Goal Solving as Operational Semantics

To combine a functional or equational programming style with logic programming, one can use an underlying logic of Horn clauses with equality (as an interpreted predicate symbol) and (typed) terms. From this point of view, the most satisfying operational semantics would search for solutions to equations or predicates. \Narrowing" and many of its variants are complete mechanisms for generating solutions. Such a melded language is more expressive than either paradigm alone: functional dependencies are explicit; \multi-valued" functions can be better expressed as predicates; nested functions can be evaluated without recourse to search (backtracking); (nonconstructor) terms can serve as arguments to predicates; functions can be inverted; nonterminating functions can be programmed in a terminating fashion; goals can be simpli ed in a \don't care" manner; \functional" negation can prune searches. Moreover, the availability of backtracking and existential (\logic") variables provides an alternative to in nite data structures (\streams").