Question

我需要在客户端之间来回做一些，并在启动更多管道之前获取Client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

以下是我最终解决的问题：

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的限制：您无法从最下游组件获取结果值。有一些解决方法：

有一个更高级的管道API，允许捕获上游终结器。您感兴趣的功能是withUpstream。请注意，这是＆＃34;权利＆＃34;解决问题的方法，但是这个更高级的API并不是主要的原因：它有六个类型参数，往往会让人感到困惑。
不是将readName与appSink融合，而是将appSink传递给readName，并在每次yield来电时将其融合。 E.g：
```
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。
创建IORef或其他可变变量，并将客户端的名称放入该可变变量中。

如何在运行管道后获取值？

1 个答案: