Question

Haskell相对较新，无法理解为什么以下不起作用。

我正在使用Aeson解析websocket查询中的对象并尝试打印它。

所有打印在所讨论的一个之前工作，但这个打印永远不会打印。我还尝试使用fromMaybe并提供默认值的变体，但这也无济于事。

Command只是一个记录对象，包含各种对象的几个（可能是Map）。所有这些都有FromJSON实例等。

通常只有一张地图存在，但我打算稍后进行一些累积，这可能意味着一次传递了几个对象。

data Command = Command { _articles :: Maybe ArticleMapMaybe
                       , _clients :: Maybe ClientMapMaybe
                       , _orders :: Maybe OrderMapMaybe } deriving (Show, Eq, Generic)

instance FromJSON Command
instance ToJSON Command


-- take a request from the queue and service it
wsApplication :: MVar ServerState
              -> WS.ServerApp
wsApplication state pending = do
  putStrLn "Processing request"
  let path = WS.requestPath (WS.pendingRequest pending)
  putStrLn $ "path: " ++ (BS.unpack $ BS.fromStrict path)

  conn <- WS.acceptRequest pending
  query <- WS.receiveData conn
  putStrLn $ "query: " ++ (T.unpack query)

  let command = (A.decode $ BS.pack . T.unpack $ query) :: Maybe Command
  print command -- this doesn't do anything!

  let client = (query, conn) :: SocketClient
  modifyMVar_ state $ return . addClient client

  serverState <- readMVar state
  putStrLn $ "connected clients: " ++ (show . length $ serverState ^. connectedClients)
  perform state client command

结果：

Processing request
path: /api
query: {"_clients":null,"_orders":null,"_articles":{"dsa141":{"_notes":"ewq","_articleName":"Stellaris: Apocalypse"}}}
Processing request
path: /api

如果我删除“打印命令”行，则还会打印以下行（“连接的客户端”）。这意味着在打印命令时功能会阻塞..？显然，解析的结果不依赖于任何MVar或任何类型（并且“连接的客户端”打印，意味着服务器状态MVar未被阻止...）。

打印使用Aeson解析的对象不会执行任何操作

0 个答案: