Combining Type Classes and Existential Types

This paper demonstrates that the novel combination of type classes and existential types adds significant expressive power to a language, requiring only a minor syntactic change. We explore this combination in the context of higher-order functional languages with static typing, parametric polymorphism, algebraic data types, and Hindley-Milner type inference. Since we have examined the underlying type-theoretic issues already, this paper focuses on the practical aspects of our extension. We first examine limitations of existing functional and object-oriented languages. We then give examples to demonstrate how our first-class abstract types with user-defined interfaces address those limitations. Finally, we give an informal description of the translation from our language to a target language without type classes. Our extension equally applies to other languages with similar type systems and is independent of strictness considerations. It has been implemented in the Chalmers Haskell B. system, and all examples from this paper have been developed using this system.