Yes, they deal with Kleisli composition (which I intuitively think of as taking regular composition and &quot;twisting&quot; it with a monad) but that is different from what I meant. You can&#x27;t take two different Kleisli arrows twisted with two different monads and combine them in an obvious way the same way you can compose&#x2F;combine regular arrows. That is why all the tricks with transformers exist. You somehow have to get everything to be a monad again because Kleisli arrows can&#x27;t cross monad boundaries.