gistfile1.txt

import qualified Data.ByteString as BS
 
  
instance FromJSON Config where
  parseJSON (Object o) = ...

parseConfig :: FilePath -> IO (Either String Config)
parseConfig fp = BS.readFile fp >>= return . eitherDecode

main :: IO ()
main = do
  optionalDecoded <- parseConfig "optional.json"
  config <- case optionalDecoded of
    (Left err) -> parseConfig "default.json"
    (Right v)  -> return optionalDecoded
  either handleError doWhatYouGonnaDo