I've lately begun to explore Parse::RecDescent and have a couple of questions. Given:

#!/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] + $it
+em[3];}
    sub_calculation: statement '-' statement {$return = $item[1] - $it
+em[3];}
    mul_calculation: statement '*' statement {$return = $item[1] * $it
+em[3];}
    div_calculation: statement '/' statement {$return = $item[1] / $it
+em[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";
    }
}
[download]

It's clear that the parentheses work, my first question is why does the lack fail? '4 + 8 + 5' working from left to right should collapse into '12 + 5' and then to '17'. Obviously using $::RD_TRACE = 1; shows that it gets as far as '12', but then fails after that. I'm fairly sure this is a beginner's error but it doesn't leap out at me.

Looking ahead, I'd want to implement some sort of precedence scheme. Pointers, brickbats and suggestions gratefully accepted!

–hsm

"Never try to teach a pig to sing…it wastes your time and it annoys the pig."