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Re: Trees and Language::AttributeGrammar

by nothingmuch (Priest)
on Mar 06, 2006 at 17:56 UTC ( #534724=note: print w/ replies, xml ) Need Help??


in reply to Trees and Language::AttributeGrammar

First of all, the fix is this:

Cons: $/.len = { $/; $<head>.len + $<tail>.len }
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});
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'); }
versus
$_AG_ATTR->get($_AG_SELF)->get('len')->set(sub { $_AG_N1->get('len', 'grammar line 4') + $_AG_N2->get('len', 'gramm +ar line 4'); }
So, what is the void thingy doing in there, you are probably wondering? Aha!
'visit' => [ sub { ... # snipped my($_AG_SELF, $_AG_ATTR) = @_; ... # snipped $_AG_ATTR->get($_AG_SELF)->get('len')->set(sub { $_AG_SELF;
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} } ) ] }; }
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


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Re^2: Trees and Language::AttributeGrammar
by rg0now (Chaplain) on Mar 06, 2006 at 21:01 UTC
    Thanks for the nice response. Let's say I understand it...:-)

    Something however tells me that both of your solutions take away something very important from the elegance of the code: in my opinion the efficiency lies in that we let the Cons stuff just emerge and go away on the fly.

    My qustion is: do you have any better idea to represent trees and attribute grammars to manipulate them than mine? Once I adopted this "functional" approach I really do not want to go back to the "imperative" world...

      Haskell?

      Seriously though, post a bug on rt.cpan.org - luqui should know about this and fix it somehow...

      luqui also said he will add support for attributes over aggregate types (e.g. @<children>.foo) eventually, if he can find a nice way.

      -nuffin
      zz zZ Z Z #!perl
        The bug has finally been fixed in version 0.08. There is also syntax for aggregate types, though it is awkward: my @child_foos = `@<children>`.foo; Luke

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