import Data.Maybe (isJust)

skipWhiteSpace = takeWithProperty IC.WhiteSpace >> pure ()

skipWhiteSpace = fmap (const ()) $ takeWithProperty IC.WhiteSpace

    Nothing -> return Nothing

    Nothing -> pure Nothing

        case f fi of
          Nothing -> return Nothing
          Just x -> do
            shiftEnv (slen l, r)
            return . Just $ x

        do
          let fres = f fi
          (if isJust fres then shiftEnv (slen l, r) else pure ())
          pure fres