Using absolute_file_name/3 to find files

A note on usage of multifile/1, since I got tripped up, and it might help others.

When predicates that are declared to be multifile/1 are added from other modules, they need to be prefixed with the module into which the declaration will be made.

E.g., if we define the multifile predicate bar/1 in foo.pl:

:- module(foo, [bar/1]).

:- multifile bar/1.

then to add new clauses for the predicate in baz.pl, we prefix the clause's head with the originating module foo, like so:

:- module(baz, []).

:- use_module(foo).

foo:bar(1).
foo:bar(2).

The effect is to add these clauses into module foo. If we had instead tried just adding the facts as bar(1), we would have hit the warning:

Local definition of baz:bar/1 overrides weak import from foo

because we'd have overwritten foo:bar/1 with the newly defined baz:bar/1.

For more discussion you can see https://swi-prolog.discourse.group/t/having-trouble-using-multifile-1-for-multi-file-predicates/4460

Also see this Logtalk blogpost:

Multifile predicates: dos and don'ts

(via Stack Overflow question A cross-module “interface” call in SWI-Prolog)

• never rely on the order of the multifile predicate clauses
• avoid cuts in multifile predicate definitions. I.e. avoid making assumptions about the predicate calling the multifile predicate and about other multifile predicate definitions.
• the body of a multifile predicate clause is compiled in the context of the entity containing the clause.
• In the presence of Prolog modules or when using Logtalk objects, multifile predicates can be declared and defined in modules and objects. In this case, there’s an entity that holds the multifile predicate primary declaration and other entities defining clauses for the predicate. Other entities defining clauses for the multifile predicate, will need to prefix the predicate functor with the name of the entity containing its primary declaration (i.e. in SWI Prolog, qualify it with the module name; but SWI-Prolog seems relaxed about this, a bit too much maybe).

YAP documentation it's a bit more explicit, I cite from there :

multifile P [ISO]

Instructs the compiler about the declaration of a predicate P in more than one file. It must appear in the first of the loaded files where the predicate is declared, and before declaration of any of its clauses.

Multifile declarations affect reconsult/1 and compile/1: when a multifile predicate is reconsulted, only the clauses from the same file are removed.

Still, a clarification about module interaction would be welcome.