Solvability = Typability + Inhabitation

Abstract

We extend the classical notion of solvability to a lambda-calculus equipped with pattern matching. We prove that solvability can be characterized by means of typability and inhabitation in an intersection type system P based on non-idempotent types. We show first that the system P characterizes the set of terms having canonical form, i.e. that a term is typable if and only if it reduces to a canonical form. But the set of solvable terms is properly contained in the set of canonical forms. Thus, typability alone is not sufficient to characterize solvability, in contrast to the case for the lambda-calculus. We then prove that typability, together with inhabitation, provides a full characterization of solvability, in the sense that a term is solvable if and only if it is typable and the types of all its arguments are inhabited. We complete the picture by providing an algorithm for the inhabitation problem of P.