Ocaml : Passing constructor type between modules

Question

I have this module type :

module type MOD =    
  sig   
    type operand
    type op
    val print : op -> string
  end;;

An implementation of MOD is :

module M1:MOD =
  struct
    type operand = Mem of int | Reg of int | Const of int
    type op = Add of operand * operand| Sub of operand * operand
    let print op = match op with
      | Add _ -> "Add"
      | Sub _ -> "Sub"
  end;;

I want to create a parametrized module witch take op type from the first module and implement function on variables of that type. like this:

module  ARCHI = functor (M : MOD) ->
  struct
    type op = M.op

    let execute o = match o with
      | Add (x,y) -> x + y
      | Sub (x,y) -> x - y
  end;;

I get error : Unbound constructor Add. How can I manage this?

Answer 1

You have declared type op to be abstract in MOD, and then you have defined your functor to take module M of type MOD. Thus, you correctly cannot have access to the implementation of op. Otherwise, your functor doesn't do what it claims to which is take any module of type MOD, not just the specific M1 you've defined.

You can expose the implementation of op in MOD by writing out the full definition of the type in the signature. However, it's not clear that you actually need a functor at all here.