Split a string up by occurrences of a substring

SplitOn.hs

import Control.Arrow (second)
import qualified Data.List as L

splitOn :: Eq a => [a] -> [a] -> [[a]]
splitOn sep = L.unfoldr f
  where
    f [] = Nothing
    f xs = Just $ second (drop l) $ breakList (L.isPrefixOf sep) xs
    l    = length sep

breakList :: ([a] -> Bool) -> [a] -> ([a], [a])
breakList p = para f ([], [])
  where
    f x ((ys, zs), xs) | p (x:xs)  = ([], x:xs)
                       | otherwise = (x:ys, zs)

-- | list paramorphism, it should be in the standard library
para :: (a -> (b, [a]) -> b) -> b -> [a] -> b
para f z []     = z
para f z (x:xs) = f x (para f z xs, xs)

-- > splitOn "bar" "foobarbazbar"
-- > ["foo","baz"]