Polymorphic subtyping in OʹHaskell

OʹHaskell is a programming language derived from Haskell by the addition of concurrent reactive objects and subtyping. Because Haskell already encompasses an advanced type system with polymorphism and overloading, the type system of OʹHaskell is much richer than what is the norm in almost any widespread object-oriented or functional language. Yet, there is strong evidence that OʹHaskell is not a complex language to use, and that both Java and Haskell programmers can easily find their way with its polymorphic subtyping system. This paper describes the type system of OʹHaskell both formally and from a programmerʹs point of view; the latter task is accomplished with the aid of an illustrative, real-world programming example: a strongly typed interface to the graphical toolkit Tk.