用讀卡器擴展ServerPartT Monad

Question

我正在寫一個Happstack服務器，並且有一個MongoDB數據庫可以連接到。爲此，我做了一個函數來創建一個連接池

type MongoPool = Pool IOError Pipe 

withMongo :: (MongoPool -> IO a) -> IO() 
withMongo f = do 
    pool <- dbPool 
    f pool 
    killAll pool 

然後一個函數運行與創建池的Action：

runDB :: (MonadIO m) => MongoPool -> Action IO a -> m (Either Failure a) 
runDB pool f = liftIO $ do 
    pipe <- runIOE $ aResource pool 
    access pipe master dbName f 

很顯然，這需要攜帶pool中的所有路線作爲參數。我想將其包裝到ReaderT中，以便runDB可以具有類似Action IO a -> ServerPart (Either Failure a)或更好的類型，Action IO a -> ServerPart a，其中故障將自動導致HTTP錯誤500。

對於如何實現這一點，我有點麻煩，我很想從Haskell monads和happstack有更多經驗的人那裏得到一些提示。

謝謝。

Answer 1

通過這個問題，我發現了另一個非常好的提示，我已經建立了這個。它似乎工作正常，我想我會分享它：

type MongoPool = Pool IOError Pipe 

type DBServerPart a = ReaderT MongoPool (ServerPartT IO) a 

hostName = "127.0.0.1" 

dbName = "test" 

defaultPoolSize = 10 

runDB :: Action IO a -> DBServerPart (Either Failure a) 
runDB f = do 
    pool <- ask 
    liftIO $ do 
     pipe <- runIOE $ aResource pool 
     access pipe master dbName f 

withMongo :: DBServerPart a -> ServerPart a 
withMongo f = do 
    pool <- liftIO $ dbPool 
    a <- runReaderT f pool 
    liftIO $ killAll pool 
    return a 

dbPool = newPool fac defaultPoolSize 
    where fac = Factory { 
      newResource = connect $ host hostName, 
      killResource = close, 
      isExpired = isClosed 
     }