module type Show = sig
  type t
  val show : t -> string
end

module ShowInt : Show with type t = int = struct
  type t = int

  let show = Int.to_string
end

module ShowFloat : Show with type t = float = struct
  type t = float

  let show = Float.to_string
end

(* with a functor *)
module F(M : Show) = struct
  let f = M.show
end

let x =
  let module F_int = F(ShowInt) in
  let module F_float = F(ShowFloat) in
  F_int.f 42, F_float.f 3.14

(* with a first-class module *)
let f (type a) (module M : Show with type t = a) x = M.show x

let x =
  f (module ShowInt) 42, f (module ShowFloat) 3.14