Define parts of a predicate across several modules

Question

I'm trying to write a predicate move/3 which handles several kinds of terms, each kind of which is defined in a separate file. I'm trying to use modules for this, because the files contain other predicates which should be namespaced appropriately.

So, I've created a module cat.prolog with contents:

:- module(cat, [move/3]).
:- multifile(move/3).

move(cat(C), P, cat(C2)) :-
   ...

Similarly for dog.prolog.

And main.prolog with:

:- use_module(['cat.prolog'], [move/3]).
:- use_module(['dog.prolog'], [move/3]).

(various predicates that use move/3 and expecting the clauses from all imported modules to be applicable.)

Trying to run this in SWI-Prolog:

?- ['main.prolog'].
%  cat.prolog compiled into cat 0.00 sec, 4,800 bytes
ERROR: Cannot import dog:move/3 into module user: already imported from cat
Warning: /home/edmund/main.prolog:2:
        Goal (directive) failed: user:use_module([dog.prolog],[move/3])
% main.prolog compiled 0.00 sec, 10,176 bytes
true.

At this point I can use dog:move/3 and cat:move/3 but not move/3. It works for the cat case but not the dog case.

I get the feeling there's a really obvious way to do this. I've tried combining modules and imports and multifile directives in many ways and still not found it yet...

Answer 1

The multifile/1 syntax is simple, but the documentation lacks a simple example...

I created 3 modules files: pets.pl, cat.pl, dog.pl.

:- module(pets, [test/0, move/3]).
:- multifile move/3.
move(A,B,C) :- writeln(pets-move(A,B,C)).
test :- forall(move(A,B,C), writeln(move(A,B,C))).

:- module(cat, []).
:- use_module(pets).
pets:move(A,B,C) :- writeln(cat-move(A,B,C)).

:- module(dog, []).
:- use_module(pets).
pets:move(A,B,C) :- writeln(dog-move(A,B,C)).

Note the relevant syntax Module:Pred :- ... in the 'dependent' files

?- [cat,dog].
%  pets compiled into pets 0.00 sec, 3 clauses
% cat compiled into cat 0.01 sec, 7 clauses
% dog compiled into dog 0.00 sec, 3 clauses
true.

?- test.
Correct to: "pets:test"? yes
pets-move(_G41,_G42,_G43)
move(_G41,_G42,_G43)
cat-move(_G41,_G42,_G43)
move(_G41,_G42,_G43)
dog-move(_G41,_G42,_G43)
move(_G41,_G42,_G43)
true.

?- 

Answer 2

A second working solution, this time without using the Logtalk extension to Prolog, following up my comment about using the include/1 directive:

----- common.pl -----
:- export(move/1).
---------------------

----- cat.pl -----
:- module(cat, []).
:- include(common).

move(cat).
---------------------

----- dog.pl -----
:- module(dog, []).
:- include(common).

move(dog).
---------------------

$ swipl
...
?- use_module(cat, []), use_module(dog, []).
% cat compiled into cat 0.00 sec, 4 clauses
% dog compiled into dog 0.00 sec, 4 clauses
true.

?- cat:move(X).
X = cat.

?- dog:move(X).
X = dog.

?- module_property(cat, exports(Exports)).
Exports = [move/1].

?- module_property(dog, exports(Exports)).
Exports = [move/1].

Like in the Logtalk solution, multifile predicates are not the answer.

It should be noted that, in most Prolog module systems, including SWI-Prolog, any module predicate can be called using explicit qualification as in the queries above. But using a file to hold the common bits have still two advantages: (1) it makes explicit in a single place the common bits and avoids source code duplication; (2) well behaved applications should only call exported predicates and there are tools that can detect violation of this principle. On the down side, an included file is only a first class entity of you consider files itself as such. Care must also be taking when loading the modules that include the common bits file as usage of e.g. consult/1 or use_module/1 will result in conflicts:

?- [cat, dog].
% cat compiled into cat 0.00 sec, 4 clauses
ERROR: import/1: No permission to import dog:move/1 into user (already imported from cat)
% dog compiled into dog 0.00 sec, 4 clauses
true.

In practice, this solution may result in most or all module predicates to be called using explicit qualification.