How to make module implement multiple signatures

You can use include when defining types. It is useful for more mathy designs, like bastet/bastet/src/Interface.ml at master · Risto-Stevcev/bastet · GitHub

module type A = {
    type a
}
module type B = {
    type b
}
module type AB = {
    include A
    include B
}

There is also extra syntax for dealing with collisions and abstractions, like include B with type a = or :=. There isn’t any rescript documentation on this, afaik, so I find it more useful to read the ocaml docs and translate. OCaml - The module system

I did this a lot when I first came from Haskell. After a handful of years, I sometimes will use include, but I don’t think I ever apply a module type at definition time (well, sometimes I do for debugging but then I delete it).

It isn’t wrong by any means, I just find that if you only limit modules in specific use cases (functors) instead of trying to track everything a module can do and immediately limiting it, you often end up with less code and a simpler design. That totally depends on the type of code you write though. I mostly write ddd style product services, so I have a specific set of goals.

When I really do want to limit the public interface of a module, I’m more likely to use a resi file, and leave the module type definitions for defining requirements for functors.