Bootstrapping Traits

Despite the undisputed prominence of inheritance as the fundamental reuse mechanism in object-oriented programming languages, the variants – single inheritance, multiple inheritance, and mixin inheritance – all suffer from conceptual and practical problems. Traits overcome the problems arising with the different variants of inheritance. Traits are essentially groups of methods that serve as building blocks for classes and are primitive units of code reuse. In this model, classes are composed from a set of traits by specifying glue code that connects the traits together and accesses the necessary state. This thesis discusses the implementation of traits. The result it presents is a new Smalltalk kernel bootstrapped with traits. The implementation is fully done in Squeak [INGA 97], an open-source dialect of Smalltalk. It is planned that the next generation of Squeak will include traits. Because traits are simple and completely backward compatible with single inheritance, implementing traits in a reflective single inheritance language like Squeak is unproblematic. However, an implementation with a sophisticated and clean design, with the robustness to be used in production and the flexibility to be used as a vehicle for future research, is not trivial. Furthermore our work is aimed at serving as a reference implementation for the introduction of traits in other languages. Hence, we focused on building a simple but powerful system for the future. Consequently following the fundamental idea of a reflective language – using the features of the language to define the behavior of the language itself – we bootstrapped the new kernel which, eventually, allowed us to fully express the system itself with traits. The refactoring of the core of the Smalltalk language as a composition of traits not only improved its quality but also enhanced its understandability. This has the advantage that it is easier maintainable and it facilitates experimentation with the language because the different aspects of the kernel are now available as traits and can therefore be recomposed to create new kernel classes with different properties.