Generalising the BETA Type System

The type system of object-oriented programming languages should enable the description of models that originate from object-oriented analysis and design. In this paper, the BETA type system is generalised, resulting in direct language support for a number of new modelling aspects. The increased expressive power is obtained from a synergy between general block structure and the generalised type hierarchy, and not from syntactic additions to the language. The type hierarchy described in this paper is a superset of the class hierarchy. In order to regain an orthogonal and internally consistent language, we investigate the impact of the new type hierarchy on other parts of the language. The resulting increase in expressive power serves to further narrow the gap between statically and dynamically typed languages, adding among other things more general generics, immutable references, and attributes with types not known until runtime.