如何在 IO 异常处理程序中保留 monad 堆栈的状态？

如何解决如何在 IO 异常处理程序中保留 monad 堆栈的状态？

考虑以下程序。

import Control.Monad.State
import Control.Monad.Catch

ex1 :: StateT Int IO ()
ex1 = do
    modify (+10)
    liftIO . ioError $ userError "something went wrong"

ex2 :: StateT Int IO ()
ex2 = do
    x <- get
    liftIO $ print x

ex3 :: StateT Int IO ()
ex3 = ex1 `onException` ex2

main :: IO ()
main = evalStateT ex3 0

当我们运行程序时，我们得到以下输出。

$ runhaskell Test.hs
0
Test.hs: user error (something went wrong)

但是，我希望输出如下。

$ runhaskell Test.hs
10
Test.hs: user error (something went wrong)

如何在异常处理程序 ex1 中保留 ex2 中的中间状态？

解决方法

改用 IORef（或 MVar 或 TVar 或其他）。

newtype IOStateT s m a = IOStateT { unIOStateT :: ReaderT (IORef s) m a }
    deriving (Functor,Applicative,Monad,MonadTrans,MonadIO)
    -- N.B. not MonadReader! you want that instance to pass through,-- unlike ReaderT's instance,so you have to write the instance
    -- by hand

runIOStateT :: IOStateT s m a -> IORef s -> m a
runIOStateT = runReaderT . unIOStateT -- or runIOStateT = coerce if you're feeling cheeky

instance MonadIO m => MonadState s (IOStateT s m) where
    state f = IOStateT $ do
        ref <- ask
        liftIO $ do
            s <- readIORef ref
            let (a,s') = f s
            writeIORef ref s'
            pure a

这种模式我已经见过很多次了，应该有一个 Hackage 包，但我不知道。