Looks like MaybeT? http://hackage.haskell.org/cgi-bin/hackage-scripts/package/MaybeT-0.1.1
> readHandle x = runMaybeT $ do > ref <- MaybeT (return x) > liftIO (readIORef ref) > readField f h = runMaybeT $ do > a <- MaybeT (readHandle h) > return (f a) (or, the short version) > readHandle x = runMaybeT (liftIO . readIORef =<< MaybeT (return x)) > readField f = runMaybeT . liftM f . MaybeT . readHandle As a bonus, readHandle and readField work in any MonadIO due to the use of liftIO (as opposed to just lift). -- ryan On Wed, Sep 3, 2008 at 3:09 AM, minh thu <[EMAIL PROTECTED]> wrote: > Hi, > > I'd like to write a data structure to be used inside the IO monad. > The structure has some handles of type Maybe (IORef a), > i.e. IORef are pointers and the Maybe is like null pointers. > > So I came up with the following functions : > > readHandle :: Maybe (IORef a) -> IO (Maybe a) > readField :: (a -> b) -> Maybe (IORef a) -> IO (Maybe b) > > readHandle Nothing = do > return Nothing > readHandle (Just r) = do > v <- readIORef r > return $ Just v > > readField f h = do > m <- readHandle h > return $ fmap f m > > Is it something usual ? > Are there any related functions in the standard libraries ? > > Thanks, > Thu > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
