Re: [Haskell-cafe] combining monads with IO

By the way, how would one write the following with Monad Transformers?

newtype IOMayfail a = IOMayfail (IO (Maybe a))

instance Monad IOMayfail where return = IOMayfail . return . return (>>=) a f = IOMayfail (bind (run a) (run . f)) fail s = trace s (IOMayfail $ return Nothing)

run :: IOMayfail a -> IO (Maybe a) run (IOMayfail a) = a

bind :: IO (Maybe a) -> (a -> IO (Maybe b)) -> IO (Maybe b) bind a f = do r <- a case r of Nothing -> return Nothing Just r' -> f r' Lift :: IO a -> IOMayfail a lift f = IOMayfail (f >>= return . return)