1

在启动更多管道之前，我需要在与客户端之间来回做一些事情并获取客户端对象或其名称字符串。

但我似乎无法让 appSink 让我有一个返回值。

我该怎么做？

checkAddClient :: Server -> ClientName -> AppData -> IO (Maybe Client)
checkAddClient server@Server{..} name app = atomically $ do
  clientmap <- readTVar clients
  if Map.member name clientmap
    then return Nothing
    else do
        client <- newClient name app
        writeTVar clients $ Map.insert name client clientmap
        return (Just client)


readName server app = go
  where
  go = do
    yield "What is your name? "
    name <- lineAsciiC $ takeCE 80 =$= filterCE (/= _cr) =$= foldC
    if BS.null name
      then go
      else do
        ok <- liftIO $ checkAddClient server name app
        case ok of
            Nothing -> do
                yield . BS.pack $ printf "The name '%s' is in use, please choose another\n" $ BS.unpack name
                go
            Just client -> do
                yield . BS.pack $ printf "Welcome, %s!\n" $ BS.unpack name
                return client -- <-- Here is the problem!!

main :: IO ()
main = do
    server <- newServer
    runTCPServer (serverSettings 4000 "*") $ \clientApp -> do
        (clientC, client) <- appSource clientApp $$+ readName server clientApp =$ appSink clientApp

更新

这是我最终得到的解决方案：

readName :: Server -> AppData -> Sink BS.ByteString IO Client
readName server app = go
  where
  go = do
    yield "What is your name? " $$ appSink app
    name <- lineAsciiC $ takeCE 80 =$= filterCE (/= _cr) =$= foldC
    if BS.null name
      then go
      else do
        ok <- liftIO $ checkAddClient server name app
        case ok of
            Nothing -> do
                yield (BS.pack $ printf "The name '%s' is in use, please choose another\n" $ BS.unpack name) $$ appSink app
                go
            Just client -> do
                yield (BS.pack $ printf "Welcome, %s!\n" $ BS.unpack name) $$ appSink app
                return client


main :: IO ()
main = do
    server <- newServer
    runTCPServer (serverSettings 4000 "*") $ \clientC -> do
        client <- appSource clientC $$ readName server clientC
        print $ clientName client
4

1 回答 1

1

这是主要管道 API 的一个限制：除了最下游的组件之外，您无法从任何东西中获取结果值。有一些解决方法：

  1. 有一个更高级的管道 API，它允许捕获上游终结器。您感兴趣的功能是withUpstream。请注意，这是解决问题的“正确”方法，但这个更高级的 API 不是主要 API 是有原因的：它有六个类型参数，并且容易混淆人们。

  2. 而不是融合readName到appSink，在每次调用时appSink传入并融合它。例如：readNameyield

    yield (BS.pack $ printf "...") $$ appSink app

    这可能是简单性和类型安全之间的最佳平衡。

  3. 创建一个IORef或其他可变变量，并将客户端的名称放入该可变变量中。

于 2014-05-29T04:01:21.097 回答