Re: [Yap-users] Strange tabling behavior with dif/2

[Eyal D. <eya...@gm...>]

Hi Ed,

I think that this is because dif/2 is part of the co-routining extension
and the tabling extension (YAPTab) doesn't support the co-routining
extension. According to the manual (section 18) this should through an
error; that it doesn't is likely a bug.

I think the reason it is not supported is because tabling in YAP depends on
being able to ask when one term is a variant of another. But in the
presence of constraints like dif/2, two terms that are syntactic variants
are no longer necessarily unifiable.

Best,
Eyal

On Tue, Feb 16, 2016 at 11:29 AM, Edward Schwartz <esc...@ce...>
wrote:

> Hi all,
>
> I am experiencing some strange behavior when I combine tabling and dif/2.
>
> Here's a very simple prolog program for computing transitivity:
>
> % 1, 2 and 3 are on the same class.
> same(1, 2).
> same(1, 3).
> % 4, 5, and 6 are the same class.
> same(4, 5).
> same(4, 6).
> :- table ruleSame/2.
> % Root from real facts.
> ruleSame(X, Y) :-
>     same(X, Y).
> % Commutative.
> ruleSame(X, Y) :-
>     dif(X, Y),
>     ruleSame(Y, X).
> % Transitive.
> ruleSame(X, Y) :-
>     dif(X, Y),
>     dif(Y, Z),
>     dif(X, Z),
>     ruleSame(X, Z),
>     ruleSame(Z, Y).
>
> Basic facts are ok:
>
>    ?- ruleSame(1,2).
> yes
>    ?- ruleSame(1,3).
> yes
>    ?- ruleSame(2,3).
> yes
>
> But then some unexpected facts are generated...
>
>    ?- ruleSame(2,4).
> yes
>
> If I debug the evaluation of these facts, I get an extremely long trace.
>
> If I move the dif sub-goals after the ruleSame sub-goals, the program
> works as expected.
>
> Is this a bug?  I think it is, but hopefully I haven't overlooked
> something.  It seems like maybe the deferred goals from dif/2 are
> interacting with tabling in a non-trivial way.
>
> I tested this both on YAP 6.2.2 and the current development git branch
> of 6.3.  Both had the same behavior.
>
> Thanks for any help,
>
> Ed
>
>
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