haskell - Haskell 是否有 `when` 和 `unless` 的组合？

Question

我正在开发一个 TCP 客户端，它从流中获取指定种类的消息，并且发现我自己使用了很多 unless 和 when 表达式。例如:

hunt :: Socket -> ThreadId -> IO ()                               
hunt conn t = do threadDelay 1000000
                 msg <- recv conn 1024
                 unless (C.isInfixOf "1b14010102" $ encode msg) $ hunt conn t
                 when (C.isInfixOf "1b14010102" $ encode msg) $ do
                     threadDelay 7000000
                     sendAll conn $ goHunt
                     msg <- recv conn 1024
                     threadDelay 3000000
                     close conn
                     killThread t

我正在尝试构建一个像这样的助手:

waitfor :: ByteString -> Socket -> t -> (ByteString -> Socket -> t -> IO ()) -> IO ()
waitfor str conn tid f = do 
    threadDelay 1000000
    msg <- recv conn 1024
    let m = C.isInfixOf str msg
    unless m $ waitfor str conn tid f
    when m $ f msg conn tid

然后我可以重新使用助手:

main = do
    ...
    tid <- myThreadId
    conn <- joinWorld u
    waitfor "1b14010102" conn tid hunt.

但是如果我有另一个函数(它需要 3 个参数，不像 hunt)

hunt' :: ByteString -> Socket -> ThreadId -> IO ()
hunt' idx conn t = do threadDelay 1000000
                      msg <- recv conn 1024
                      unless (C.isInfixOf "0300aa0801" $ encode msg) $ hunt' conn t
                      when (C.isInfixOf "0300aa0801" $ encode msg) $ do
                          threadDelay 1000000
                          sendAll conn $ goHunt' idx
                          threadDelay 3000000
                          close conn
                          killThread t

那我就不能用waitfor了，需要再用when/unless。那么，Haskell 是否有 when/unless 的组合？如果不是，那么对我的情况来说更好的方法是什么？

Answer 1

您可以为此使用 if ... then ... else。

例如，

waitfor :: ByteString -> Socket -> t -> (ByteString -> Socket -> t -> IO ()) -> IO ()
waitfor str conn tid f = do 
    threadDelay 1000000
    msg <- recv conn 1024
    if C.isInfixOf str msg
    then waitfor str conn tid f
    else f msg conn tid