Extending HCLP Languages: Nonmonotonicity and Inter-Hierarchy Comparison

Authors: Molly Wilson and Alan Borning

Published in Proceedings of the 1989 North American Logic Programming Conference, Cleveland, October 1989, pages 3-19.


Hierarchical Constraint Logic Programming languages extend Constraint Logic Programming to include constraint hierarchies. These languages provide both required constraints and default constraints of various strengths. In the original definition of HCLP, alternate solutions to a given constraint hierarchy were compared, and only the "best" solutions were returned. However, there was no attempt to compare solutions arising from different choices of rules in the logic program. In many practical applications of HCLP, to rule out unintuitive solutions we do need to make such inter-hierarchy comparisons. Such comparisons introduce nonmonotonic behavior in HCLP programs. We define two related nonmonotonicity properties of HCLP languages, and compare these properties with those of standard nonmonotonic logics. The nonmonotonicity properties create novel implementation problems, which we discuss, while at the same time extending the usefulness of HCLP languages.

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