FPH: First-class Polymorphism for Haskell Declarative, Constraint-free Type Inference for Impredicative Polymorphism

Languages supporting polymorphism typically have ad-hoc restrictions on where polymorphic types may occur. Supporting "first class" 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.