## 5.8 Dynamic Modules

So far, we discussed modules that were created by loading a module-file. These modules have been introduced on 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. Example:

    ?- assert(world_a:consistent),
       world_a:unknown(_, fail).

These calls create a module called `world_a' and make the call `world_a:consistent' succeed. Undefined predicates will not start the tracer or autoloader for this module (see unknown/2).

Import and export from dynamically created world is arranged via the predicates import/1 and export/1:

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