-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 8/29/10 18:56 , 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
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?
You don't really need one; use the existing plumbing.
ioE :: e -> IO (Maybe a) -> IO a ioE e a = a >>= \r -> case r of Just r' -> r' Nothing -> fail e
io :: IO (Either e a) -> IO a io a = a >>= \r -> case r of Right v -> v Left e -> fail $ show e
main = do initialize iDeviceId <- ioE "No default input device" getDefaultInputDeviceId oDeviceId <- ioE "No default output device" getDefaultOutputDeviceId inputStream <- io $ openInput inputDeviceId outputStream <- io $ openOutput outputDeviceId 0 runTranslationLoop inputStream outputStream
If you really want a monad transformer, ErrorT is probably the one to look at (Control.Monad.Error). But it won't be pretty unless you rewrite the functions getDefault(...) and open(...) to use it as well. - -- brandon s. allbery [linux,solaris,freebsd,perl] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.10 (Darwin) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAkx70+cACgkQIn7hlCsL25V+7QCgkBPSUIzfpnRX69utD7o8d4oz 8FkAn3Sn47biInZg7dp56rThwamVmzu9 =tPWe -----END PGP SIGNATURE-----