Abstract

Constraint Logic Programming (CLP) is a general scheme for extending logic programming to include constraints. It is parameterized by D, the domain of the constraints. However, CLP(D) languages, as well as most other constraint systems, only allow the programmer to specify constraints that must hold. In many applications, such as interactive graphics, page layout, and decision support, one needs to express preferences as well as strict requirements. If we wish to make full use of the constraint paradigm, we need ways to represent these defaults and preferences declaratively, as constraints, rather than encoding them in the procedural parts of the language. We describe a scheme for extending CLP(D) to include both required and preferential constraints, with an arbitrary number of strengths of preference. We present some of the theory of such languages, and an algorithm for executing them. To test our ideas, we have implemented an interpreter for an instance of this language scheme with D equal to the reals. We describe our interpreter, and outline some examples of using this language.

Citations