Ah, I misunderstood! And yes, absolutely. An even easier example data Nullity a This is a perfectly valid functor, too. Ignoring undefined, there are no v

Ah, I misunderstood! And yes, absolutely. An even easier example data Nullity a This is a perfectly valid functor, too. Ignoring undefined, there are no values of it, but fmap cannot be used to create values of the mapped type, just manipulate them.