Towards a type theory for active objects

A key property of object-oriented languages is that they promote software reuse through inter-changeability or plug compatibility of software components that conform to the same message-passing interface. A characterization of such an interface is a type, and can be viewed as a constraint on the behaviour of an object. An object that conforms to the type meets the constraint. A subtype, in this view, is simply a stronger constraint: all objects that conform to a subtype also conform to the supertype. The nature of these constraints may vary, however, as may the rules for determining when one type is a subtype of another. The choice of characterization will depend on the computational model of a particular language and the way in which objects interact. We seek to develop a notion of type that will serve to characterize concurrent, active objects whose behaviour may not conform to a strict client/server model of interaction and communication.