Haskell Functors Example: Maybe

data Maybe a = Just a | Nothing
instance Functor Maybe where
    fmap :: (a -> b) -> (Maybe a -> Maybe b)
    fmap f (Just a) = Just (f a)
    fmap f Nothing = Nothing

fmap (+1) (Just 1) == Just 2
fmap (+1) Nothing  == Nothing
fmap head (Just [1,2,3]) == Just 1