Issues in the Design of an Object Oriented Programming Language

The object oriented paradigm, which advocates bottom-up program development, appears at rst sight to run counter to the classical, top-down approach of structured programming. The deep requirement of structured programming, however, is that programming should be based on well-de ned abstractions with clear meaning rather than on incidental characteristics of computing machinery. This requirement can be met by object oriented programming and, in fact, object oriented programs may have better structure than programs obtained by functional decomposition. The de nitions of the basic components of object oriented programming, object, class, and inheritance, are still su ciently uid to provide many choices for the designer of an object oriented language. Full support of objects in a typed language requires a number of features, including classes, inheritance, genericity, renaming, and rede nition. Although each of these features is simple in itself, interactions between features can become complex. For example, renaming and rede nition may interact in unexpected ways. In this paper, we show that appropriate design choices can yield a language that ful lls the promise of object oriented programming without sacri cing the requirements of structured programming.