Use cases for GADT in OCaml/Reason

gadt as singleton type.re

type typ(_) =
  | Int: typ(int)
  | String: typ(string)
  | Pair(typ('a), typ('b)): typ(('a, 'b));

let rec to_string: type t. (typ(t), t) => string =
  (t, x) =>
    switch (t) {
    | Int => string_of_int(x)
    | String =>
      Js.log(x);
      x;
    | Pair(t1, t2) =>
      let (x1, x2) = x;
      let text = "(" ++ to_string(t1, x1) ++ "," ++ to_string(t2, x2) ++ ")";
      Js.log(text);
      text;
    };