管道單子whileJust_問題

Question

考慮利用鏡頭，管道和單子，循環以下Haskell代碼：

type Broadcast = Int 
type BroadcastChan = TChan Broadcast 

data SessionState = SessionState 
    { _broadcastChan :: BroadcastChan 
    } 

makeLenses ''SessionState 

type Session m = StateT SessionState m 
type SessionIO = Session IO 

-- This function requires the state and IO... 
processBroadcast :: Broadcast -> Conduit Broadcast SessionIO Packet 
processBroadcast = undefined 

-- | Handles networking 
protocol :: Conduit Packet SessionIO Packet 
protocol = do 
    ch <- lift $ use broadcastChan 
    -- line 51: 
    whileJust_ (liftIO . atomically $ tryReadTChan ch) (\x -> yield $ processBroadcast x) 
    liftIO $ putStrLn "End" 

我已經無法解決以下類型的錯誤，通過大量豐富解除：

src\Burrito\Network.hs:51:61: 
    Couldn't match expected type `Packet' 
       with actual type `Conduit Broadcast SessionIO Packet' 
    Expected type: ConduitM Packet Packet SessionIO b0 
     Actual type: ConduitM 
        Packet (Conduit Broadcast SessionIO Packet) m0() 
    In the expression: yield $ processBroadcast x 
    In the second argument of `whileJust_', namely 
     `(\ x -> yield $ processBroadcast x)' 

一些指導或建議，將不勝感激。請記住，processBroadcast要求狀態由Session進行處理。

---

• 單子 - 環文檔：http://hackage.haskell.org/packages/archive/monad-loops/0.4.2/doc/html/Control-Monad-Loops.html

• 導管文檔： http://hackage.haskell.org/packages/archive/conduit/1.0.6/doc/html/Data-Conduit.html

• STM（TChan）文檔： http://hackage.haskell.org/packages/archive/stm/2.4.2/doc/html/Control-Concurrent-STM-TChan.html

Answer 1

這工作：

import Control 
type Broadcast = Int 
type BroadcastChan = TChan Broadcast 

data SessionState = SessionState 
    { _broadcastChan :: BroadcastChan 
    } 

makeLenses ''SessionState 

type Session m = StateT SessionState m 
type SessionIO = Session IO 

-- This function requires the state and IO... 
processBroadcast :: Broadcast -> Conduit a SessionIO Packet 
processBroadcast b = undefined 

-- | Handles networking 
protocol :: Conduit Packet SessionIO Packet 
protocol = do 
    ch <- lift $ use broadcastChan 
    -- line 51: 
    whileJust_ (liftIO . atomically $ tryReadTChan ch) processBroadcast 
    liftIO $ putStrLn "End" 

由於功能processBroadcast只處理單個廣播（我認爲），沒有必要限制管道的輸入型播出。爲了在協議中使用它，該通道的輸入類型必須與協議Conduit的輸入類型（即Packet）相匹配，因此在協議中，processBroadcast中的a被實例化爲Packet。