To get the symmetric difference of two lists I think it would be a good idea to use list comprehension. But I don't know what I could do to remove the common elements of the lists and get the elements that are different.
For example
symDiff [1,2,3,4] [1,3,5] -- should return [2,4,5].
So far I have
symDiff :: [a] -> [a] -> [a]
symDiff xs ys =