On 08/24/2011 09:02 AM, Michael Snoyman wrote:
Hi all,
Max asked earlier[1] how to create a new instance of a class in Persistent using a monad transformer. Without getting into the specific details of persistent, I wanted to pose a question based on a much more general question: how can we lift the inner monad of an enumerator? We can easily do so for an Iteratee[2], but there is nothing to allow it for an Enumerator.
I faced the same problem a few weeks back, but for ReaderT. I tried for a while to get it working for all transformers, but couldn't get it to work. After spending time with this a few weeks ago, I think perhaps you could write liftEnum :: (Monad m, MonadTrans t, MonadCont m) => Enumerator a m b -> Enumerator a (t m) b That is, use callCC to return the step from the inner iteratee to be able to execute the step in the correct monad. But I didn't take the time to get it to work, since I got the ReaderT working. In any case, here is what I wrote for ReaderT. John newtype MemcacheBackend m a = MemcacheBackend { unMemBackend :: ReaderT MemcacheConnection m a } deriving (Monad, MonadIO, MonadTrans, Functor, Applicative, Alternative, MonadPlus, MonadCatchIO, MonadControlIO) lower :: Monad m => MemcacheConnection -> Iteratee a (MemcacheBackend m) b -> Iteratee a m b lower c i = Iteratee $ do step <- runReaderT (unMemBackend $ runIteratee i) c case step of (Error ex) -> return $ Error ex (Yield b s) -> return $ Yield b s (Continue f) -> return $ Continue $ lower c . f liftEnum :: (Monad m) => Enumerator a m b -> Enumerator a (MemcacheBackend m) b liftEnum e (Yield b s) = liftTrans $ e $ Yield b s liftEnum e (Error err) = liftTrans $ e $ Error err liftEnum e (Continue f) = Iteratee $ do r <- MemcacheBackend ask step <- lift $ runIteratee $ e $ Continue $ lower r . f case step of (Yield b s) -> return $ Yield b s (Error err) -> return $ Error err (Continue f') -> return $ Continue $ \x -> liftTrans $ f' x