[[!comment format=mdwn
 username="https://www.joachim-breitner.de/"
 nickname="nomeata"
 avatar="http://cdn.libravatar.org/avatar/a2112893817513537c6a2c228c04c138a2f68bba57121ab7f267de58fc5171d7"
 subject="comment 3"
 date="2017-05-07T03:20:38Z"
 content="""
Still not good, I think, as the instance `Hashable Hash` has `hash = id`, so

       hash Nothing
    = hash ()
    = id (hash ())
    = hash (Just (hash ())

 and we have a collision at type `Maybe Hash`.

What would work is to do the same that `Hashable []` does, i.e. has the hash again:

    hash (Just v) = hash (val (hashValue v))
    hash Nothing = hash (mempty :: B.ByteString)

"""]]