[Haskell-beginners] Searching Maybe lists

[aditya siram]

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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