What about creating a MonadState instance for the TVar?
newtype SharedStateT m a = SharedStateT (ReaderT (TVar s) m a)
instance MonadIO m => MonadState s (SharedStateT s m) where
get = SharedStateT $ liftIO . readTVarIO =<< ask
put s = SharedStateT $ do
tvar <- ask
liftIO . atomically $ writeTVar tvar s
state f = SharedStateT $ do
tvar <- ask
liftIO . atomically $ stateTVar tvar f
On Thu, 2 Sep 2021, 12:50 am Olaf Klinke,
Dear Café,
I have a sequence of state-modifying computations
action :: MonadUnliftIO m => StateT s m ()
where the state s should be visible to other threads. Thus I created a TVar in which I keep track of this state s for other threads to read.
The type system tells me it can't be done, which suggests I am using the wrong abstraction. The following type-checks but may be unsafe.
import Control.Concurrent.STM import Control.Monad.IO.Unlift updateStateTVar :: MonadUnliftIO m => TVar s -> StateT s m () -> m () updateStateTVar var action = withRunInIO (\inIO -> do s0 <- atomically (readTVar var) s1 <- inIO (execStateT action s0) atomically (writeTVar var s1))
Yet the splitting into readTVar and writeTVar is dangerous if several threads have read/write access to the same TVar. I was hoping to write the above using modifyTVar. However, the action essentially gives me a Kleisli map s -> m s which I somehow have to turn into an m (s -> s) but it is not possible in general. (The reverse works for any functor.)
What should I do? * Switch to WriterT (Endo s) m? This is not as powerful as StateT s m. * Do everything in the STM monad? But this disallows arbitrary IO because it would facilitate nested STM transactions.
The code above is safe, I believe, if only one thread has read/write access and the others are read-only. Are there type-level abstractions of this kind? (I suppose one could easily make them with some newtypes.)
Thanks in advance for any thoughts on this. Olaf
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