[Haskell-cafe] generating Maybe

[Jonathan Cast]

On 7 Nov 2007, at 12:40 PM, Tim Newsham 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.

guarded p f x = guard (p x) >> f x

which I would expect looks even better in practice:

[1..10] = unfoldr (\ n -> guard (n <= 10) >> return (n, n + 1)) 1

vs.

[1..10] = unfoldr (guarded (<= 10) (\ n -> return (n, n + 1)) 1

For example.

jcc