On Tuesday 28 March 2006 07:29, Andrew Pimlott wrote:
MonadError is not up to this task as far as I can tell.
Why not? All that needs to be done is write the missing instances, eg
instance MonadError () Maybe where throwError x = Nothing Nothing `catchError` f = f () Just x `catchError` f = Just x
instance Error () where noMsg = () strMsg s = ()
How would you go about writing the Maybe based analogue of ErrorT? What do you give to the handler in the instance of MonadError? newtype ErrMaybeT e m a = ErrMaybeT { runErrMaybeT :: m (Maybe a) } instance (Monad m, Error e) => Monad (ErrMaybeT e m) where return a = ErrMaybeT $ return (Just a) m >>= k = ErrMaybeT $ do a <- runErrMaybeT m case a of Nothing -> return Nothing Just r -> runErrMaybeT (k r) fail msg = ErrMaybeT $ return Nothing instance (Monad m, Error e) => MonadError e (ErrMaybeT e m) where throwError l = ErrMaybeT $ return Nothing m `catchError` h = ErrMaybeT $ do a <- runErrMaybeT m case a of Nothing -> runErrMaybeT (h ???) --what to do here? Just r -> return (Just r) f :: (MonadError String m) => Bool -> m Int f b = if b then return 42 else throwError "The boolean was false." test1 b = do r <- runErrorT $ f b putStrLn (show r) --Left "..." or Right 42 return () test2 b = do r <- runErrMaybeT $ f b putStrLn (show r) --Nothing or Just 42 return () Daniel