(* Code of Figure 9.14, page 382 from
   Kenneth C. Louden, Programming Languages
   Principles and Practice 2nd Edition
   Copyright (C) Brooks-Cole/ITP, 2003
   (Test code from page 383 added, too.)
*)

structure Queue1: QUEUE =
   struct
   datatype 'a Queue = Q of 'a list
   val createq = Q [];
   fun enqueue(Q lis, elem) = Q (lis @ [elem]);
   fun frontq (Q lis) = hd lis;
   fun dequeue (Q lis) = Q (tl lis);
   fun emptyq (Q []) = true
     | emptyq (Q (h::t)) = false;
   end;

val q = Queue1.enqueue(Queue1.createq,3);
Queue1.frontq q;
val q1 = Queue1.dequeue q;
Queue1.emptyq q1;

open Queue1;

val q = enqueue (createq,3);
frontq q;
val q1 = dequeue q;
emptyq q1;