Ocaml version of Haskell's newtype?

--Haskell version
newtype Id1 = Id1 {unId1 :: Int}
newtype Id2 = Id2 {unId2 :: Int}


(* Ocaml version *)
module Id1 : sig
  type t
  val of_int : int -> t
  val to_int : t -> int
end = struct
  type t = int
  let of_int = ident
  let to_int = ident
end

module Id2 : sig
  type t
  val of_int : int -> t
  val to_int : t -> int
end = struct
  type t = int
  let of_int = ident
  let to_int = ident
end