Getting a hang of Haskell Types

Perhaps.hs

data Perhaps a = Nil | Only a

instance (Eq a) => Eq (Perhaps a) where
    Only x == Only y = x == y
    Nil == Nil = True

instance (Show a) => Show (Perhaps a) where
    show Nil = "Nil is Nothing"
    show (Only x) =  " It is Only " ++ show x

instance Functor Perhaps where
    fmap f (Only x) = Only $ f x
    fmap f Nil  = Nil