maybeoptions.hs

import Data.Maybe
import System.IO (readFile)

-- Open a file and process its contents with action, or return the default value 'deflt'
maybeWithFile :: Maybe FilePath -> b -> (String -> b) -> IO b
maybeWithFile fname deflt action = maybe (return deflt) (fmap action . readFile) fname

main :: IO ()
main = do
  let inactiveWhitelist = Just "whitelist.txt"
  whitelist <- maybeWithFile inactiveWhitelist [] $ \contents -> words contents
  print whitelist