Extensible and modular generics for the masses

A generic function is a function that is defined on the structure of data types: with a single definition, we obtain a function that works for many data types. In contrast, an ad-hoc polymorphic function requires a separate implementation for each data type. Previous work by Hinze on lightweight generic programming has introduced techniques that allow the definition of generic functions directly in Haskell. A severe drawback of these approaches is that generic functions, once defined, cannot be extended with ad-hoc behaviour for new data types, precluding the design of an extensible and modular generic programming library based on these techniques. In this paper, we present a revised version of Hinze’s Generics for the masses approach that overcomes this limitation. Using our new technique, writing an extensible and modular generic programming library in Haskell 98 is possible.