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.
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