Haskell:使用镜头修改状态时 TChan 包装重复

Question

考虑以下代码(和错误):

import Control.Lens
import Control.Monad.STM
import Control.Monad.IO.Class
import Control.Monad.State.Lazy
import Control.Concurrent.STM.TChan

data Broadcast = Broadcast Int

data ImmutableState = ImmutableState
    { _broadcastChan :: TChan Broadcast
    , _encryption    :: Int
    }

makeLenses ''ImmutableState

helper :: MonadIO m => StateT ImmutableState m a
helper = do
    broadcastChan <~ (liftIO . atomically $ dupTChan $ use broadcastChan)
    {- ^
        Test.hs:
            Couldn't match expected type `Broadcast'
                        with actual type `TChan Broadcast'
            Expected type: StateT ImmutableState m (TChan Broadcast)
              Actual type: StateT ImmutableState m (TChan (TChan Broadcast))
            In the second argument of `(<~)', namely
              `(liftIO . atomically $ dupTChan $ use broadcastChan)'
            In a stmt of a 'do' block:
              broadcastChan
              <~ (liftIO . atomically $ dupTChan $ use broadcastChan)
    -}

forkImmState :: MonadIO m => ImmutableState -> m ImmutableState
forkImmState s = evalStateT helper s

有人可以解释一下(TChan (TChan Broadcast))是如何产生的吗？

Answer 1

dupTChan 需要一个像 TChan a 这样的参数，并且正在应用像 MonadState ImmutableState m => m (TChan Broadcast) 这样的东西。如果您直接尝试，您会得到稍微不同类型的错误

*Main> :t dupTChan $ use broadcastChan

<interactive>:1:12:
    No instance for (MonadState ImmutableState TChan)
      arising from a use of `use'
    Possible fix:
      add an instance declaration for (MonadState ImmutableState TChan)
    In the second argument of `($)', namely `use broadcastChan'
    In the expression: dupTChan $ use broadcastChan

GHC 尝试通过 a ~ TChan Broadcast 和 m ~ TChan 来统一 TChan a 和 m(TChan Broadcast) ，但它失败了，因为 TChan 不是 MoandState 的实例。您已经可以看到 TChan(TChan Broadcast) 错误是如何形成的，但它无法统一，因此停止了。

只有当我们声称我们知道结果的类型时，GHC 才会告诉我们它在幕后尝试的内容

*Main> :t (dupTChan $ use broadcastChan) :: STM (TChan Broadcast)

<interactive>:1:2:
    Couldn't match type `TChan Broadcast' with `Broadcast'
    Expected type: STM (TChan Broadcast)
      Actual type: STM (TChan (TChan Broadcast))
    In the expression:
        (dupTChan $ use broadcastChan) :: STM (TChan Broadcast)