tree structure

tree_structure_test.pl

use Marpa::XS;
use Data::Dumper;

my $grammar = Marpa::XS::Grammar->new(
    {   start   => 'Expression',
        actions => 'My_Actions',
        rules   => [

    # This rebuilds the tree
            { lhs => 'Expression', rhs => [qw/Term/],            action => 'Return_0' },
            { lhs => 'Term',       rhs => [qw/+ Term Term/],     action => 'All'},
            { lhs => 'Term',       rhs => [qw/Factor/],          action => 'Return_0' },
            { lhs => 'Factor',     rhs => [qw/* Factor Factor/], action => 'All' },
            { lhs => 'Factor',     rhs => [qw/Number/],          action => 'Return_0' },

    # Simplication rules
        # n + 0 ==> n
            { lhs => 'Term',       rhs => [qw/+ 0 Term/],        action => 'Return_2'},
            { lhs => 'Term',       rhs => [qw/+ Term 0/],        action => 'Return_1'},

        # n * 1 ==> n
            { lhs => 'Factor',     rhs => [qw/* 1 Factor/],      action => 'Return_2' },
            { lhs => 'Factor',     rhs => [qw/* Factor 1/],      action => 'Return_1' },

        # n * 0 ==> 0
            { lhs => 'Factor',     rhs => [qw/* 0 Factor/],      action => 'Zero' },
            { lhs => 'Factor',     rhs => [qw/* Factor 0/],      action => 'Zero' },
        ],
    }
);
$grammar->precompute();

sub My_Actions::Return_0 { shift; return $_[0]; }
sub My_Actions::Return_1 { shift; return $_[1]; }
sub My_Actions::Return_2 { shift; return $_[2]; }
sub My_Actions::All      { shift; return \@_; }
sub My_Actions::Zero     { shift; return 0; }

my $rec = Marpa::XS::Recognizer->new({grammar => $grammar});

my $expr = [ '+', [ '*', 10, 2 ], 0 ];

$rec->alternative('Term', $expr, 5);
$rec->alternative('+', '+', 1);
$rec->earleme_complete;

$rec->alternative('Factor', $expr->[1], 3);
$rec->alternative('*', '*', 1);
$rec->earleme_complete;

$rec->alternative($expr->[1][1], $expr->[1][1], 1);
$rec->alternative('Number', $expr->[1][1], 1);
$rec->earleme_complete;

$rec->alternative($expr->[1][2], $expr->[1][2], 1);
$rec->alternative('Number', $expr->[1][2], 1);
$rec->earleme_complete;

$rec->alternative($expr->[2], $expr->[2], 1);
$rec->alternative('Number', $expr->[2], 1);
$rec->earleme_complete;

$rec->end_input;

while (my $v = ${$rec->value}) {
    print Dumper($v);
}