Simulating quantified class constraints

Abstract

Defining nontrivial class instances for irregular and exponential datatypes in Haskell is challenging, and as a solution it has been proposed to extend the language with quantified class constraints of the form ∀a. C a ⇒ C' (f a) in the contexts of instance declarations. We show how to express the equivalent of such constraints in vanilla Haskell 98, but their utility in this language is limited. We also present a more flexible solution, which relies on a widely-supported language extension.