This is a report of ongoing work

Concurrent and distributed programming is notoriously hard. Many languages and software toolkits address the challenge by offering high-level abstractions, such as message-passing processes and actors; they allow for intuitive reasoning, but do not formally ensure that a program implements a given specification.

We address this challenge with Effpi: a toolkit for strongly-typed concurrent programming in Dotty (a.k.a. the future Scala 3 programming language). Effpi allows to specify the intended behaviour of a message-passing application using types: i.e., if a program type-checks and compiles, then it will run and communicate as prescribed by its types.

The foundation of Effpi is a concurrent functional language with a novel blend of behavioural types and a form of dependent types, extending to higher-order interaction (i.e., sending/receiving mobile code). This design has three main advantages. First, it allows to statically verify programs through a combination of type checking and model checking techniques. Second, it is directly implemented (via deep embedding) in Dotty, including a simplified API for strongly-typed actor-based programming. And third, its functional nature allows for an efficient runtime system, supporting highly concurrent programs with millions of interacting processes / actors.