#!/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} });
}

##</code><code>##

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


##</code><code>##

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";


##</code><code>##

Deep recursion on subroutine "Language::AttributeGrammar::Thunk::get" at (eval 29) line 5.

##</code><code>##

# find the global minimum and propagate it back down the tree
ROOT:   $/.gmin        = { $/.min }
Branch: $<left>.gmin   = { $/.gmin }
      | $<right>.gmin) = { $/.gmin }
                     ^
                     |
                    this is superfluous