FPH: first-class polymorphism for Haskell

Abstract

Languages supporting polymorphism typically have ad-hoc restrictions on where polymorphic types may occur. Supporting "firstclass" polymorphism, by lifting those restrictions, is obviously desirable, but it is hard to achieve this without sacrificing type inference. We present a new type system for higher-rank and impredicative polymorphism that improves on earlier proposals: it is an extension of Damas-Milner; it relies only on System F types; it has a simple, declarative specification; it is robust to program transformations; and it enjoys a complete and decidable type inference algorithm.