test2.ml

module type ToStr = sig
  type t
  val to_string : t -> string
end

module Str (T:ToStr) = struct
  let stringify t = T.to_string t
end

let to_string (type t) (module M : ToStr with type t = t) (x : t) =
  M.to_string x

module Int = struct
  type t = int
  let to_string = string_of_int
end

let _ = to_string (module Int) 3