list zipper

ListZipper.hs

-- little helper function
-- f3 (f2 (f1 x)) == x -: f1 -: f2 -: f3
(-:) x f = f x

-- the list zipper simply split the list in two parts.
type ListZipper a = ([a], [a])

makeListZipper :: [a] -> ListZipper a
makeListZipper l = (l, [])

forward :: ListZipper a -> ListZipper a
forward (x:xs, zs) = (xs, x:zs)

back :: ListZipper a -> ListZipper a
back (xs, z:zs) = (z:xs, zs)

toHead :: ListZipper a -> ListZipper a
toHead (xs, []) = (xs, [])
toHead z = toHead (back z)

toEnd :: ListZipper a -> ListZipper a
toEnd ([], zs) = ([], zs)
toEnd z = toEnd (forward z)

modify :: (a -> a) -> ListZipper a -> ListZipper a
modify f (x:xs, zs) = (f x : xs, zs)

attach :: [a] -> ListZipper a -> ListZipper a
attach l (_, zs) = (l, zs)

main = do
    let sample = [1..8]
    let l = makeListZipper sample -: forward -: forward -: (modify $ const 42) -: forward -: forward -: (attach [99, 88]) -: toHead
    mapM_ putStrLn [ show sample
                   , "------------------------------------------------------"
                   , "change 3 to 42   and   replace [5,6,7,8] with [99, 88]"
                   , "------------------------------------------------------"
                   , show $ fst l
                   ]