quick sort in haskell

-- quick sort
qsort :: (Num a, Ord a) => [a] -> [a]
qsort [] = []
qsort (x:xs) = (qsort less) ++ [x] ++ (qsort greater)
                    where less = [ n | n <- xs, n < x ]
                          greater = [n | n <- xs, n > x]

a = [5, 2, 7, 8, 2, 1, 6]
main = print $ qsort a