checkResult :: forall (m :: * -> *). (Monad m) => Text -> m Bool -> m (Maybe Text)
checkResult msg f = do
   result <- f
   return $ if result then Nothing else (Just msg)

class Validatable a where
    validate :: forall master. (SqlBackend ~ YesodPersistBackend master,
                                YesodPersist master,
                                ~{moduleName m}Interface master) 
             => a -> HandlerT master IO [Text]

class Yesod master => ~{moduleName m}Interface master where