[Chapter 4] Is it possible to have an instance of Functor for "Trap e" of "Secret e a"?

[IceAsteroid]

Hi, guys, greetings!

This is a very confusing part of typeclass to me.

I can define an instance for "Secret e a", which the "Trap e" is being fixed.

But I can't figure out the way to also define a Functor instance for the first type variable, which is the value "Trap e" of "Secret e a"

For example:

data Secret e a
    = Trap e
    | Reward a
    deriving (Show, Eq)

-- This one for the value "Reward a" works.
-- instance Functor (Secret e) where
--   fmap :: (a -> b) -> Secret e a -> Secret e b
--   fmap _ (Trap a) = Trap a
--   fmap f (Reward a) = Reward (f a)

But how do I possibly have the value "Trap e" also to work for fmap, is it for some reason that such cases are not necessary?