Session and Union Types for Object Oriented Programming

In network applications it is crucial to have a mechanism to guarantee that communications evolve correctly according to the agreed protocol. Session types offer a method for abstracting and validating structured communication sequences (sessions). In this paper we propose union types for refining and enhancing the flexibility of session types in the context of communication centred and object oriented programming. We demonstrate our ideas through an example and a calculus formalising the main issues of the present approach. The type system garantees that, in well-typed executable programs, after a session has started, the values sent and received will be of the appropriate type, and no process can get stuck forever.