[maleldil]

> Ocaml's module system means that you can describe the async runtime as a signature and make your entire library generic to the async runtime it runs on top of

Is this the program describing the interface it expects, and the compiler matching that with the implementations structure (e.g. Go interfaces, Python Protocols), or is it something someone declares and the libraries implement?

[yawaramin]

Mostly the former. The compiler infers the type of the implementation and ensures it conforms to the interface. But, you can further restrict the interface's type to hide details from the outside world.