First of all, the fix is this: 
Cons: $/.len            = { $/; $<head>.len + $<tail>.len }

[download]
As you can see mentioning $/, the attribute's invocant fixes this.

You're probably asking yourself why... 

use Data::Dumper;
$Data::Dumper::Deparse = 1;
warn Dumper($grammar->{engine}{cases}{Cons});

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Comparing the two versions shows us something interesting: 
$_AG_ATTR->get($_AG_SELF)->get('len')->set(sub {
    $_AG_SELF;
    $_AG_N1->get('len', 'grammar line 4') + $_AG_N2->get('len', 'gramm
+ar line 4');
}

[download]
versus 
$_AG_ATTR->get($_AG_SELF)->get('len')->set(sub {
    $_AG_N1->get('len', 'grammar line 4') + $_AG_N2->get('len', 'gramm
+ar line 4');
}

[download]
So, what is the void thingy doing in there, you are probably wondering? Aha! 
'visit' => [
    sub {
... # snipped
        my($_AG_SELF, $_AG_ATTR) = @_;
... # snipped
        $_AG_ATTR->get($_AG_SELF)->get('len')->set(sub {
            $_AG_SELF;

[download]
Basically, $_AG_SELF causes the value to remain non-garbage collected (since it's captured in the closure it's reference count stays up). Since luqui is using a lazy evaluation strategy to produce results, and this probably means that somewhere inside $_AG_ATTR or whatever there's a table of objects to their attr values, the moment that $_AG_SELF dies (and it will die, because you are creating Cons thingies on the fly inside 'children') it can no longer be referenced in a thunk, because it's StrVal cannot be reproduced. Anyway, long story short, the evaluation scheme is somehow landing on an edge case with two children in the Cons/Nil thing, and thus it's causing the value to be recalculated over and over and over and over again. I think.

The more elegant fix is to cache your linked list, something like 

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

[download]
so that the values don't go away. Remember to invalidate _ll_cache whenever new children are added.

Ciao!

-nuffin
zz zZ Z Z #!perl

In reply to Re: Trees and Language::AttributeGrammar by nothingmuch
in thread Trees and Language::AttributeGrammar by rg0now

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