Complex use cases of meta_predicate declaration

[hurufu]

What are the correct meta-predicate declarations for the following examples:

1. my_pred(foo(G_1), X) :- call(G_1, X).

2. call_all([], _, _).
   call_all([G_2|T], A, B) :- call(G_2, A, B), call_all(T, A, B).

[triska]

Case (2) is #2444, isn't it? According to that logic, the first argument would be [2], which I think makes sense.

[UWN]

You are over-stretching the capacities of meta-predicate declarations (as they exist in SICStus). You seem to expect that they have the full capacity of a type system. But they can cover only a small part of it. In both cases, a meta-predicate declaration does not make sense. That means that their G_1 and G_2 should already be explicitly qualified. (I am somewhat tempted to say that the ensuing call/2/3 should best insist on such an explicit qualification. But this would probably break a lot of code...)

In a way, meta-predicate declarations help to add the missing explicit qualifications. And, as long as everything can be expressed with them, they help to cross-reference code as well. Your cases are such that cross-referencing would be more complex.

There have been some ad hoc extensions to meta-predicate declarations to allow lists of goals such as yours. I'd say staying on very shaky grounds.

[hurufu]

Understood, so given the following module (foo.pl):

:- module(foo, [call_foo/3]).
call_foo(foo(G_2), X, Y) :- call(G_2, X, Y).

And the following user code:

:- use_module(library(lists)).
:- use_module(foo).
:- meta_predicate(goal_qualified_2(2,-)).
goal_qualified_2(G_2, G_2).

This query must throw an exception:

?- call_foo(foo(length), A, B).
   error(existence_error(procedure,length/2),length/2).

And this is the only correct way of using that predicate:

?- goal_qualified_2(length, QG_2), call_foo(foo(QG_2), A, B).

Sad, I would've expect as you correctly have noticed a "type system". It can be unintuitive to novices though.

[UWN]

Very well observed! Also note that using goal_qualified_2/2 helps to alert some cross-referencer of that very goal, so it would make sense to indicate a warning, should that length not be defined.

Ideally, the colon is never used explicitly, but one produces it only implicitly. Ideally, I said...