管道和分叉與Control.Concurrent.MonadIO

Question

在下面的代碼中，我試圖將2個Producer s組合成1.所有類型都相同。 它們將由兩個輸入Producer的每一個組合在一個單獨的線程中運行，並由Consumer ，將Consumer放入一個unagi chan（我使用unagi chan來提高性能）。 返回一個從chan讀取的生產者。 我希望能夠使用任何Producer ，只要它可以運行IO所以我將monad的類限制為MonadIO和HasFork 。

import Pipes (Producer, Consumer, (>->), yield, await, runEffect)
import Control.Concurrent.Chan.Unagi (InChan, OutChan, newChan, readChan, writeChan)
import Control.Monad (forever, void)
import Control.Concurrent.MonadIO (MonadIO, HasFork, liftIO, fork)

combine :: (MonadIO m, HasFork m) => Producer a m r -> Producer a m r -> Producer a m r
combine p1 p2 = do
  (inChan, outChan) <- liftIO $ newChan
  t1 <- fork . void . runEffect $ p1 >-> (consumer inChan)
  t2 <- fork . void . runEffect $ p2 >-> (consumer inChan)
  producer outChan

producer :: (MonadIO m) => OutChan a -> Producer a m r
producer outChan = forever $ do
  msg <- liftIO $ readChan outChan
  yield msg

consumer :: (MonadIO m) => InChan a -> Consumer a m r
consumer inChan = forever $ do
  msg <- await
  liftIO $ writeChan inChan msg

但是我收到以下錯誤：

    • Couldn't match type ‘m’
                 with ‘Pipes.Internal.Proxy Pipes.Internal.X () () a m’
  ‘m’ is a rigid type variable bound by
    the type signature for:
      combine :: forall (m :: * -> *) a r.
                 (MonadIO m, HasFork m) =>
                 Producer a m r -> Producer a m r -> Producer a m r
    at src/Pipes/Unagi.hs:8:12
  Expected type: Pipes.Internal.Proxy
                   Pipes.Internal.X () () a m GHC.Conc.Sync.ThreadId
    Actual type: m GHC.Conc.Sync.ThreadId

我正在嘗試做什么？

Answer 1

在fork添加lift 。

t1 <- lift . fork . ...

在你的函數中， fork . ... fork . ...有類型m ThreadId ，但是do塊在monad Producer am 。

此外， lifted-base有一個更新的fork 。