如何从处理程序访问全局MongoDB连接？

我正在尝试使用Snap制作一个简单的web应用程序。我希望有一个全局MongoDB连接，可以跨处理程序使用

我在

appInit

中打开此连接，它可以正常工作

data App = App { _mongoDB :: Pipe -- connection
               }

appInit :: SnapletInit App App
appInit = makeSnaplet "site" "My Site" Nothing $ do
    db <- liftIO $ do
        pipe <- runIOE $ connect (host "127.0.0.1")
        return pipe
    return $ App db

它给了我以下错误：

No instance for (MonadState App IO)
  arising from a use of `gets'
Possible fix: add an instance declaration for (MonadState App IO)
In a stmt of a 'do' block: pipe <- gets _mongoDB
In the second argument of `($)', namely
  `do { pipe <- gets _mongoDB;
        results <- access pipe master "db" (find $ select [] "movies");
        return results }'
In a stmt of a 'do' block:
  res <- liftIO
         $ do { pipe <- gets _mongoDB;
                results <- access pipe master "db" (find $ select [] "movies");
                return results }

没有（MonadState应用程序IO）的实例
由使用“get”引起的
可能的修复方法：为（MonadState应用程序IO）添加实例声明
在'do'块的stmt:pipe中，
liftIO
块仅用于IO操作，需要在
处理程序
monad本身中访问应用程序状态

watchHandler :: Handler App App ()
watchHandler = do
    pipe <- gets _mongoDB
    res <- liftIO $ do
        results <- access pipe master "db" (find $ select [] "movies")
        return results

    writeBS "test"

watchHandler:：Handler应用程序（）
watchHandler=do
管
watchHandler :: Handler App App ()
watchHandler = do
    pipe <- gets _mongoDB
    res <- liftIO $ do
        results <- access pipe master "db" (find $ select [] "movies")
        return results

    writeBS "test"

watchHandler :: Handler App App ()
watchHandler = do
    pipe <- gets _mongoDB
    res <- liftIO $ access pipe master "db" (find $ select [] "movies")

    writeBS "test"