The eXchange Calculus (XC): A functional programming language design for distributed collective systems

Distributed collective systems are systems formed by homogeneous dynamic collections of devices acting in a shared environment to pursue a joint task or goal. Typical applications emerge in the context of wireless sensor networks, robot swarms, groups of wearable-augmented people, and computing infrastructures. Programming such systems is notoriously hard, due to requirements of scalability, concurrency, faults, and difficulty in making desired collective behaviour ultimately emerge: ad-hoc languages and mechanisms have been proposed threads like spatial computing, macro-programming, and field-based coordination. In this paper we present the eXchange Calculus (XC), formalising a tiny set of key mechanisms, usable across many different languages and platforms, allowing to express the overall interactive behaviour of distributed collective systems in a declarative way. In this approach, computation (executed in asynchronous rounds), communication (which is neighbour-based), and state over time, are all expressed by a single declarative construct, called exchange. We provide a formalisation of XC in terms of syntax, device-level and network-level semantics, prove a number of properties of the calculus, and discuss applicability considering a smart city scenario. XC is implemented as a DSL in Scala and in C＋＋, with different trade-offs in terms of productivity and platform targetting.