#!/usr/bin/perl
use warnings;
use strict;
use Marpa::R2;
use Tree::Binary;

package Construct;


sub newNode {
    my ($value, $left, $right) = @_;
    my $node = Tree::Binary->new($value);

    $node->setLeft($left)   if $left;
    $node->setRight($right) if $right;
    return $node;
}


sub doOp {
    my @params = @_;

    return newNode(@params[2, 1, 3],);
}


sub doConst {
    my @params = @_;

    return newNode($params[1]);
}


package main;

my $syntax = <<'SYNTAX';
lexeme default = latm => 1
 
expression ::= expression '+' term action => doOp | term action => ::first
 
term ::= term '*' factor action => doOp | factor action => ::first

factor ::= constant action => doConst

constant ~ digits
digits ~ [\d]+
:discard ~ spaces
spaces ~ [\s]+
SYNTAX

my $grammar = Marpa::R2::Scanless::G->new({source => \$syntax});
my $input   = '1 * 2 + 3 * 4';
my $root    = $grammar->parse(\$input, 'Construct');

($$root)->traverse(sub {print "  " x $_[0]->{_depth}, $_[0]->getNodeValue(), "\n"});