Foundations for Extensible Objects with Roles

Giorgio Ghelli, Pisa

Object-oriented database systems are an emerging technology which is very 
promising, and which lays its foundations on the integration of ideas from 
object-oriented languages with specific needs of database applications.

One basic motivation for the adoption of such systems is the ability of
modelling any real-world entity with one object which matches its structure and
behavior. To this end, the standard notion of object has to be augmented 
to be able to model the fact that an entity may acquire new pieces of
structure and behavior during its existence, without changing its identity.
To allow this extensibility in a statically typed system, a notion of
context-dependent behavior (``role playing'') has to be added to the basic
features of object-oriented languages. This feature turns out to be a
useful modeling device too.

Languages with role mechanisms have already been proposed. However, their
design is full of choices which cannot be easily justified. 
A strong foundation for the object-with-role notion would be extremely 
helpful to justify these choices and to understand, and prove, the properties
of such a mechanism. In this talk we describe such a foundation, building
on the object model proposed by Abadi and Cardelli.