(* Example of polymorphic stacks
   in ML from pages 244-246 of
   Kenneth C. Louden, Programming Languages
   Principles and Practice 2nd Edition
   Copyright (C) Brooks-Cole/ITP, 2003
*)

datatype 'a Stack = EmptyStack | Stack of 'a * ('a Stack);

val empty = EmptyStack; (* empty has type 'a Stack *)
val x = Stack (3, EmptyStack); (* x has type int Stack *)

fun top (Stack p) = #1(p);