Defeasible constraint solving

Abstract

Hierarchical Constraint Solving has been proposed as an adequate scheme to specify over-constrained problems where some of the constraints might remain unsatisfied. However it is often not clear which criteria should be adopted to select the “best” combination of constraints to be relaxed. In previous work we proposed IHCS — an Incremental Hierarchical Constraint Solver — but only with a single criterium. This paper presents IHCS as a general scheme to incrementally handle a hierarchy of constraints for a class of comparators using different criteria. The scheme is further extended with a satisfaction mode — given a threshold, this mode enables the search of “satisfactory” solutions to large problems whose optimization is not possible in acceptable time. This scheme can be integrated with different programming environments. In particular, we have integrated it with Prolog to produce an instance of an HCLP language. Because of its portability and incremental nature, IHCS is well suited for reactive systems, allowing the interactive introduction and removal of preferred constraints, illustrated in the examples presented in the paper.