Some interesting fold example

fold.hs

-- Try to guess what divide does!
divide:: (Integral a) => [a] -> [[a]]
divide = f1 . f2 where
    f1 = foldr f3 []
    f2 = map (:[])
    f3 x acc
        | null acc = x : acc
        | (head x * head (head acc)) > 0 = (x ++ head acc) : tail acc
        | otherwise = x : acc