Type classes with more higher-order polymorphism

Abstract

We propose an extension of Haskell's type class system with lambda abstractions in the type language. Type inference for our extension relies on a novel constrained unification procedure called guided higher-order unification. This unification procedure is more general than Haskell's kind-preserving unification but less powerful than full higher-order unification.The main technical result is the soundness and completeness of the unification rules for the fragment of lambda calculus that we admit on the type level.