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:
Some examples:
| ?- [user].
| :- meta_predicate p(:), q(:).
| r(X) :- p(X).
| q(X) :- p(X).
| ^D
% consulted user in module user, 0 msec 424 bytes
| ?- listing.
r(A) :-
p(user:A).
q(A) :-
p(A).
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.