7 Nov
2007
7 Nov
'07
9:46 p.m.
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