Dan Doel
On Thursday 02 July 2009 6:36:09 am Jon Fairbairn wrote:
check :: (MonadPlus m) => (a -> Bool) -> a -> m a check p a | p a = return a | otherwise = mzero
I've often noticed the need for a similar function in conjunction with unfoldr:
-- This is overly general for unfoldr, but it lines up with check stopAt :: (MonadPlus m) => (a -> Bool) -> (a -> b) -> a -> m b stopAt p f x | p x = mzero | otherwise = return (f x)
Yes, I've had occasion to use something like that too, eg things similar to: reverse . unfoldr (stopAt (==0) (swap . flip divMod 10)) where swap (a,b) = (b,a)
And of course: check = flip stopAt id . not
or, equally, "stopAt p f = fmap f . check (not . p)" Granted, reverse . unfoldr (fmap (swap . flip divMod 10) . check (/=0)) isn't /quite/ as nice as the first version, but I imagine one could get used to it. -- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk