Type Checking Issues for Dynamically Changing Types in Object-Oriented Database Sysems

Abstract

The association between an instance and a class is exclusive and permanent in many class-based, object-oriented (OO) database systems. Therefore, these systems habe serious diffculties ofr applications in which objects take on diffculties ofr applications in which objects take on different and multiple roles over time. Recently, sone researchers have tried to relax this restriction by allowing an object to have multiple most specific types and be able to change to different types furing its lifetime. However, alth ough many of these researchers have realized th importance and difficulties of soving the type problems cused by their proposed extensions, formal semantics and corresponding type issues have not been addressed. In particular, a type checking mechanism, which is to ensure the correctness fo program writing form the type aspect, has never been proposed for these kinds of OO extensions. In thisp paper, we develop an expressive yet semantically sound type calculus ofr objetcs with multiple types (well them roles). While the rich modeling constructs are introduced, the calculus can still be expressed in neat way, by using the polymorphism of overloaded functions. Both static and dynamic type checking frameworks are included and discussed individually. Furthermore, we show the important properties of our calculus, which included Subject Reduction, Strong Normalization and Confluence. The calculus described in this paper provieds a foundation for dynamic type changing and for objects with multiple most-specific types. We show that the calculus is gengeral enough to be applied to various dynamic type and role models with little modification.