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Design pattern: Many worlds

Description: Allow reasoning about different worlds, where a world can be e.g. a dataset, a knowledge base, a set of examples.

Related examples: The Metagol port, an inductive logic programming (ILP) system is a good example. In this case, each world is composed by examples and background language and reasoning results on learning a set of rules. See the ports/metagol directory for details. Another, simpler, example of this design pattern is examples/family.

This is a pattern that is trivial in Logtalk but cumbersome at best using Prolog modules. There are two sensible implementations of this design pattern: using inheritance or using parametric objects. Both solutions are illustrated. Load the loader.lgt file to load the sample implementations and look into the SCRIPT.txt file for sample calls.

In the parametric solution, we use a parametric object for the reasoning code and pass the dataset or knowledge base to reason about as a parameter. A simple implementation of this solution is provided in the parametric.lgt file.

In the inheritance solution, the key Logtalk feature (of the reasoning code that we want to apply to many worlds) are messages to self (which allow calling from the reasoning code the predicates that describe a specific world). A simple implementation of this solution is provided in the inheritance.lgt file.

A common scenario is porting an original Prolog module application that assumes a single world defining in "user". Porting consist mainly of applying the following steps:

1. Replacing Prolog module directives with Logtalk directives.
2. Changing calls and updates to predicates in "user" into messages to self.
3. Declaring those predicates as public or protected as needed.