A zipper for traversables.

zipper.hs

import Data.Traversable
import Control.Monad.Free
import Control.Comonad
import Control.Comonad.Trans.Store

type Zipper t a = Free (Store a) (t a)

zipper :: Traversable t => t a -> Zipper t a
zipper = traverse (wrap . store return)

unzipper :: Zipper t a -> t a
unzipper = iter extract

step :: Maybe a -> Zipper t a -> Zipper t a
step _  (Pure t) = Pure t
step ma (Free s) = maybe extract peek ma s

test :: [Int]
test = unzipper . step (Just 10) . step Nothing . step (Just 20) $ zipper [1..10]