It's widely known that the classic WriterT tends to leak space, and that in some cases this leak can be resolved by using StateT instead. The implementations I've seen of this idea both use the module system to prevent the computation from gaining unauthorized access to the state. I believe I've found a way to avoid this. Does this look right? {-# language RankNTypes, FlexibleInstances, MultiParamTypeClasses #-} import qualified Control.Monad.Writer.Class as W import Control.Monad.State.Strict import Control.Monad.Reader import Control.Applicative -- The key idea is that the computation -- can't inspect the state because it doesn't know the type newtype WriterT w m a = WriterT { unWriterT :: forall s. ReaderT ((w -> w) -> s -> s) (StateT s m) a } runWriterT :: Monoid w => WriterT w m a -> m (a, w) runWriterT m = runStateT (runReaderT (unWriterT m) id) mempty instance Functor m => Functor (WriterT w m) where fmap f m = WriterT $ fmap f (unWriterT m) instance Monad m => Applicative (WriterT w m) where pure a = WriterT (pure a) liftA2 f m n = WriterT $ liftA2 f (unWriterT m) (unWriterT n) instance Monad m => Monad (WriterT w m) where m >>= f = WriterT $ unWriterT m >>= unWriterT . f instance MonadTrans (WriterT w) where lift m = WriterT $ lift . lift $ m tell :: (Monad m, Semigroup w) => w -> WriterT w m () tell w = WriterT $ do p <- ask modify' $ p (<> w) pass :: Monad m => WriterT w m (a, w -> w) -> WriterT w m a pass m = WriterT $ do p <- ask (a, ww) <- unWriterT m modify' (p ww) pure a instance (Monoid w, Monad m) => W.MonadWriter w (WriterT w m) where tell = tell listen m = do aw@(_a, w) <- lift $ runWriterT m tell w pure aw pass = pass