1 Type Classes Without Types ∗

Data-directed programs consist of collections of generic functions, functions whose underlying implementation differs depending on properties of their arguments. Scheme’s flexibility lends itself to developing generic functions, but the language has some shortcomings in this regard. In particular, it lacks both facilities for conveniently extending generic functions while preserving the flexibility of ad-hoc overloading techniques and constructs for grouping related generic functions into coherent interfaces. This paper describes and discusses a mechanism, inspired by Haskell type classes, for implementing generic functions in Scheme that directly addresses the aforementioned concerns. Certain properties of Scheme, namely dynamic typing and an emphasis on block structure, have guided the design toward an end that balances structure and flexibility. We describe the system, demonstrate its function, and argue that it implements an interesting approach to polymorphism and, more specifically, overloading.