My solution is this transformer: newtype ConsumerT c m a = ConsumerT { runConsumerT :: [c] -> m (a, [c]) } instance (Monad m) => Monad (ConsumerT c m) where return x = ConsumerT $ \cs -> return (x, cs) m >>= f = ConsumerT $ \cs -> do ~(x, cs') <- runConsumerT m cs runConsumerT (f x) cs' fail msg = ConsumerT $ const (fail msg) consume :: (Monad m) => ConsumerT c m (Maybe c) consume = ConsumerT $ \css -> case css of [] -> return (Nothing, []) (c:cs) -> return (Just c, cs) consumeAll :: (Monad m) => ConsumerT c m [c] consumeAll = ConsumerT $ \cs -> return (cs, []) -- Früher hieß es ja: Ich denke, also bin ich. Heute weiß man: Es geht auch so. - Dieter Nuhr