Module re-export not visible across SWI-Prolog and SICStus

ISO Standard and Re‑export Visibility

ISO/IEC 13211‑2 does not mandate that a predicate re‑exported through a module chain becomes an exported predicate of the importing module. The standard leaves re‑export semantics implementation‑defined, so portable code cannot rely on transitive visibility.

Behavior in SWI‑Prolog and SICStus

SWI‑Prolog

In SWI‑Prolog, a predicate is callable as A:q/1 from a client only if q/1 appears in the export list of module A (i.e., in the :- module(A, [p/1, q/1]). declaration) or is defined in A. Merely importing q/1 from B does not make it available as A:q/1 unless it is also exported by A.

SICStus Prolog

SICStus requires an explicit export or re‑export declaration for q/1 in module A. Without it, A:q/1 and call(A:q(X)) will fail or resolve only to B:q/1.

Portable Pattern

To guarantee that clients can call A:p/1 and A:q/1 on both systems:

1. In module A, declare both predicates in the export list: :- module(A, [p/1, q/1]).
2. Define p/1 locally in A.
3. Define a wrapper for q/1 that forwards to B: q(X) :- B:q(X).

Clients then use A:p/1 and A:q/1 (or call(A:q(X))) without modification on either system.

Caveats

• The wrapper q(X) :- B:q(X). executes q/1 in B’s module context. If q/1 depends on predicates local to A or on meta‑predicate context (e.g., call/1, !), qualify those calls inside B or pass goals explicitly.
• Exact syntax for export/re‑export lists varies by version; test the pattern on your target versions rather than assuming compatibility.

Verification Steps

1. Load module A and a client module in SWI‑Prolog; run A:p(1), A:q(1), and call(A:q(1)). All should succeed.
2. Repeat in SICStus; if A:q/1 fails, ensure the wrapper and export list are present and retry.
3. Inspect each system’s module export list for A (e.g., module_property(A, exports(Exports)) in SWI) to confirm q/1 is listed.