Mixed sessions

Abstract

Session types describe patterns of interaction on communicating channels. Traditional session types include a form of choice whereby servers offer a collection of options, of which each client selects exactly one. Mixed choices blur the distinction between servers and clients (that is, external and internal choice) by allowing options to be both offered and selected in the same choice. We introduce mixed choices in the context of session types and argue that they increase the flexibility of program development at the same time that they reduce the number of synchronisation primitives down to exactly one. We present a type system incorporating subtyping and prove preservation and absence of runtime errors for well-typed processes. We further show that classical (conventional) sessions can be faithfully and tightly embedded in mixed choices, and conversely that there is a minimal encoding from mixed choices to classical sessions. Finally, we discuss algorithmic type checking and a runtime system built on top of a conventional (choice-less) message-passing architecture.