You can't have two operators next to each other, it's invalid syntax. This holds true for all operators. So xs !! $ length xs - 1
just will not work. There isn't really a good way to get rid of the parens in that statement without resorting to some tricks that aren't worth it and make the code less readable.
The $
operator does not have to appear after function names, though, you can do
> let add x y = x + y
> add 1 $ add 2 $ add 3 4