added Zipper

Data/List/Zipper.hs

@@ -0,0 +1,24 @@
+module Data.List.Zipper where
+
+data Zipper a = Zipper [a] [a]
+
+sinistra z@(Zipper [] _) = z
+sinistra (Zipper (x:xs) ys) = Zipper xs (x:ys)
+
+destra z@(Zipper _ [x]) = z
+destra (Zipper xs (y:ys)) = Zipper (y:xs) ys
+
+mkZipper x = Zipper [] [x]
+valore (Zipper _ (x:_)) = x
+
+inserisci f (Zipper xs (y:ys)) = Zipper (y:xs) ((f y):ys)
+
+elimina (Zipper [] [y]) = Nothing 
+elimina (Zipper (x:xs) [y]) = Just (Zipper xs [x])
+elimina (Zipper xs (y:ys)) = Just (Zipper xs ys)
+
+modifica f (Zipper xs (y:ys)) = Zipper xs (f y : ys)
+elementi (Zipper xs ys) = xs ++ ys
+
+
+