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|Importing Predicates into a Module|
Predicates can be added to a module by importing them from another module. Importing adds predicates to the namespace of a module. An imported predicate can be called exactly the same as a locally defined predicate, although its implementation remains part of the module in which it has been defined.
Importing the predicates from another module is achieved using the directives use_module/1 or use_module/2. Note that both directives take filename(s) as arguments. That is, modules are imported based on their filename rather than their module name.
The imported predicates act as weak symbols in the module
into which they are imported. This implies that a local definition of a
predicate overrides (clobbers) the imported definition. If the flag
true (default), a warning is printed. Below is an
example of a module that uses library(lists), but redefines flatten/2,
giving it a totally different meaning:
:- module(shapes, ). :- use_module(library(lists)). flatten(cube, square). flatten(ball, circle).
Loading the above file prints the following message:
Warning: /home/janw/Bugs/Import/t.pl:5: Local definition of shapes:flatten/2 overrides weak import from lists
This warning can be avoided by (1) using use_module/2
to only import the predicates from the
lists library that
are actually used in the `shapes' module, (2) using the
option of use_module/2,
before the local definition or (4) setting
false. Globally disabling this warning is only
recommended if overriding imported predicates is common as a result of
design choices or the program is ported from a system that silently
overrides imported predicates.
importoption of load_files/2. The first example below loads member/2 from the
listslibrary and append/2 under the name
list_concat, which is how this predicate is named in YAP. The second example loads all exports from library
optionexcept for meta_options/3. These renaming facilities are generally used to deal with portability issues with as few changes as possible to the actual code. See also section C and section 6.7.
:- use_module(library(lists), [ member/2, append/2 as list_concat ]). :- use_module(library(option), except([meta_options/3])).
In most cases a module is imported because some of its predicates are being used. However, sometimes a module is imported for other reasons, e.g., for its declarations. In such cases it is best practice to use use_module/2 with empty ImportList. This distinguishes an imported module that is used, although not for its predicates, from a module that is needlessly imported.
The module/2, use_module/1 and use_module/2 directives are sufficient to partition a simple Prolog program into modules. The SWI-Prolog graphical cross-referencing tool gxref/0 can be used to analyse the dependencies between non-module files and propose module declarations for each file.