Abstract

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. 1 Introduction Constraint Handling Rules [3] (CHR) are a powerful rule based language forspecifying constraint solvers. Constraint handling rules operate on a global multiset (conjunction) of constraints. A constraint handling rule defines a rewritingfrom one multi-set of constraints to another.

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