meta_predicate/1 directive is sometimes not applied

[triska]

I am filing the short example from #2624 (comment) here as a simpler issue. With:

:- meta_predicate(p(0)).

p(t).

We get:

?- p(X).
   X = t, unexpected.
   false. % expected

Reason: The call is expected to be user:X, and thus not unifiable with t.

[hurufu]

Meta-predicate argument expansion is applied only per goal, and not on a term level – this is the root of a problem. You can check it for example by adding the following to your snippet:

:- dynamic(t/0).
t :- p(t).

And then doing following query.

?- listing(t/0).
t :-
   p(user:t).
   true.

[triska]

@hurufu: What you posted does not correspond to the example in the issue, please try it with:

t :- p(_).

[hurufu]

Ah yes, but it is still quite hard to solve with current loader in a systematic way and without pilling-up workarounds on top of workarounds.

I think I can do a warning in this case, as was suggested recently.

[hurufu]

Interestingly enough the following code:

t :- p(_).

Already triggers a warning:

% Warning: Meta-predicate detected, but no qualified variables found at line 29 of warnings.pl

This happens because p/1 calls an unbound variable that doesn't appear in the head of the clause.

[hurufu]

Actually I've tested this situation a little bit more, and I think the problem is more subtle. The following code behaves as expected:

:- module(m, [p/0]).

:- meta_predicate(b(:)).
b(t).

p :- b(X).

Query:

?- p.
   false.

[hurufu]

Because there is no distinction between t and user:t I'm even starting to think that behavior is correct...

[hurufu]

By "no distinction" I mean that call(t) is the same as call(user:t) if executed in the user context, which is the case for this bug.

[hurufu]

I have found a workaround:

:- meta_predicate(q(0,-)).
q(A,A).

p(A) :- q(t, A).

Now it works as expected:

?- p(A).
   A = user:t.

Because module qualifier are added in the place of call, this means that for original example, there was no enough information whether to add or not those qualifiers.

[hurufu]

I've actually have spent some time trying to write a warning, but I'm kinda stuck. Consider the following code:

:- meta_predicate(g(0,-)).
g(A,A).

:- meta_predicate(p(0)).
p(A) :- g(t, A).
p(A) :- A = t.
p(t).

o :- p(A), portray_clause(A).

First clause of p/1 is correct and shouldn't trigger a warning, but other two clauses should trigger it, and I don't know how to distinguish between them.