On Mon, Jul 6, 2009 at 8:49 PM, Dan Doel
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)
-- stopAt p f x = guard (not $ p x) >> return (f x) -- stopAt p f = liftM2 (>>) (guard . not . p) (return . f) -- etc.
Then you can write:
unfoldr (stopAt p $ f)
I have the following function sitting around: unfoldUntil :: (b -> Bool) -> (b -> (a, b)) -> b -> [a] unfoldUntil p f n = unfoldr g n where g m | p m = Nothing | otherwise = Just $ f m But I don't remeber where I picked it up from. It looks like it fills a similar niche. Antoine