MPI-I-93-243

Narrowing is the operational principle of languages that integrate functional and logic programming. We propose a notion of a needed narrowing step that, for inductively sequential rewrite systems, extends the Huet and Levy notion of a needed reduction step. We define a strategy, based on this notion, that computes only needed narrowing steps. Our strategy is sound and complete for a large class of rewrite systems, is optimal w.r.t. the cost measure that counts the number of distinct steps of a derivation, computes only independent unifiers, and is efficiently implemented by pattern matching.