Proving Termination of Constraint Solver Programs

We adapt and extend existing approaches to termination in rule-based languages (logic programming and rewriting systems) to prove termination of actually implemented CHR constraint solvers. CHR (Constraint Handling Rules) are a declarative language especially designed for writing constraint solvers. CHR are a concurrent constraint logic programming language consisting of multi-headed guarded rules that rewrite constraints into simpler ones until they are solved. The approach allows to prove termination of many constraint solvers, from Boolean and arithmetic to terminological and path-consistent constraints. Because of multi-heads, our termination orders must consider conjunctions, while atomic formulas suffice in usual approaches. Our results indicate that in practice, proving termination for concurrent constraint logic programs may not be harder than for other classes of logic programming languages, contrary to what has been feared in the literature.