CODEWITHSUNDEEP
moduleprologswi-prolog
Matthias Berndt
Matthias Berndt

Reputation: 4577

Prolog: consider clauses from a dynamic module when calling a predicate

In the SWI Prolog manual, I found the following remark:

For example, assume an application that can reason about multiple worlds. It is attractive to store the data of a particular world in a module, so we extract information from a world simply by invoking goals in this world.

This is actually a very good description of what I'm trying to achieve. However I ran into a problem. While I do want to model many different worlds, there are also things that I want to share across all of them. So my idea is to have an allworlds module for things that are true in every world, and one module for every world that I want to reason about, and the latter imports from the former. So I'd do something like this in the REPL:

allworlds:asserta(grandparent(X, Z) :- (parent(X, Y), parent(Y, Z))).
allworlds:dynamic(parent/2).
add_import_module(greece, allworlds, start).
greece:asserta(parent(kronos, zeus)).
greece:asserta(parent(zeus, ares)).

Now I'd like to query greece:grandparent(kronos, X) and get X = ares, but all I get is false. When allworlds:grandparent calls parent, it doesn't call greece:parent like I want it to, but allworlds:parent. My research seems to indicate that I need to make the grandparent predicate module-transparent. But calling allworlds:module_transparent(grandparent/2). didn't fix the issue, and it's also deprecated. This is where I'm stuck. How can I get this working? Is meta_predicate/1 part of the solution? Unfortunately I can't make heads or tails of its documentation.

Upvotes: 3

Views: 140

Answers (2)

Matthias Berndt
Matthias Berndt

Reputation: 4577

I ultimately solved this by passing the module around explicitly and invoking predicates in it with the : operator. It reminds me a bit of doing OOP in C, where you do things like obj->vtable->method(obj, params) (note how obj is mentioned twice, just like the M in my code below).

Similar to the Logtalk solution, I need to explicitly call into the imported module when I want to consider its clauses. As an example, I've added the fact that a father is also a parent to the allworlds module.

allworlds:assertz(grandparent(M, X, Z) :- (M:parent(M, X, Y), M:parent(M, Y, Z))).
allworlds:assertz(parent(M, X, Y) :- M:father(M, X, Y)).
add_import_module(greece, allworlds, start).
greece:assertz(parent(_, kronos, zeus)).
% need to call into allworlds explicitly
greece:assertz(parent(M, X, Y) :- allworlds:parent(M, X, Y)). 
greece:assertz(father(_, zeus, ares)).

After making these assertions, I can call greece:grandparent(greece, kronos, X). and get the expected result X = ares.

Upvotes: 0

Paulo Moura
Paulo Moura

Reputation: 18663

Prolog modules don't provide a good solution for the "many worlds" design pattern. Notably, making the predicates meta-predicates (or module transparent or multifile) would be a problematic hack. But this pattern is trivial with Logtalk, which is a language extends Prolog and can use most Prolog systems as a backend compiler. A minimal (but not unique) solution for your problem is:

:- object(allworlds).

    :- public(grandparent/2).
    grandparent(X, Z) :-
        ::parent(X, Y),
        ::parent(Y, Z).

    :- public(parent/2).

:- end_object.


:- object(greece,
    extends(allworlds)).

    parent(kronos, zeus).
    parent(zeus, ares).

:- end_object.

Here, we use inheritance (the individual worlds inherit the common knowledge) and messages to self (the ::/1 control construct) when common predicates need to access world specific predicate definitions (self is the object/world that received the message - grandparent/2 in the example).

Assuming the code is saved in a worlds.lgt file and that you're using SWI-Prolog as the backend:

$ swilgt
...
?- {worlds}.
% [ /Users/pmoura/worlds.lgt loaded ]
% (0 warnings)
true.

?- greece::grandparent(kronos, X).
X = ares.

P.S. If running on windows, use the "Logtalk - SWI-Prolog" shortcut from the Start Menu after installing Logtalk.

Upvotes: 3

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