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
Thanks to lazy evaluation, this is not necessarily the case, see also http://en.wikibooks.org/wiki/Haskell/Performance_Introduction#Time You will have to write orMaybe as orMaybe Nothing y = y orMaybe x _ = x though. (By the way, your original code for orMaybe doesn't seem to do what you want.) This function has already been implemented for you, it's called mplus Regards, apfelmus -- http://apfelmus.nfshost.com