biwalk.hs

data Pick a b c = PickLeft (a -> c -> c) | PickRight (b -> c -> c) | PickBoth (a -> b -> c -> c)

biwalk :: (a -> b -> Pick a b c) -> ([a] -> c) -> ([b] -> c) -> c -> [a] -> [b] -> c
biwalk b l r z = go
  where
    go [] [] = z
    go xs [] = l xs
    go [] ys = r ys
    go (x:xs) (y:ys) = case b x y of
      PickLeft u -> u x (go xs (y:ys))
      PickRight v -> v y (go (x:xs) ys)
      PickBoth w -> w x y (go xs ys)