6.3 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.

use_module(+Files)

Load the file(s) specified with Files just like ensure_loaded/1. The files must all be module files. All exported predicates from the loaded files are imported into the module from which this predicate is called. This predicate is equivalent to ensure_loaded/1, except that it raises an error if Files are not module files.

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 warn_override_implicit_import is 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 except([flatten/2]) option of use_module/2, (3) use :- abolish(flatten/2). before the local definition or (4) setting warn_override_implicit_import to 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.

Note that it is always an error to import two modules with use_module/1 that export the same predicate. Such conflicts must be resolved with use_module/2 as described above.

use_module(+File, +ImportList)

Load File, which must be a module file, and import the predicates as specified by ImportList. ImportList is a list of predicate indicators specifying the predicates that will be imported from the loaded module. ImportList also allows for renaming or import-everything-except. See also the import option of load_files/2. The first example below loads member/2 from the lists library 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 option except 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 6.8.

:- 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.