I've managed to create the sort of nested case code that all the monad tutorials warn you against. I haven't found a way to work myself out of it, though. If I've understood monad transformers conceptually, this seems like the sort of thing they'd be good for, but I don't know how to actually make what I need. Here's the code I have. getDefaultInputDeviceID and getDefaultOutputDeviceID are from PortMidi, and return IO (Maybe Int). openInput and openOutput return IO (Either PMStream PMError). At this point, I'd be happy to get the second half of the function refactored into monads. main :: IO () main = do initialize ideviceId <- getDefaultInputDeviceID case ideviceId of Nothing -> putStrLn "No default input device" Just inputDeviceId -> do odeviceId <- getDefaultOutputDeviceID case odeviceId of Nothing -> putStrLn "No default output device" Just outputDeviceId -> do openInputResult <- openInput inputDeviceId case openInputResult of Right err -> putStrLn $ show err Left inputStream -> do openOutputResult <- openOutput outputDeviceId 0 case openOutputResult of Right err -> putStrLn $ show err Left outputStream -> runTranslationLoop inputStream outputStream It seems like I ought to be able to change the second half of the function to something like this: openStreamsAndLoop :: (Num a) => a -> a -> IO (Maybe err) openStreamsAndLoop inputDeviceId outputDeviceId = do inputStream <- openInput inputDeviceId outputStream <- openOutput outputDeviceId 0 runTranslationLoop inputStream outputStream return Nothing But how do I create an IO (Maybe err) monad? -- Brian