An object-oriented inference engine for PROLOG

This article describes an object-oriented inference engine for PROLOG. The inference engine is part of the Graphically Oriented Development Environment for Logic (Gödel) programming. Gödel incrementally translates source clauses to a persistent clause base in which each clause is an object. The inference engine is a distributed one in which each clause object knows how to unify and execute itself. This means that Gödel supports multiple queries at various points of execution. That is, although multiple queries cannot actually execute concurrently, they can be suspended at any point in their execution and reactivated at any time. This is a major advantage during debugging and exploratory programming. In addition, the inference engine supports primitive clauses written in the implementation language, SMALLTALK-80. This provides a simple interface between PROLOG clauses and SMALLTALK objects that puts the entire SMALLTALK80 class hierarchy in the hands of PROLOG programmers. Despite these advantages, an objectoriented interface engine has one drawback. In its most general form, the object granularity is too small, so that too many objects must be created and destroyed during program execution. This reduces execution speed. For this reason, several optimizations have been performed in the implementation to consolidate objects to a more reasonable size. With these optimizations, Gödel's interpreter has execution speeds that are still significantly slower than compiled PROLOG but acceptable for exploration, development, and debugging. In exchange, this approach provides integration with the object-oriented paradigm, including all the power that comes from objectoriented programming environments, including browsers, incremental translation, inspectors, debuggers, and multiple query execution, as well as the ability to easily experiment with both the environment and the PROLOG language itself.