A Session Object Calculus for Structured Communication-Based Programming

We present an object calculus for structured concurrent programming, facilitating the implementation of typed binary protocols, known as sessions. Session types are based on a concept of duality of interactions, providing a powerful type-theoretic foundation for guaranteeing well-behaved parallel composition of communicating code. Our formalism extends the Abadi and Cardelli imperative object calculus with concurrency and typed queue-based primitives for asynchronous communication, each session consisting two queue end-points. It incorporates an iso-recursive formulation for recursive sessions for objects, naturally integrating session-based choices with method invocations. Linear objects enable mobile code to contain active sessions, while mobility of queue end-points realises higher-order session communication. The typing system of our session object calculus ensures type soundness, communication safety and the minimum typing property with respect to a new session subtyping relation for asynchrony in the presence of spawning, cloning and updating objects.