EitherIO Monad met do-notatie

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data EitherIO e a = EitherIO { runEitherIO :: IO (Either e a) }

instance Functor (EitherIO e) where
    fmap f x = EitherIO $ do either <- runEitherIO x
                             return undefined

instance Applicative (EitherIO e) where
    pure    = EitherIO . return . Right
    f <*> x = EitherIO $ do f' <- runEitherIO f
                            x'  <- runEitherIO x
                            return undefined


instance Monad (EitherIO e) where
    return  = pure
    m >>= k = EitherIO $ do m' <- runEitherIO m
                            return undefined

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