The SWI-Prolog module system is largely derived from the Quintus
Prolog module system, which is also adopted by SICStus, Ciao and YAP.
Originally, the mechanism for defining meta-predicates in SWI-Prolog was
based on the module_transparent/1
directive and strip_module/3.
Since 5.7.4 it supports the de-facto standard meta_predicate/1
directive for implementing meta-predicates, providing much better
compatibility.
The support for the meta_predicate/1
mechanism, however, is considerably different. On most systems, the caller
of a meta-predicate is compiled differently to provide the required <module>:<term>
qualification. This implies that the meta-declaration must be available
to the compiler when compiling code that calls a meta-predicate. In
practice, this implies that other systems pose the following
restrictions on meta-predicates:
- Modules that provide meta-predicates for a module to be compiled
must be loaded explicitly by that module.
- The meta-predicate directives of exported predicates must follow the module/2
directive immediately.
- After changing a meta-declaration, all modules that call
the modified predicates need to be recompiled.
In SWI-Prolog, meta-predicates are also module-transparent,
and qualifying the module-sensitive arguments is done inside the
meta-predicate. As a result, the caller need not be aware that it is
calling a meta-predicate and none of the above restrictions hold for
SWI-Prolog. However, code that aims at portability must obey the above
rules.
Other differences are listed below.
- If a module does not define a predicate, it is searched for in the
import modules. By default, the import module of any
user-defined module is the
user module. In turn, the
user module imports from the module system
that provides all built-in predicates. The auto-import hierarchy can be
changed using add_import_module/3
and delete_import_module/2.
This mechanism can be used to realise a simple object-oriented system
or a hierarchical module system.
- Operator declarations are local to a module and may be exported. In
Quintus and SICStus all operators are global. YAP and Ciao also use
local operators. SWI-Prolog provides global operator declarations from
within a module by explicitly qualifying the operator name with the
user
module. I.e., operators are inherited from the
import modules (see above).
:- op(precedence, type, user:(operatorname)).
