Modifying a random String stored in the clipboard

Question

I wrote a simple program, that will make a string 'noisy' if the clipboard contains one. What disappoints me, is that I should manually check what I got from getClipboardString — in the Nothing case, we simply return from program, otherwise we modify the string. Is there a better way to do this kind of check?

import Data.Char (toUpper)
import System.Random (randomIO)
import System.Clipboard (setClipboardString, getClipboardString)
import Control.Monad (join)

main :: IO ()
main = do 
 join $ fmap (test doNoise) getClipboardString
 where
 test :: (String -> IO ()) -> (Maybe String) -> IO ()
 test _ Nothing = return ()
 test f (Just s) = f s

 doNoise :: String -> IO ()
 doNoise s = do
  capsed <- (sequence $ map randCap s)
  setClipboardString capsed

 randCap :: Char -> IO Char
 randCap x = fmap ($ x) $ fmap choice (randomIO :: IO Bool)

 choice :: Bool -> (Char -> Char)
 choice x = if x then toUpper else id

Answer 1

main :: IO ()
main = traverse_ doNoise =<< getClipboardString where
 doNoise = setClipboardString <=< traverse randCap
 randCap x = bool id toUpper <$> (randomIO :: IO Bool) <*> pure x

Edit: Each change was arrived at through pattern-matching from my experience; compact code is usually easier to work with further. Looking at this again makes me think there ought to be a more lensy solution, one that fuses getting and setting, and indeed:

main = modifyClipboardString =<<
  zipWith (bool id toUpper) <$> getRandoms

We are fortunate to not actually need side effects depending on clipboard contents, but if we did, I would recommend writing a monadic variant of modifyClipboardString.