To fmap over a Data.Set just use Coyoneda!

SetFunctor.hs

module SetFunctor where

import Data.Functor.Coyoneda
import Data.Set

setF :: Coyoneda Set Int
setF = liftCoyoneda $ fromList [1, 2, 3]

runCoyendaSet :: Ord a => Coyoneda Set a -> Set a
runCoyendaSet (Coyoneda f s) = fromList . fmap f $ toList s

main :: IO ()
main = print . runCoyendaSet . fmap (+1) $ fmap (`div` 2) setF
-- fromList [1,2]