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Implementation of Constraint Based Inference Mechanisms through Semantic Unification
- Christian Holzbaur
- We present an extension to unification as a general means to smoothly integrate a variety of control and constraint-based inference mechanisms into PROLOG: freeze, forward checking, CLP(B), CLP(R), ... The proposed extension entails virtually no overhead to programs which do not use it, provided the hook into unification is installed properly. The extension does not disable the standard optimization techniques like clause indexing and last call optimization. A distinguishing feature of our approach is that extended theories themselves are implemented in PROLOG - in contrast to the majority of current implementations in C. The advantages are obvious - the time needed for implementation and modification is reduced, and the resulting programs are in a high-level, transparent language. On the other hand, the efficiency reduction is much lower then expected. We outline our implementation of the extended unification in a standard PROLOG interpreter, and an implementation of two extended theories: forward checking and CLP(R). Our experiments with a partial implementation of CLP(R) indicate that a modern compiler with decent garbage collection would achieve almost the same efficiency as a dedicated implementation in C.