Symmetric set difference in Haskell

gistfile1.hs

module Example (
    (∆)
) where

unique :: (Eq a) => [a] -> [a]
unique     [] = []
unique (x:xs) = x:(unique (filter (/= x) xs))

without :: (Eq a) => [a] -> [a] -> [a]
xs `without` ys = [ x | x <- xs, not (x `elem` ys)]

(∆) :: (Eq a) => [a] -> [a] -> [a]
(∆) xs ys =
    let uniqueXs = unique xs
        uniqueYs = unique ys
    in  (uniqueXs `without` uniqueYs) ++ (uniqueYs `without` uniqueXs)

main :: IO ()
main = print $ [1, 2, 3, 4, 5] ∆ [3, 4, 5, 6, 7]