Alice or Bob?: Process Polymorphism in Choreographies

TL;DR

PolyChorλ is a new higher-order functional choreographic language with type and process polymorphism, enabling runtime participant determination and distributed value communication, while ensuring deadlock-free distributed implementations.

Contribution

It introduces PolyChorλ, the first higher-order functional choreographic language with process polymorphism and runtime participant flexibility, expanding the expressiveness of choreographic programming.

Findings

Supports distributed value communication and delegation.

Ensures deadlock-free projected programs.

Enables runtime participant determination.

Abstract

We present PolyChor$\lambda$, a language for higher-order functional \emph{choreographic programming} -- an emerging paradigm by which programmers write the desired cooperative behaviour of a system of communicating processes and then compile it into distributed implementations for each process, a translation called \emph{endpoint projection}. Unlike its predecessor, Chor$\lambda$, PolyChor$\lambda$ has both type and \emph{process} polymorphism inspired by System F$_\omega$. That is, PolyChor$\lambda$ is the first (higher-order) functional choreographic language which gives programmers the ability to write generic choreographies and determine the participants at runtime. This novel combination of features also allows PolyChor$\lambda$ processes to communicate \emph{distributed values}, leading to a new and intuitive way to write delegation. While some of the functional features of PolyChor$\lambda$ give it a weaker correspondence between the semantics of choreographies and their endpoint-projected concurrent systems than some other choreographic languages, we still get the hallmark end result of choreographic programming: projected programs are deadlock-free by design.