It appears to me that the MonadPlus instance for ListT breaks the following MonadPlus law m >> mzero = mzero I've tried with every implementation of ListT I could find - the one in mtl, transformers, List, even "ListT done right" and it's alternative. They all seem to violate the above law. Following is a ghci session that shows this with the mtl impl. Prelude> import Control.Monad.List Prelude Control.Monad.List> let nt = ListT Nothing :: ListT Maybe Int Prelude Control.Monad.List> let e = mzero :: ListT Maybe Int Prelude Control.Monad.List> runListT nt Nothing Prelude Control.Monad.List> runListT e Just [] Prelude Control.Monad.List> runListT $ nt >> e Nothing According to the MonadPlus law above, that last line should evaluate to `Just []`. So is ListT a valid MonadPlus? Or am I missing something? If not, can a valid MonadPlus instance for ListT be written? I've tried a few things and come up short. Thanks, Rich