Designing an object-oriented programming language with behavioural subtyping

Abstract

This paper describes the design of the parallel object-oriented programming language POOL-I. We concentrate on the type system of the language and specifically on the aspects of subtyping and genericity. POOL-I is the first language we know of that includes subtyping and inheritance as completely separate language mechanisms. By decoupling these two, which have been strongly tied together in other statically typed object-oriented languages with inheritance, a much cleaner language design can be obtained and a much more flexible use of both mechanisms can be made in actual programs. In POOL-I subtyping is based only on the externally observable behaviour of objects. This includes not only their signature (the names of available methods and their parameter and result types) but also more detailed information about this behaviour. We also present a preliminary formalism in which these properties can be specified. Finally we introduce the more advanced features of the POOL-I type system, such as bounded genericity and dynamic type manipulation.