The usual point of Cons is that it's instructive to reconstruct the list type from first principles. You would never do this in a real program, since it's exactly the same as the built-in list type --- except that it doesn't come with all the predefined list stuff, so it's easier for you to build it yourself without interfering with (or being interfered with by!) the standard library. It also doesn't come with the convenient syntactic sugar. And even if for some reason you did need to restrict lists that way in a real program, you would probably use a newtype instead so that you can still use the standard list operations and syntax after unwrapping.

    data List a = Nil | Cons a (List a)

    data []   a = []  | (:)  a ([]   a) -- the builtin list type, for comparison