This is a suggestion I initially ran by Ross and he recommended that I post it to this mailing list. Here's the problem: WriterT always leaks space. I've tested three ways to fix the space leak, and only one works: * [DOES NOT WORK] Add a strictness annotation to the Monad instance for WriterT * [DOES NOT WORK] Use the StateT monad transformer *without* a strictness annotation * [WORKS] Use the StateT monad transformer *with* a strictness annotation I've hpasted the three different approaches and their test cases so you all can test it for yourselves: http://hpaste.org/75837 I'm requesting this because I need a WriterT monad transformer that does not leak space for defining folds for my pipes library. I can temporarily define the correct one within my own library but in the long run I believe it belongsin transformers. I also discovered another subtle advantage of using StateT to simulate WriterT. The traditional WriterT monad instance punishes the case where you use a free monad as the base monad because of the following bind in its monad instance: instance (Monad m, Monoid w) => Monad (WriterT w m) where return a = WriterT2 $ return (a, mempty) m >>= f = WriterT2 $ do (a, w) <- runWriterT m (b, w') <- runWriterT (k a) <= This bind is the problem return (b, w `mappend` w') This bind triggers a quadratic blowup when the base monad is a free monad, whereas the StateT simulation of WriterT runs in linear time. For these reasons, I suggest adding a "Control.Monad.Trans.Writer.Stricter" module to "transformers" that implements WriterT using the "WriterT4" implementation from the above hpaste.