## 6.10 Dynamic importing using import modules

Until now we discussed the public module interface that is, at least to some extent, portable between Prolog implementations with a module system that is derived from Quintus Prolog. The remainder of this chapter describes the underlying mechanisms that can be used to emulate other module systems or implement other code-reuse mechanisms.

In addition to built-in predicates, imported predicates and locally defined predicates, SWI-Prolog modules can also call predicates from its import modules. Each module has a (possibly empty) list of import modules. In the default setup, each new module has a single import module, which is `user` for all normal user modules and `system` for all system library modules. Module `user` imports from `system` where all built-in predicates reside. These special modules are described in more detail in section 6.11.

In general, the import relations between modules form an acyclic directed graph. The import relation affects the following mechanisms:

Predicate visibility
  When looking for a specific predicate definition the system starts in the target module. If the predicate is undefined there it walks the module import relations depth-first left-to-right searching for a module that defines the predicate. The first encountered definition is used. Note that using the default setup this means it searches the `user` and `system` modules (in that order).
Operators
  Operators are also searched through the import relations. System operators are defined in the module `system`. The user may define operators in `user` to make them globally visible for compatibility with e.g., SICStus Prolog that has no local operators. Normally operators are defined in a module and, when applicable, exported using the module/2 module header.
The unknown flag
  This flag controls the response to encountering an undefined predicate in the target module.
Term and goal expansion
  The hooks term_expansion/2 and goal_expansion/2 (see section 4.3.1) are chained over the import modules that define these hooks. This implies we collect all modules that provide definitions for these hook predicates by traversing the import module relation depth-first and left-to-right. Next, we perform the transformations in a pipeline, starting at the target module.

The list of import modules for a specific module can be manipulated and queried using the following predicates, as well as using set_module/1.

[nondet]import_module(+Module, -Import)
  True if Module inherits directly from Import. All normal modules only import from `user`, which imports from `system`. The predicates add_import_module/3 and delete_import_module/2 can be used to manipulate the import list. See also default_module/2.
[multi]default_module(+Module, -Default)
  True if predicates and operators in Default are visible in Module. Modules are returned in the same search order used for predicates and operators. That is, Default is first unified with Module, followed by the depth-first transitive closure of import_module/2.
add_import_module(+Module, +Import, +StartOrEnd)
  If Import is not already an import module for Module, add it to this list at the `start` or `end` depending on StartOrEnd. See also import_module/2 and delete_import_module/2.
delete_import_module(+Module, +Import)
  Delete Import from the list of import modules for Module. Fails silently if Import is not in the list.