多相バリアントで Show 的な

show.ml

type 'a show = [`Show of (string * 'a)]
type my_int = [`Show of (string * int)]
type my_float = [`Show of (string * float)]
type my_str = [`Show of (string * string)]

let show (`Show (pretty, x) : [< `Show of (string * 'a) ]) = pretty

let print x = print_endline (show x)

let mk_my_int x = `Show((string_of_int x), x)
let mk_my_float x = `Show((string_of_float x), x)
let mk_my_str x = `Show(("\""^x^"\""), x)

let () = begin
  let x = mk_my_int 42 in
  let y = mk_my_float 3.14 in
  let z = mk_my_str "answer" in

  print x;
  print y;
  print z;
end