module M where

data ListZipper a = ListZipper [a] a [a]
  deriving (Functor)

instance Foldable ListZipper

instance Traversable ListZipper where
  traverse f (ListZipper left focus right) =
    ListZipper <$> traverseRev f left <*> f focus <*> traverse f right

traverseRev :: Applicative f => (a -> f b) -> [a] -> f [b]
traverseRev _ [] = pure []
traverseRev f (x : xs) = flip (:) <$> traverseRev f xs <*> f x