Re: [Haskell-beginners] Mixing IO and other monads

30 Aug 2010


      On Mon, 30 Aug 2010 00:56:06 +0200, Brian Victor   
wrote:
...
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
This can be made more readable if main is split up:
...
main :: IO ()
main = do
    initialize
    ideviceId <- getDefaultInputDeviceID
    case ideviceId of
        Nothing -> putStrLn "No default input device"
        Just inputDeviceId -> main2 inputDeviceId
    where
      main2 inputDeviceId =        do
          odeviceId <- getDefaultOutputDeviceID
          case odeviceId of
            Nothing -> putStrLn "No default output device"
            Just outputDeviceId -> main3 inputDeviceId outputDeviceId
main3 inputDeviceId outputDeviceId =
        do
          openInputResult <- openInput inputDeviceId
          case openInputResult of
            Right err -> putStrLn $ show err
            Left inputStream -> main4 inputDeviceId outputDeviceId  
inputStream
main4 inputDeviceId outputDeviceId inputStream =
        do
          openOutputResult <- openOutput outputDeviceId 0
          case openOutputResult of
            Right err -> putStrLn $ show err
            Left outputStream -> runTranslationLoop inputStream  
outputStream
This can be made more compact with the aid of the following two functions:
...
ifLeft x f =
  x' <- x
  case x' of
    Right err -> putStrLn $ show err
    Left  y   -> f y
...
ifJust x f msg =
  x' <- x
  case x' of
    Nothing -> putStrLn msg
    Just y  -> f y
The main program than becomes:
...
main :: IO ()
main =  do
    initialize
    ifJust getDefaultInputDeviceID
      main2      "No default input device"
where
      main2 inputDeviceId =        ifJust getDefaultOutputDeviceID 
          (main3 inputDeviceId)          "No default output device"
main3 inputDeviceId outputDeviceId =
        ifLeft (openInput inputDeviceId)
          (main4 inputDeviceId outputDeviceId)
main4 inputDeviceId outputDeviceId inputStream =
        ifLeft (openOutput outputDeviceId 0) 
          (runTranslationLoop inputStream)
Of course, another thing to prevent long lines, is to use two spaces for  
indentation.

Met vriendelijke groet,
Henk-Jan van Tuyl


-- 
http://Van.Tuyl.eu/
http://members.chello.nl/hjgtuyl/tourdemonad.html
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