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).
?- world_b:export(solve/2). % exports solve/2 from world_b ?- world_c:import(world_b:solve/2). % and import it to world_c