Using failWith with Servant and custom monad stack

Question

I'm using Servant with custom monad stack:

newtype AppHandler a = AppHandler { runHandler :: ReaderT Config (ExceptT ServantErr IO) a }
  deriving (Functor, Applicative, Monad, MonadReader Config, MonadError ServantErr, MonadIO)

data Config = Config
    { getPool :: ConnectionPool }

Now, in many handlers I just need to do fetch some data (Persistent) from db and act upon it, so I've got:

runDb :: (MonadReader Config m, MonadIO m) => SqlPersistT IO b -> m b
runDb query = do
  pool <- asks getPool
  liftIO $ runSqlPool query pool

As it turns out, when fetching from db, you're bound to work with Maybe, and quite often when the Maybe is Nothing, you just want to throw error so that Servant server will turn it into proper HTTP response. This led me to the discovery of Control.Error.Util and the (!?) :: Applicative m => m (Maybe a) -> e -> ExceptT e m a helper. So I tried following:

someHandler :: AppHandler NoContent
someHandler = do
  entity <- (runDb $ getCompanyByName companyName) !? err400
  -- some more logic
  return NoContent

But this does not compile, the result of !? here is ExceptT ServantErr m0 (Entity SomeEntity) but I no longer work with such handler type, it requires the AppHandler (Entity SomeEntity). How would I convert such value back to my handler type?

Answer 1

You want a variant of (!?) that is polymorphic in which monad it returns. For example:

(!??) :: MonadError e m => m (Maybe a) -> e -> m a
act !?? err = act >>= maybe (throwError err) return

Then, provided err400 :: ServantError -- which is the type of errors you declared AppHandler to have -- you will be able to write

runDb (getCompanyByName companyName) !?? err400