Folding Maybe

IM.hs

import System.Directory

orElse :: Maybe a -> Maybe a -> Maybe a
orElse x y = maybe y Just x

orElseM :: (Monad m) => m (Maybe a) -> m (Maybe a) -> m (Maybe a)
orElseM x y = maybe y (return . Just) =<< x

findJust :: [Maybe a] -> Maybe a
findJust = foldr orElse Nothing

findJustM :: (Monad m) => [m (Maybe a)] -> m (Maybe a)
findJustM = foldr orElseM (return Nothing)

tryReadFile :: FilePath -> IO (Maybe String)
tryReadFile fp = do
  exists <- doesFileExist fp
  if exists then fmap Just (readFile fp) else return Nothing

-- *Main> tryReadFile "foo" `orElseM` tryReadFile "bar"
-- Nothing
-- *Main> :! echo 1 > bar
-- *Main> tryReadFile "foo" `orElseM` tryReadFile "bar"
-- Just "1\n"
-- *Main> findJustM [tryReadFile "foo", tryReadFile "bar", tryReadFile "baz"]
-- Just "1\n"
-- *Main> findJustM [tryReadFile "foo", tryReadFile "baz", tryReadFile "quux"]
-- Nothing