Extensible Object-Orientation

. Object_oriented languages seem to be ideal to implement extensible systems. However, the fact that extensible systems can never be considered "closed" has consequences for the used language and its implementation. The challenge is to master for an open system correctness in general and type safety in particular. The following discussion is based on experiences with the design of the hybrid OOL Oberon_2, and its use to implement the extensible OO document editor Write and the extensible OO operating system Ethos. 1 Situation One of the great potentials of object_oriented languages is the possibility to create truely extensible systems. However, at a second glance it is not at all clear what extensibility really means. After all, even assembly programs are extensible; in a sense even better than anything else! Also, extending an operating system by loading an application is quite an old achievement. Hence, if arbitrary code manipulations are allowed, arbitrary extensions are easy. On ..