Sorry, you're right. I wasn't thinking with subtyping. Rockostrich demonstrated the other weakness of this approach, but I'll say it outright: if you return a s

Sorry, you're right. I wasn't thinking with subtyping. Rockostrich demonstrated the other weakness of this approach, but I'll say it outright: if you return a supertype then you've lost information. It's very important for the type of `fmap f x` to be identical to `x` except in that its "inner" type has been modified. Without that, these kinds of interfaces lose all of their value.