GADT ReScript

gadt.res

%%raw(
  "/*
    * this js function will work under both [string] and [float]
   */
  function add (x,y){
    return x + y;
}")

type rec t<_> = | String : t<string> | Float : t<float>

external add : (@ignore t<'a>, 'a, 'a) => 'a = "add"

Js.log(add(String, "abc", "def"))
Js.log(add(Float, 1. , 2.))

@unboxed
type rec k = Any('a) : k

external add: (k, k) => k = "add"
Js.log(add(Any("a"), Any("b")))

Js.log(add(Any(10), Any(20)))
Js.log(add(Any(10.1), Any(20.1)))