gistfile1.txt

module type Univ = sig
  type t
  val embed: unit -> ('a -> t) * (t -> 'a option)
end

module U1 = struct
    type t = exn
    let embed (type a) () =
        let open struct
            exception E of a
        end in
        let inject e = E e in
        let project e =
            match e with
            | E a -> Some a
            | _ -> None
        in
        (inject, project)
end

module Test (U : Univ) = struct
  let (of_int, to_int) = U.embed ()
  let (of_string, to_string) = U.embed ()
  let r : U.t ref = ref (of_int 13)
  let () = begin
    assert (to_int !r = Some 13);
    assert (to_string !r = None);
    r := of_string "foo";
    assert (to_int !r = None);
    assert (to_string !r = Some "foo");
  end
end

module _ = Test(U1)