如何在运行导管后获得价值？ [英] How can I get a value after running a conduit?

问题描述

但我可以在客户端之间来回切换，并在启动更多管道之前获取Client对象或其名称字符串。 t似乎得到appSink让我有一个返回值。

我该怎么做？

checkAddClient :: Server - >客户名称 - > AppData - > IO（也许客户端）
checkAddClient服务器@服务器{..}名称app =原子地$ do
clientmap< - readTVar客户端
如果Map.member名称clientmap
然后返回Nothing
else else
client < - newClient name app
writeTVar clients $ Map.insert name client clientmap
return（Just client）


readName server app = go
where
go = do
yield你叫什么名字？
name< - lineAsciiC $ takeCE 80 = $ = filterCE（/ = _cr ）= $ = foldC
如果BS.null名称
然后转到
else else
ok< - liftIO $ checkAddClient服务器名称应用程序
确定
没有 - >做
收益。 BS.pack $ printf名称'％s'正在使用中，请选择另一个\\\
$ BS.unpack名称
去
只需客户端 - >做
收益。 BS.pack $ printfWelcome，％s！\\\
$ BS.unpack name
return client - < - 这是问题！
$ b $ main :: IO（）
main = do
服务器< - newServer
runTCPServer（serverSettings 4000*）$ \clientApp - >
（clientC，client）< - appSource clientApp $$ + readName server clientApp = $ appSink clientApp


UPDATE

以下是我最终得出的解决方案： class =lang-hs prettyprint-override> readName :: Server - > AppData - > Sink BS.ByteString IO Client
readName server app = go
where
go = do
yield你叫什么名字？$$ appSink app
name< - lineAsciiC $ takeCE 80 = $ = filterCE（/ = _cr）= $ = foldC
如果BS.null名称
然后去
else else
ok< - liftIO $ checkAddClient服务器名称应用程序
确定
没有 - >做
收益（BS.pack $ printf名称'％s'正在使用中，请选择另一个\\\
$ BS.unpack名称）$$ appSink app
go
只需客户端 - >
yield（BS.pack $ printfWelcome，％s！\\\
$ BS.unpack name）$$ appSink app
return client


main :: IO（）
main = do
server< - newServer
runTCPServer（serverSettings 4000*）$ \clientC - >
客户端< - appSource客户端C $$读取名称服务器客户端C
打印$客户端客户端


解决方案

这是主要API的局限性：除了最下游的组件外，您无法从其他任何结果中获取结果值。有几种解决方法：

1. 有一个更高级的管道API，它可以捕获上游终结器。您感兴趣的功能是 withUpstream 。请注意，这是解决问题的正确方法，但这种更高级的API并不是主要API的原因：它有六个类型参数，并且容易让人们感到困惑。

$ b

2. 不是将 readName 与 appSink 合并，而是将 appSink 转换为 readName ，并在每个产生调用时将其熔合。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，并将客户名称放入该可变变量中。

I need to do a little back and forth between with client and get either the Client object or their name string before starting up more pipelines.

But I can't seem to get appSink to let me have a return value.

How should I do this?

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

UPDATE

Here is the solution I ended up with:

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

解决方案

This is a limitation of the primary conduit API: you're not able to get the result value from anything but the most downstream component. There are a few workarounds:

1. There's a more advanced conduit API which does allow capturing upstream finalizers. The function you'd be interested in is withUpstream. Note that this is the "right" approach to the problem, but there's a reason this more advanced API isn't the primary one: it has six type parameters, and tends to confuse people.

2. Instead of fusing readName to appSink, pass appSink into readName and fuse it at each yield call. E.g.:

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

   This is probably the best balance between simplicity and type safety.

3. Create an IORef or other mutable variable, and put the client's name into that mutable variable.

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