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
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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