Understanding how Either is an instance of Functor

Question

In my free time I\'m learning Haskell, so this is a beginner question.

In my readings I came across an example illustrating how Either a is made an instance

Answer 1

Now, I'm trying to understand why the implementation maps in the case of a Right value constructor, but doesn't in the case of a Left?

Plug in here and it might make sense.

Assume a = String (an error message) You apply Either a to an Float.

So you have an f: Float -> Integer say for example roundoff.

(Either String) (Float) = Either String Float.

now (fmap f):: Either String Float -> Either String Int So what are you going to do with f? f doesn't have a clue what to do with strings so you can't do anything there. That is obviously the only thing you can act on are the right values while leaving the left values unchanged.

In other words Either a is a functor because there is such an obvious fmap given by:

• for Right values apply f
• for Left values do nothing