MonadTransControl 是否支持 STM？

Question

我正在寻找在 STM.atomically 中使用 monad 转换器的标准方法。我觉得这听起来很奇怪，因为到目前为止我发现的所有用例都只是 liftIO 。原子地和直接传递 "STM a" 而不将其包装到任何 monad 转换器中。我猜，因为原子操作通常不是很复杂 - 只是一个行和通过局部变量传递参数更容易也更有效，但在我的情况下交易很大，我想顺利通过主堆栈。

在熟悉了 monad-control 库后，我倾向于发表意见如果 source 和 result base runInBase 不能解除 monad 堆栈 monad 是不同的，但我不确定。

inNestedState :: MyData -> StateT MyState STM ()

loadCounterA :: MyData -> StateT MyState IO ()
loadCounterA md = do
   control $ \runInBase -> atomically (runInBase (inNestedState md))

第一个错误

monad-control-demo.hs:29:4: error:
    • Couldn't match type ‘STM’ with ‘IO’
      Expected type: StateT MyState IO ()
        Actual type: StateT MyState STM ()
    • In a stmt of a 'do' block:
        control $ \ runInBase -> atomically (runInBase (inNestedState md))
      In the expression:
        do control
             $ \ runInBase -> atomically (runInBase (inNestedState md))
      In an equation for ‘loadCounterA’:
          loadCounterA md
            = do control
                   $ \ runInBase -> atomically (runInBase (inNestedState md))
   |
29 |    control $ \runInBase -> atomically (runInBase (inNestedState md))

与此同时，我最终得到了有限但方便的自制解决方案：

class MonadRebase m n where
    rebase :: (m a) -> (n a)

instance (MonadRebase m n) => MonadRebase (ReaderT r m) (ReaderT r n) where
    rebase t = ReaderT $ \r -> rebase (runReaderT t r)

instance (MonadRebase m n) => MonadRebase (StateT s m) (StateT s n) where
    rebase t = StateT $ \s -> rebase (runStateT t s)

instance MonadRebase STM IO where
    rebase = atomically

data MyState = MyState {
      msCounter1 :: TVar Int,
      msCounter2 :: TVar Int,
      ops :: Int
    }

inStm :: ReaderT Int (StateT MyState STM) ()
inStm = do
  rInt <- ask
  lift $ do
          mySt <- get
          modify (\st -> st {ops = rInt + ops st })
          lift $ do
            c1Val <- (readTVar (msCounter1 mySt))
            c2Val <- (readTVar (msCounter2 mySt))
            writeTVar (msCounter1 mySt) 0
            writeTVar (msCounter2 mySt) (c1Val + c2Val)

foo :: ReaderT Int (StateT MyState IO) ()
foo = do
  rebase inStm

任何关于如何对现有库执行相同操作的想法都将受到赞赏。

Answer 1

我将你的问题解释为 "How can I convert a StateT MyState STM () to a StateT MyState IO ()?"。答案是mapStateT:

loadCounterA = mapStateT atomically . inNestedState

如第二个示例所示，要深入转换器堆栈的多层，只需嵌套转换器各自的 map 函数的应用程序：

foo = mapReaderT (mapStateT atomically) inStm

当你有一个大的转换器堆栈时，这可能有点繁琐，但由于类型检查器，这是一种你不会出错的代码。

MonadTransControl 是否支持 STM？

Does MonadTransControl support STM?

haskell

monads

monad-transformers