Active Objects Based on Algebraic Effects

Algebraic effects are a long-studied programming language construct allowing the implementation of complex control flow in a structured way. With OCaml 5, such features are finally available in a mainstream programming language, giving us a great opportunity to experiment with varied concurrency constructs implemented as simple libraries. In this article, we explore how to implement concurrency features such as futures and active objects using algebraic effects, both in theory and in practice. On the practical side, we present a library of active objects implemented in OCaml, with futures, cooperative scheduling of active objects, and thread-level parallelism. On the theoretical side, we formalise the compilation of a future calculus that models our library into an effect calculus similar to the primitives available in OCaml; we then prove the correctness of the compilation scheme.