https://gist.github.com/leque/39a66a2badb0999aee9e12e9303d6237 の写経

show.sml

signature SHOW =
sig
  type t
  val show : t -> string
end

structure ShowInt : SHOW where type t = int =
struct
  type t = int
  val show = Int.toString
end

structure ShowReal : SHOW where type t = real =
struct
  type t = real
  val show = Real.toString
end

(* with a functor *)
functor F (M : SHOW) = struct
  val f = M.show
end

local
  structure F_int = F (ShowInt)
  structure F_real = F (ShowReal)
in
val x = (F_int.f 42, F_real.f 3.14)
end