运行管道后如何获得值？

Question

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。

我应该怎么做？

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

Answer 1

这是主要管道 API 的一个限制：除了最下游的组件之外，您无法从其他任何地方获取结果值。有一些解决方法：

1. 有一个更高级的管道 API，确实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 不是主要的 API 是有原因的：它有六个类型参数，容易让人感到困惑。

2. 不要将readName 融合到appSink，而是将appSink 传递到readName 并在每次yield 调用时融合它。例如：

   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。