Trees and Language::AttributeGrammar

rg0now has asked for the wisdom of the Perl Monks concerning the following question:

Dear Monks!

I have largish data structures, organized into trees, and I have to perform rather complex transformations on this data. When I first encounterd with attribute grammars I immediately fell in love with the concept.

So I installed luqui's Language::AttributeGrammar, wrote a little tree data representation:

#!/usr/bin/perl

use strict;
use warnings;

use Language::AttributeGrammar;

package Leaf;

sub new{
  return bless { value => $_[1] }, 'Leaf';
}

package Branch;

sub new{
  return bless { value => $_[1], _list => undef }, 'Branch';
}

sub add_child{
  my $self = shift;
  push @{ $self->{_list} }, @_;
  return $self;
}

sub list {
  if (@_) {
    bless { head => $_[0], tail => list(@_[1..$#_]) }, 'Cons';
  }
  else {
    bless {}, 'Nil';
  }
}

sub children{
  return list( @{ $_[0]->{_list} });
}
[download]

and a simple attribute grammar to count the number of nodes in a tree

package main;

my $grammar = new Language::AttributeGrammar <<'EOG';

Branch: $/.len          = { 1 + $<children>.len }
Leaf:   $/.len          = { 1 }
Cons: $/.len            = { $<head>.len + $<tail>.len }
Nil:  $/.len            = { 0 }

EOG
[download]

and an example

my $tree = Branch->new(3)->add_child(
     Branch->new(1.1)->add_child(
          Leaf->new(1),
          Leaf->new(1.2)
    ),
     Branch->new(2.0)->add_child(
          Leaf->new(2.1),
#         Leaf->new(2.15),
          Leaf->new(2.2),
    )
    );

my $result = $grammar->apply($tree, 'len');
print "$result\n";
[download]

Everything works like charm as long as the line that is commented out remains commented out. As soon as I add the node back to the tree, I get

Deep recursion on subroutine "Language::AttributeGrammar::Thunk::get" 
+at (eval 29) line 5.
[download]

and the script runs into a seemingly infinite loop. Does anyone have a clue why this happens? I would greatly appreciate any ideas.

Update: sorry I mixed up: it works with 3 leafs, but stops working as soon as I comment out the offending line...

rg0now

p.s.: if luqui reads this: there is a typo in the SYNOPSIS of Language::AttributeGrammar:

# find the global minimum and propagate it back down the tree
ROOT:   $/.gmin        = { $/.min }
Branch: $<left>.gmin   = { $/.gmin }
      | $<right>.gmin) = { $/.gmin }
                     ^
                     |
                    this is superfluous
[download]

Back to Seekers of Perl Wisdom