Why Data.Set (containers) doesn't have Functor and Monad instance?

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For efficiency reasons, Set has an Ord constraint, but Functor has not!

On top of that, even if the Ord constraint were not a problem, Set would still not satisfy

fmap (f . g)  ==  fmap f . fmap g

for some peculiar Eq instances. E.g.:

import Data.Set as S

newtype A = A Double
    deriving Show

instance Eq A where
    (A a) == (A b) = round a == round b

instance Ord A where
    (A a) <= (A b) = a <= b

prova :: Set A
prova = fromList [A 11, A 12]

main = do
        print (S.map f . S.map g $ prova)
        print (S.map (f . g) prova)
        putStrLn "Ooops!"
    where
        f (A n) = A (n * 10)
        g (A n) = A (n / 10)
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