type typ(_) =
  | Unit: typ(unit)
  | Int: typ(int)
  | Float: typ(float)
  | String: typ(string)
  | Pair((typ('a), typ('b))): typ(('a, 'b))
  | Arrow((typ('a), typ('b))): typ('a => 'b);

let hello: type a. (typ(a), a) => a =
  (ty, v) =>
    switch (ty) {
    | String => "hello"
    | _ => v
    };