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+LookupModule : +Goal

Call the procedure visible in LookupModule rather than the caller module
Goal
Callable term (atom or compound).
LookupModule
Atom.

Description

This predicate provides a means to invoke a procedure which is not visible. Normally, when a procedure is called, the system looks for a visible definition (local or imported) in the caller module. This primitive on the other hand allows to specify a different lookup module.

Two conditions must be satisfied for the lookup to succeed:

The purpose of this is to allow calling procedures whose definition is not visible in the caller module. The two main uses of this facility are:
  1. If there are several definitions of a procedure with the same name in different modules, :/2 can be used to specify which one to call.
  2. If a module wants to define a procedure, but needs to call another procedure of the same name (but from a different module), :/2 can be used to call that one instead of the locally defined one.
Note that :/2 does not affect the caller (context) module. The following table summarises the different idioms:
	Call within module(m)         lookup module    caller module

	..., twice(X), ...               m               m
	..., lm : twice(X), ...         lm               m
	..., twice(X) @ cm, ...          m              cm
	..., lm : twice(X) @ cm, ...    lm              cm
	..., call(twice(X)) @ cm, ...   cm              cm

Note: In earlier versions of Eclipse the left hand side argument of :/2 was required to be the module where the procedure was defined, rather than just visible.

Modules

This predicate is sensitive to its module context (tool predicate, see @/1).

Fail Conditions

Fails if Goal fails

Resatisfiable

Resatisfiable if Goal is resatisfiable

Exceptions

(4) instantiation fault
Goal is not instantiated.
(4) instantiation fault
Module is not instantiated.
(5) type error
Goal is neither an atom nor a compound term.
(5) type error
Module is not an atom.
(68) calling an undefined procedure
Goal is an undefined procedure in Module.

Examples

% two definitions are visible:

    :- lib(ria).    % exports #>= / 2
    :- lib(eplex).  % exports #>= / 2

	..., ria:(X #>= Y), ...
	..., eplex:(X #>= Y), ...


% the library predicate is hidden by the local definition:

    :- lib(lists).     % exports print_list/1

    print_list(List) :-
	writeln("This is the list:"),
	lists:print_list(List).

See Also

call / 1, @ / 2, export / 1