structure Example : sig
val foldl : ('a -> 'b -> 'a) -> 'a -> 'b list -> 'a
val rev : 'a list -> 'a list
datatype 'a queue = AQueue of 'a list * 'a list
val empty : 'a queue
val dequeue : 'a queue -> 'a option * 'a queue
val enqueue : 'a -> 'a queue -> 'a queue
end = struct
fun foldl f a [] = a
| foldl f a (x :: xs) = foldl f (f a x) xs;
fun rev xs = foldl (fn xsa => fn x => x :: xsa) [] xs;
datatype 'a queue = AQueue of 'a list * 'a list;
val empty : 'a queue = AQueue ([], []);
fun dequeue (AQueue ([], [])) = (NONE, AQueue ([], []))
| dequeue (AQueue (xs, y :: ys)) = (SOME y, AQueue (xs, ys))
| dequeue (AQueue (v :: va, [])) =
let
val y :: ys = rev (v :: va);
in
(SOME y, AQueue ([], ys))
end;
fun enqueue x (AQueue (xs, ys)) = AQueue (x :: xs, ys);
end; (*struct Example*)