structure Example =
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;
fun list_case f1 f2 (a :: lista) = f2 a lista
| list_case f1 f2 [] = f1;
datatype 'a queue = Queue of 'a list * 'a list;
val empty : 'a queue = Queue ([], []);
fun dequeue (Queue ([], [])) = (NONE, Queue ([], []))
| dequeue (Queue (xs, y :: ys)) = (SOME y, Queue (xs, ys))
| dequeue (Queue (v :: va, [])) =
let
val y :: ys = rev (v :: va);
in
(SOME y, Queue ([], ys))
end;
fun enqueue x (Queue (xs, ys)) = Queue (x :: xs, ys);
end; (*struct Example*)