You're doing two filters with list comprehensions which you could switch with one span: quicksort (x:xs) = let (a, b) = span (< x) xs in (quicksort a) ++ [x] ++ (quicksort b)
Actually, this is incorrect. span will split the list
as soon as it finds the predicate to be false on an item, instead of finding the smaller elements of the
whole list. So, for instance, if we try your function as in the following, we get improper results:
> quicksort [1, 2, 3, 2, 1]
[1,2,1,2,3]
> quicksort [1, 2, 3, -2, -1]
[1,2,-2,-1,3]
> quicksort [1, 2, 3, -2, 0, -1]
[1,2,-2,-1,0,3]