As mentioned above, SWI-Prolog contains two special modules. The
first one is the module system. This module contains all
built-in predicates. Module system has no import module,
i.e., is a
root of the module graph. The second special module is the
module user. This module forms the initial working space of
the user. Initially it is empty.179Unfortunately
some hooks are traditionally defined in the user module.
The import module of module user is system,
making all built-in predicates available.
All normal application modules import from the module user.
This implies they can use all predicates imported into user
without explicitly importing them. If an application loads all modules
from the
user module using use_module/1,
one achieves a scoping system similar to the C-language, where every
module can access all exported predicates without any special
precautions.
All library modules (see module_property/2)
import directly from system. Library modules are modules
loaded from the SWI-Prolog installation. As they import from system,
the functionality of a library is not affected by operator or predicate
definitions in the user module.