Compiling Ask Constraints

Abstract

In this paper we investigate how to extend a generic constraint solver that provides not only tell constraints (by adding the constraint to the store) but also ask tests (by checking whether the constraint is entailed by the store), with general ask constraints. Ask constraints are important for implementing constraint implication, extensible solvers using dynamic scheduling and reification. While the ask-test must be implemented by the solver writer, the compiler can extend this to provide ask behaviour for complex combinations of constraints, including constraints from multiple solvers. We illustrate the use of this approach within the HAL system. © Springer-Verlag 2004.