Derivable Type Classes

Generic programming allows you to write a function once, and use it many times at different types. A lot of good foundational work on generic programming has been done. The goal of this paper is to propose a practical way of supporting generic programming within the Haskell language, without radically changing the language or its type system. The key idea is to present generic programming as a richer language in which to write default method definitions in a class declaration. On the way, we came across a separate issue, concerning type-class overloading where higher kinds are involved. We propose a simple type-class system extension to allow the programmer to write richer contexts than is currently possible.