+newtype MaybeT m a = MaybeT {runMaybeT :: m (Maybe a)}
+
+instance Functor m => Functor (MaybeT m) where
+ fmap f x = MaybeT $ fmap (fmap f) $ runMaybeT x
+
+instance Monad m => Monad (MaybeT m) where
+ return = MaybeT . return . Just
+ x >>= f = MaybeT $ runMaybeT x >>= maybe (return Nothing) (runMaybeT . f)
+ fail _ = MaybeT $ return Nothing