Awesome! This seems to be an analog of the Data.List.find function. I
originally didn't implement mine this way because I thought it went through
the entire list, but I was obviously mistaken!
Thanks for your help!
-deech
On Tue, May 19, 2009 at 8:39 AM, Thomas Friedrich
Hi Aditya,
Please try the following:
findJust :: (Eq a) => [Maybe a] -> Maybe a findJust xs = case (dropWhile (==Nothing) xs) of [] -> Nothing cs -> head cs
yourFunction :: (Eq b) => (a -> Maybe b) -> [a] -> Maybe b yourFunction f xs = findJust (map f xs)
It only uses functions from the Prelude, and as Haskell evaluates lazy, it just does exactly what you wants.
Happy Hacking, Thomas
aditya siram wrote:
Hi all, I would like to define a function that takes a list and a function that evaluates each member of the list to a Maybe value and output the first element in the list that evaluates to 'Just y', or 'Nothing' once the list has been completely processed. So something like:
findMaybe :: [a] -> (a -> Maybe b) -> Maybe b
The problem is that I don't want it to go through the entire list, but short-circuit when it hits a 'Just ...'. So far I have:
orMaybe :: Maybe a -> Maybe a -> Maybe a orMaybe m1 m2 = case (m1,m2) of (_, Just a) -> Just a (Just a, _) -> Just a _ -> Nothing findMaybe :: [a] -> (a -> Maybe b) -> Maybe b findMaybe as f = foldr (\a sofar -> sofar `orMaybe` (f a)) Nothing as
'findMaybe', as far as I can tell, traverses the entire input list which is undesirable for long lists. How can I fix it?
Curiously, the regular 'Data.List.find' function that applies a Boolean predicate to each member of the list also seems to first traverse the entire list using 'filter' and then grabs the head of the result.
Thanks ... -deech ------------------------------------------------------------------------
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