(* Code of Figure 9.1, page 364 from
   Kenneth C. Louden, Programming Languages
   Principles and Practice 2nd Edition
   Copyright (C) Brooks-Cole/ITP, 2003
   (Test code from page 365 added, too.)
*)

abstype 'element Queue = Q of 'element list
with
   val createq = Q [];
   fun enqueue (Q lis, elem) = Q (lis @ [elem]);
   fun dequeue (Q lis) = Q (tl lis);
   fun frontq (Q lis) = hd lis;
   fun emptyq (Q []) = true | emptyq (Q (h::t)) = false;
end;

val q = enqueue(createq,3);
val q2 = enqueue(q,4);
frontq q2;
val q3 = dequeue q2;
frontq q3;