Semantics-Driven Language Design: Statically type-safe virtual types in object-oriented languages

Virtual classes [MMP89] are a very useful language construct introduced in the language Beta which provide for a mechanism similar to that of parametric polymorphism. Unfortunately, the virtual class construct in Beta is not statically type-safe. In this paper we show how a generalization of the semantics of object-oriented languages with a MyType construct led to the discovery of a variant of virtual classes which need no run-time checks. This results in an object-oriented language in which both parametric types and virtual classes (or types) are well-integrated, and which is statically type-safe. Keywords: Language design, semantics, virtual classes, parametric polymorphism, static type checking 1 Introduction In this paper we show how the examination of the semantics of object-oriented languages led to the discovery of a statically type-safe construct which can replace "virtual classes", a construct that, as originally designed and implemented, required dynamic type checking. This..