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

[Jon Fairbairn]

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 at cl.cam.ac.uk