What are principal typings and what are they good for?

Abstract

We demonstrate the pragmatic value of the principal typing property, a property distinct from ML's principal type property, by studying a type system with principal typings. The type system is based on rank 2 intersection types and is closely related to ML. Its principal typing property provides elegant support for separate compilation, including 'smartest recompilation' and incremental type inference. Moreover, it motivates a new rule for typing recursive definitions that can type some interesting examples of polymorphic recursion.