Declarative Meta Level Control for Logic Programs

In conventional logic programming systems, control information is expressed by clause and goal order and by purely procedural constructs, e.g., the Prolog cut. This approach destroys the equivalence of declarative and procedural semantics in logic programs. In this paper, we argue that in order to comply with the logic programming paradigm, control information should also be expressed declaratively. A program should be divided into a logical theory that speciies the problem to be solved and control information that speciies the strategy of the deduction process. Control information is expressed through meta level control clauses. These control clauses are evaluated dynamically in order to select the subgoal that will be resolved next and to select the resolving clause. Program clauses have guards that allow clause determinism to be expressed. A major design goal for the presented work is to keep the declarative and the procedural semantics of logic programs equivalent. The emphasis lies on the precise speciication of the introduced meta level constructs.