Trac ticket #3453. Two week time frame. Add check function to Control.Monad check :: (MonadPlus m) => (a -> Bool) -> a -> m a check p a | p a = return a | otherwise = mzero Rationale: The example that suggested the function to me is this: readMaybe :: Read a => String -> Maybe a readMaybe = join . fmap no_trailing_garbage . listToMaybe . reads where no_trailing_garbage = fmap fst . check (all isSpace . snd) but check is clearly more generally useful than guard. (guard = flip (check . const) ()) Discussion: I also note in the comments to check that we can define List.filter like this filter = (concat .) . map . check Now, concat is just join specialised to lists, and map [is fmap, but...] is liftM, so we would expect a function mfilter = (join .) . liftM . check to be useful. Should that also be added? -- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk