A Declarative View of Modes

Mode information in logic programming is concerned with such things as inputs and outputs of procedures, producers and consumers of variable bindings, instantiation states of calls during execution and the order of execution. Modes seem inextricably tied to the procedural rather than the declarative view of logic programs. Despite this, we argue that purely declarative information can actually express the essence of modes remarkable well. The declarative view allows a high level notion of correctness which is independent of how the information is used. We start from a framework which includes types and show how a set of ground atoms related to the success set can be used to express mode information. We introduce constrained regular trees to deene such sets and show how they can be used as the basis for a polymorphic mode system. The mode system can express directional types, back communication and linearity and can be used to infer lower level mode information for languages such as Mercury.