CLP(ℤ): Attribute is sometimes unexpectedly not present

[triska]

With clpz7.pl from https://www.metalevel.at/clpz/scryer/clpz7.pl, I get:

?- X #= Y+Y+Y, X = 3.
false.

Expected result: Success.

The reason for this seems to be that in insert_queue//2 (defined in clpz7.pl), the goal get_atts/2 fails on the third invocation of insert_queue//2 in the example above:

insert_queue(Element, Which) -->
        state(Queue),
        { arg(Which, Queue, Arg),
          get_atts(Arg, queue(Head0,Tail0)),
          (   Head0 == [] ->
              Head = [Element|Tail]
          ;   Head = Head0,
              Tail0 = [Element|Tail]
          ),
          put_atts(Arg, +queue(Head,Tail)) }.

However, the attribute queue/2 is expected to be present on the third invocation. There is nothing that would remove the attribute between the second and third invocation of insert_queue//2 in this example. For reference,Head0 ought to be [], and Tail0 is expected to be a free variable on the third invocation of insert_queue//2.

I will try to find a simpler example that exhibits this issue.

[mthom]

I inserted some print statements around the code where the queue attribute is handled, and I saw that it was deleted from the third variable, which is why insert_queue code fails at the third invocation. clear_queue is where it's deleted.

[triska]

Yes, it is true that clear_queue/1 deletes attributes. However, it also reinstates them, and in this concrete case, after clear_queue/1 has finished, the above get_atts/2 goal succeeds once more (for the second time in total), and only on its third invocation, the above get_atts/2 goal fails. Between the second and the third time, clear_queue/1 is not invoked.

So we have, in total, the following sequence:

1. the above get_atts/2 succeeds
2. clear_queue/1 removes attributes and also reinstates some
3. the above get_atts/2 succeeds
4. the above get_atts/2 fails unexpectedly.

On a general note, while I try to construct a simpler test case, please focus on #95, since partial strings are the more fundamental feature that will make Scryer Prolog truly unique as soon as it is available, and which will also be automatically tested too when we then discuss other issues.

Meanwhile, I am continuing to post CLP(ℤ) related issues in case you need a short creative pause, and also for later reference.

[mthom]

Yes, I think I'll take a break from working on CLP(ℤ), and focus on partial strings.

[mthom]

I did a little more digging last night, and found that the queue/2 attribute of the variable had an empty list as the first argument. This caused the predicate responsible for reinstating the predicate to fail. I think it was this:

queue_get_arg_(Queue, Which, Element) :-
        arg(Which, Queue, Arg),
        get_atts(Arg, +queue([Element|Elements],Tail)),
        (   var(Elements) ->
            put_atts(Arg, +queue([],_))
        ;   put_atts(Arg, +queue(Elements,Tail))
        ).

[triska]

Please note that between (3) and (4) of the steps outlined above, the attribute somehow vanishes (i.e., (4) fails unexpectedly), even though there is nothing between these steps that would remove the attribute.

The value of the concrete arguments of queue/2 is immaterial, since get_atts/2 asks for the most general case (queue(Head0,Tail0), where both Head0 and Tail0 are free variables), and that should always succeed as long as an attribute of this form is present at all.

So, we have that (3) succeeds, and (4) fails even though it is exactly the same goal, which we expect to succeed as long as a queue/2 attribute is present.

[triska]

This works correctly now, thank you a lot!