Add check function to Control.Monad
check :: (MonadPlus m) => (a -> Bool) -> a -> m a check p a | p a = return a | otherwise = mzero
I agree that such a function is occasionally useful. But I doubt your rationale:
Rationale: [...]
but check is clearly more generally useful than guard. (guard = flip (check . const) ())
This is a bit like saying that concatMap is more generally useful than map and concat because: map f = concatMap ((:[]).f) concat = concatMap id Although this is correct, map and concat are smaller pieces that can be easily combined to concatMap: concatMap f = concat . map f So the question is whether check is useful enough to be included as a shortcut for a combination of simpler primitives (as was decided for concatMap). If 'check' is added then I would prefer this definition: check f x = guard (f x) >> return x It emphasises how 'check' is combined from simpler parts.
so we would expect a function
mfilter = (join .) . liftM . check
to be useful. Should that also be added?
I would not object and prefer this definition: mfilter f m = m >>= check f It seems simpler. Cheers, Sebastian -- Underestimating the novelty of the future is a time-honored tradition. (D.G.)