Actor languages such as Erlang and Elixir are widely used for implementing scalable and reliable distributed applications, but the informally-specified nature of actor communication patterns leaves systems vulnerable to costly errors such as communication mismatches and deadlocks.

\emph{Multiparty session types} (MPSTs) rule out communication errors early in the development process, but until now, the nature of actor communication has made it difficult for actor languages to benefit from session types.

This paper introduces Maty, the first actor language design supporting both \emph{static} multiparty session typing and the full power of actors taking part in \emph{multiple sessions}. Our main insight is to enforce session typing through a flow-sensitive type-and-effect system, combined with an event-driven programming style and first-class message handlers. Using MPSTs allows us to guarantee communication safety: a process will never send or receive an unexpected message, nor will it ever get stuck waiting for a message that will never arrive.

We extend Maty to support Erlang-style supervision and cascading failure, and show that this preserves Maty’s strong metatheory. We implement Maty in Scala using an API generation approach, and evaluate our implementation on a series of microbenchmarks, a factory scenario, and a chat server.