Order-Sorted Generalization

Generalization, also called anti-unification, is the dual of unification. Given terms t and t^′, a generalization is a term t^″ of which t and t^′ are substitution instances. The dual of a most general unifier (mgu) is that of least general generalization (lgg). In this work, we extend the known untyped generalization algorithm to an order-sorted typed setting with sorts, subsorts, and subtype polymorphism. Unlike the untyped case, there is in general no single lgg. Instead, there is a finite, minimal set of lggs, so that any other generalization has at least one of them as an instance. Our generalization algorithm is expressed by means of an inference system for which we give a proof of correctness. This opens up new applications to partial evaluation, program synthesis, and theorem proving for typed reasoning systems and typed rule-based languages such as ASF+SDF, Elan, OBJ, Cafe-OBJ, and Maude.