Question

I have already defined an interface and a module:

module type BASE =
  sig
    type 'a base
    val ...
    ...
  end

module base =
  (struct
    type 'a base
    let ...
    ...
  end: BASE) 

I would like to define another interface DERIVED inherited from BASE, which includes all the types and signatures of BASE, and has its own types and declarations. Then I could define another module Derived whose type is DERIVED.

Could anyone tell me how to do it? Is it necessarily the object feature of OCaml?

Going further, is it possible to change the names of the types of DERIVED: for instance, it is called 'a derived instead of ‘a base?

Answer 1

You can use include on both signature and implementations.

module type DERIVED = sig 
  include BASE
  val foo : ...
end

module Derived : DERIVED = struct
  include Base
  let foo = ...
end

This is a simple inclusion of structure. It is not “(implementation) inheritance” in the sense of OOP, because there is no late-binding (open recursion) involved. If you define a value f in Derived that was previously defined in Base, it will shadow the previous f, but call to f from Base will still use the old value, not the new implementation.