A few comments on your recent commit of a proposed new monad structure (fptools/libraries/base/Control.Monad.X.*): I never liked the old "diagonal" structure, but a lot of people use these modules, and they'll notice the change of interface. In the old scheme, Control.Monad.Fix was portable, and now it isn't. (This will ripple through to my favourite module, Control.Arrow.) I'd prefer that the MonadFix instances for the new monad transformers be defined in the transformer modules, so they could import a portable Fix as before. You defined the classes in Trans, the types in Types and the instances in the *T modules, which makes them orphan instances, I believe (cf GHC User's Guide 4.9.8). Why not just get rid of Types and move the type definitions into the relevant *T modules? Use of type synonyms, like type Reader r = R.ReaderT r Identity is more economical, but will lead to more complex error messages. Resumptions may be generalized to newtype ResumeT f m a = Re { unRe :: m (Res f m a) } data Res f m a = Value a | Delay (f (ResumeT f m a)) where f is a functor.