I think given AMP that transformers should use CPP to relax Monad constraints to Applicative on newer versions of base. As an example: except :: (Monad m) => Either e a -> ExceptT e m a except m = ExceptT (return m) could become except :: (Applicative m) => Either e a -> ExceptT e m a except m = ExceptT (pure m) Similarly some typeclasses in mtl could do with some relaxation. A related issue: https://github.com/haskell/mtl/issues/35