Non-empty stacks.

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/Pure/General/stack.ML	Tue Sep 13 22:19:53 2005 +0200
@@ -0,0 +1,37 @@
+(*  Title:      Pure/General/stack.ML
+    ID:         $Id$
+    Author:     Makarius
+
+Non-empty stacks.
+*)
+
+signature STACK =
+sig
+  type 'a T = 'a * 'a list
+  val level: 'a T -> int
+  val init: 'a -> 'a T
+  val top: 'a T -> 'a
+  val map: ('a -> 'a) -> 'a T -> 'a T
+  val push: 'a T -> 'a T
+  val pop: 'a T -> 'a T      (*exception Empty*)
+end;
+
+structure Stack: STACK =
+struct
+
+type 'a T = 'a * 'a list;
+
+fun level (_, xs) = length xs;
+
+fun init x = (x, []);
+
+fun top (x, _) = x;
+
+fun map f (x, xs) = (f x, xs);
+
+fun push (x, xs) = (x, x :: xs);
+
+fun pop (_, x :: xs) = (x, xs)
+  | pop (_, []) = raise Empty;
+
+end;