[Haskell-cafe] golf, predicate check function for MonadPlus (was Re: How to read safely?)

[Antoine Latter]

On Mon, Jul 6, 2009 at 8:49 PM, Dan Doel<dan.doel at gmail.com> wrote:
>
> 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