(*
 * apples_oranges.sml - generic programming example
 *   Ronald Garcia
 *   $Id: apples_oranges.sml,v 1.2 2003/05/02 20:03:36 jewillco Exp $
 *)

(* Copyright 2003, Trustees of Indiana University
 * Please see the license in the file ../LICENSE
 *)


(* Comparable concept *)
signature Comparable =
sig
 type value_t
 val better : value_t * value_t -> bool
end

structure Apple =
struct
  datatype value_t = AppleT of int
  fun create n = AppleT n
  fun better ((AppleT x),(AppleT y)) = x > y
end

structure Orange =
struct
  datatype value_t = OrangeT of string
  fun create s = OrangeT s
  fun better ((OrangeT x),(OrangeT y)) = x > y
end


(* pick - generic algorithm *)
functor MakePick(C : Comparable) :
sig
    type value_t
    val pick : value_t -> value_t -> value_t
end
=
struct
  type value_t = C.value_t
  fun pick x y = if C.better(x,y) then x else y
end

structure ApplePick = MakePick(Apple);
structure OrangePick = MakePick(Orange);

val a1 = Apple.create 5;
val a2 = Apple.create 6;
ApplePick.pick a1 a2;

val o1 = Orange.create "Miller";
val o2 = Orange.create "Portokalos";
OrangePick.pick o1 o2;