Flexible Search Strategies in Prolog CHR

We extend the refined operational semantics of the Constraint Handling Rules language to support the implementation of different search strategies. Such search strategies are necessary to build efficient Constraint Logic Programming systems. This semantics is then further refined so that it is more suitable as a basis for a trailing based implementation. We propose a source to source transformation to implement breadth first search in CHR with Prolog as a host language. Breadth first is chosen because it exhibits the main difficulties in the implementation of search strategies, while being easy to understand. We evaluate our implementation on some benchmarks and give directions for future work.