Reader/Writer/State are handled in extensible-effects inside Control.Eff.State. Cont is handled in Control.Eff.Coroutine (although could use better documentation, patches welcome!). RWS isn't done, but I don't see why it can't be implemented in terms of State! Error is provided by Control.Eff.Exception, and a substitute for monad-supply is provided by Control.Eff.Fresh. Random numbers are in a new package: system-random-effect.

Eff is an instance of applicative, but not MonadPlus. I don't immediately see a way to make it an instance of MonadPlus, especially since we can have an 'IO' effect.