Module Name Expansion

Some predicates take goals as arguments (i.e. meta-predicates). These arguments must include a module specification stating which module the goal refers. Some other predicates also need module information i.e. compile/1. The property of needing module information is declared with a meta-predicate declaration (see Meta Decl). Goals for these predicates are module name expanded to ensure the module information. Goals appearing in queries and meta-calls are expanded prior to execution while goals in the bodies of clauses and directives are expanded at compile time. The expansion is made by preceding the relevant argument with Module:. If the goal is prefixed by Module:, Module is used for the expansion; otherwise, the source/type-in module is used. An argument is not expanded if:

• It already has a module prefix, or
• It is a variable which appears in an expandable position in the head of the clause.

Some examples:

     | ?- [user].
     | :- meta_predicate p(:), q(:).
     | r(X) :- p(X).
     | q(X) :- p(X).
     | ^D
     % consulted user in module user, 40 msec 1088 bytes
     
     yes
     | ?- listing.
     
     r(A) :-
             p(user:A).
     
     q(A) :-
             p(A).
     
     yes
     

Here, p/1 and q/1 are declared as meta-predicates while r/1 is not. Thus the clause r(X) :- p(X) will be transformed to r(X) :- p(M:X), by item 2 above, where M is the type-in module, whereas q(X) :- p(X) will not.

     | ?- m:assert(f(1)).
     

Here, assert/1 is called in the module m. However, this does not ensure that f(1) is asserted into m. The fact that assert/1 is a meta-predicate makes the system module name expand the goal, transforming it to m:assert(m:f(1)) before execution. This way, assert/1 is supplied the correct module information.