You might try

    import Data.Maybe (catMaybes, listToMaybe)
    fmap (listToMaybe . catMaybes) . sequence $ [tryA, tryA', tryA'']

listToMaybe is another word for safeHead or headMay, ie head that returns a maybe if the list is empty.

or alternatively

    fmap (maybe defaultA id . listToMaybe . catMaybes) . sequence $ [tryA, tryA', tryA'']

to return a default value if you fail to get A.

On Tue, Jan 29, 2013 at 5:30 AM, Libor Wagner <liborwagner@gmail.com> wrote:
Hi,

say I have a number of functions tryGetA, tryGetA', tryGetA'' … with the type tryGetA :: IO (Maybe A), what I want is to get A, trying these function one by one until a get Just a. The first shot code I came up with is using if:

getA :: IO A
getA = do
a <- tryGetA
if isJust a
then return $ fromJust a
else do
a' <- tryGetA'
if isJust a
then return $ fromJust a'
…

there would be some default value at the end, this looks really ugly so I have tried another shot:

firstJust :: a -> [Maybe a] -> a
firstJust a [] = a
firstJust a (x:xs) = if isJust x
then fromJust x
else firstJust a xs

getA :: IO A
getA = do
r <- sequence [tryGetA, tryGetA', tryGetA'', …]
return $ firstJust "OK" r

which is a lot better, but I will appreciate any comment or suggestion.

Thanks,
Libor



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