Integration and Optimization of Rule-Based Constraint Solvers

One lesson learned from practical constraint solving applications is that constraints are often heterogeneous. Solving such constraints requires a collaboration of constraint solvers. In this paper, we introduce a methodology for the tight integration of CHR constraint programs into one such program. CHR is a high-level rule-based language for writing constraint solvers and reasoning systems. A constraint solver is wellbehaved if it is terminating and confluent. When merging constraint solvers, this property may be lost. Based on previous results on CHR program analysis and transformation we show how to utilize completion to regain well-behavedness. We identify a class of solvers whose union is always confluent and we show that for preserving termination such a class is hard to find. The merged and completed constraint solvers may contain redundant rules. Utilizing the notion of operational equivalence, which is decidable for well-behaved CHR programs, we present a method to detect redundant rules in a CHR program.