ReasonML adhoc polymorphism

Show.re

module type SHOW = {
    type t
    let show: t => string
};

type show_impl('a) = (module SHOW with type t = 'a)

module Show_int: SHOW with type t = int = {
    type t = int;
    let show = string_of_int
};

module Show_bool: SHOW with type t = bool = {
    type t = bool;
    let show = string_of_bool
};

let show_int: show_impl(int) = (module Show_int: SHOW with type t = int)
let show_bool: show_impl(bool) = (module Show_bool: SHOW with type t = bool)

let print = (type a, show, x) => {
    let module Show = (val (show: (module SHOW with type t = a)));
    print_endline @@ Show.show(x);
};

true |> print(show_bool);
false |> print(show_bool);