Observable confluence for constraint handling rules

Authors:
Gregory J. Duck;Peter J. Stuckey;Martin Sulzmann
Affiliations:
NICTA Victoria Laboratory, Department of Computer Science and Software Engineering, University of Melbourne, Australia;NICTA Victoria Laboratory, Department of Computer Science and Software Engineering, University of Melbourne, Australia;School of Computing, National University of Singapore, Singapore
Venue:
ICLP'07 Proceedings of the 23rd international conference on Logic programming
Year:
2007

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Abstract

Constraint Handling Rules (CHR) are a powerful rule based language for specifying constraint solvers. Critical for any rule based language is the notion of confluence, and for terminating CHR programs there is a decidable test for confluence. But many CHR programs that are in practice confluent fail this confluence test. The problem is that the states that illustrate non-confluence are not observable from the initial goals of interest. In this paper we introduce the notion of observable confluence, a more general notion of confluence which takes into account whether states are observable. We devise a test for observable confluence which allows us to verify observable confluence for a range of CHR programs dealing with agents, type systems, and the union-find algorithm.