(* Code of Figure 9.15, page 384 from
   Kenneth C. Louden, Programming Languages
   Principles and Practice 2nd Edition
   Copyright (C) Brooks-Cole/ITP, 2003
   (Test code from page 384 added, too.)
*)

structure Queue2: QUEUE = 
   struct
   datatype 'a Queue = Createq 
                 | Enqueue of 'a Queue * 'a ;
   val createq = Createq;
   fun enqueue(q,elem) = Enqueue (q,elem);
   fun frontq (Enqueue(Createq,elem)) = elem
                 | frontq (Enqueue(q,elem)) = frontq q;
   fun dequeue (Enqueue(Createq,elem)) = Createq
                 | dequeue (Enqueue(q,elem)) 
                         = Enqueue(dequeue q, elem);
   fun emptyq Createq = true | emptyq _ = false;
   end;

open Queue2;
val q = enqueue(createq,3);
frontq q;
val q1 = dequeue q;
emptyq q1;