Try to further understanding the interface/module of OCaml

Question

I understand in OCaml there are concepts of interfaces and module.

And I understand how to use them now.

However, what I don\'t under

Answer 1

The most direct correspondence between your Java example and OCaml is using a functor (what OCaml calls a static function from a module to a module). So suppose you have the following implemented in OCaml:

module type Map = sig
  (* For simplicity assume any key and value type is allowed *)
  type ('k, 'v) t

  val make : unit -> ('k, 'v) t
  val put : ('k, 'v) t -> ~key:'k -> ~value:'v -> unit
end

module Hashtable : Map = struct ... end
module HashMap : Map = struct ... end

Then you would write a functor like this:

module MyFunctor(Map : Map) = struct
  let my_map =
    let map = Map.make () in
    Map.put map ~key ~value;
    map
end

Then you would instantiate a module using the functor:

module MyModule = MyFunctor(Hashtable)

And voila, changing the implementation is a one-line diff because both the module implementations conform to the Map signature:

module MyModule = MyFunctor(HashMap)