Proposal (Trac ticket #3524): Add mfilter to Control.Monad

Following on from the discussion of #3453, I propose the addition of mfilter to Control.Monad. It's just List.filter for any MonadPlus. I'd rather that we didn't have List.filter, and called this filter, but (a) I can't see that happening and (b) the naming scheme for Control.Monad is quite explicit. Time limit for discussion: 1 October. -- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk