Zip from end of list

RZip.hs

rzipWith :: (a -> b -> c) -> [a] -> [b] -> [c]
rzipWith f xs0 ys0 = loop 0 0 xs0 ys0
  where
    loop x y = curry \ case

      (_:xs, _:ys) -> loop (succ x) (succ y) xs ys

      (xs,   [])   -> done (x + length xs) y
      ([],   ys)   -> done x               (y + length ys)

    done x y = zipWith f (dx xs0) (dy ys0)
      where
        d = y - x
        (dx, dy)
          | d < 0     = (drop (-d), id)
          | otherwise = (id,        drop d)

newtype Zip a b = Zip { runZip :: a -> (Zip a b -> b) -> b }

-- A version that fuses away both input lists!
-- Adapted from <https://www.reddit.com/r/haskell/comments/f3z18s/zipping_from_the_end_of_a_list/g14cgmh/>
rzipWith' :: forall a b c. (a -> b -> c) -> [a] -> [b] -> [c]
rzipWith' f xs ys
  = foldl xf (const id) xs
    (Zip (foldl yf (const (const id)) ys))
    []
  where
    xf
      :: (Zip x y -> y)
      -> x
      -> Zip x y
      -> y
    xf k x = \ a -> runZip a x k
    yf
      :: (x -> (Zip x y -> y) -> y)
      -> b
      -> a
      -> (Zip x y -> [c] -> r)
      -> [c]
      -> r
    yf r2 y = \ x r1 zs -> r1 (Zip r2) (f x y : zs)