运行导管后如何获取值

Question

在启动更多管道之前，我需要在与客户端和获取客户端对象或其名称字符串之间来回做一些操作。

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 anothern" $ 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? " $$ appSink app
    name <- lineAsciiC $ takeCE 80 =$= filterCE (/= _cr) =$= foldC
    if BS.null name
      then go
      else do
        ok <- liftIO $ checkAddClient server name app
        case ok of
            Nothing -> do
                yield (BS.pack $ printf "The name '%s' is in use, please choose anothern" $ BS.unpack name) $$ 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的限制：除了最下游的组件之外，您无法从任何东西获得结果值。有几个解决办法：

1. 有一个更高级的管道API，允许捕获上游终结器。您感兴趣的函数是Upstream。请注意，这是解决问题的"正确"方法，但这种更高级的API并不是主要的方法，这是有原因的：它有六个类型参数，容易混淆人们。

2. 不是将readName融合到appSink，而是将appSink传递到readName，并在每次yield调用时进行融合。例如：

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

   这可能是简单性和类型安全性之间的最佳平衡。

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。