#!/usr/bin/perl
# parse.pl -- learning Parse::RecDescent ex.
use strict;
use warnings;
use diagnostics;
use Parse::RecDescent;

my $parser = new Parse::RecDescent( q{
    startrule: calculation eofile {$return = $item[1];}
    statement: grouped_op | number
    grouped_op: '(' calculation ')' {$return = $item[2];}
    calculation: add_calculation | sub_calculation | mul_calculation | div_calculation
    add_calculation: statement '+' statement {$return = $item[1] + $item[3];}
    sub_calculation: statement '-' statement {$return = $item[1] - $item[3];}
    mul_calculation: statement '*' statement {$return = $item[1] * $item[3];}
    div_calculation: statement '/' statement {$return = $item[1] / $item[3];}
    number: /\d+/
    eofile: /^\Z/
});

my @tests = (
'4 + 8',
'6 + (3 - 2)',
'(6 + (3 - 2)) + 1',
'4 + 8 + 5',
'5 * 5',
'5 / 5',
);
my $result;

foreach my $test (@tests) {
    if ( $result = $parser->startrule($test) ) {
        print "'$test' = $result\n";
    }
    else {
        print "'$test' failed parser\n";
    }
}