[Haskell-cafe] generating Maybe

[Stuart Cook]

On 11/8/07, Tim Newsham <newsham at lava.net> wrote:
> Data.Maybe has functions for processing Maybe's but nothing useful
> for creating maybe.  I think the following would be a very useful
> addition, a guarded function:
>
>      guarded :: (a -> Bool) -> (a -> b) -> a -> Maybe b
>      guarded p f x | p x       = Just (f x)
>                    | otherwise = Nothing
>
> such a function in the std libs would make functions like "unfoldr"
> more attractive -- uses of foldr nearly always encapsulate this
> notion.

How about this variant:

  ensure :: (MonadPlus m) => (a -> Bool) -> a -> m a
  ensure p x | p x       = return x
             | otherwise = mzero

Which as Jonathan points out, could also be written:

  ensure p x = guard (p x) >> return x

Now we can define

  guarded p f x = ensure p x >>> fmap f


Stuart