PolyTOIL is a new statically-typed polymorphic object-oriented programming language that is provably type-safe. By separating the de nitions of subtyping and inheritance, providing a name for the type of self, and carefully de ning the type-checking rules, we have obtained a language that is very expressive while supporting modular type-checking of classes. The matching relation on types, which is related to F-bounded quanti cation, is used both in stating type-checking rules and expressing the bounds on type parameters for polymorphism. The design of PolyTOIL is based on a careful formal de nition of type-checking rules and semantics.

Over the last several years, much interesting work has been done in modelling object-oriented programming languages in terms of extensions of the bounded second-order lambda calculus, F . Unfortunately, it has recently been shown by Pierce ([Pie92]) that type checking F is undecidable. Moreover, he showed that the undecidability arises in the seemingly simpler problem of determining whether one type is a subtype of another. In [Bru93a, Bru93b], the first author introduced a statically-typed, functional, object-oriented programming language, TOOPL, which supports classes, objects, methods, instance variables, subtypes, and inheritance. The semantics of TOOPL is based on F , so the question arises whether type checking in this language is decidable. In this paper we show that type checking for TOOPLE, a minor variant of TOOPL (Typed Object-Oriented Programming Language), is decidable. The proof proceeds by showing that subtyping is decidable, that all terms of TOOPLE have minimum types...