Types that Reflect Changes of Object Usability

                          Franz Puntigam
    Institut für Computersprachen, Technische Universität Wien
           Argentinierstraße 8, A-1040 Vienna, Austria
             E-mail: franz@complang.tuwien.ac.at

Strong, static typing is useful because it can improve the readability,
reliability and optimizability of programs. The usability of an object
may change over time. However, conventional type systems for
object-oriented languages presume that objects are always used in the
same way. In this paper an extension of Ada's type concept is presented:
Possible changes of object usabilities can be specified as part of type
declarations for passive and active objects. This extension is
compatible with inheritance and supports static type checking.