So far, we discussed modules that were created by loading a module
file. These modules have been introduced to facilitate the development
of large applications. The modules are fully defined at load-time of the
application and normally will not change during execution. Having the
notion of a set of predicates as a self-contained world can be
attractive for other purposes as well. For example, assume an
application that can reason about multiple worlds. It is attractive to
store the data of a particular world in a module, so we extract
information from a world simply by invoking goals in this world.
Dynamic modules can easily be created. Any built-in predicate that
tries to locate a predicate in a specific module will create this module
as a side-effect if it did not yet exist. For example:
?- assert(world_a:consistent),
set_prolog_flag(world_a:unknown, fail).
These calls create a module called‘world_a’and make the
call‘world_a:consistent’succeed. Undefined predicates will
not raise an exception for this module (see unknown).
Import and export from a dynamically created world can be achieved
using
import/1
and export/1
or by specifying the import module as described in
section 6.10.
?- world_b:export(solve/2). % exports solve/2 from world_b
?- world_c:import(world_b:solve/2). % and import it to world_c
