Hi, I've been digging around the source code of reactive-banana and I found this code https://github.com/HeinrichApfelmus/reactive-banana/blob/master/reactive-ban...: data Cached m a = Cached (m a) runCached :: Cached m a -> m a runCached (Cached x) = x -- | An action whose result will be cached. -- Executing the action the first time in the monad will -- execute the side effects. From then on, -- only the generated value will be returned. {-# NOINLINE cache #-} cache :: (MonadFix m, MonadIO m) => m a -> Cached m a cache m = unsafePerformIO $ do key <- liftIO $ newIORef Nothing return $ Cached $ do ma <- liftIO $ readIORef key -- read the cached result case ma of Just a -> return a -- return the cached result. Nothing -> mdo liftIO $ -- write the result already writeIORef key (Just a) a <- m -- evaluate return a I'm trying to understand the reasom behind the use of mdo. Can't it be like this: do a <- m liftIO $ writeIORef key (Just a) return a Removing the need for a recursive definition? Yotam