[Haskell-cafe] broadcasting stateful computations

Bryan Richter b at chreekat.net
Wed Sep 1 15:12:01 UTC 2021

You've maybe already considered this, but what if the TVar is part of the
state s?

(StateT IO) can't give any guarantees about when or how the state is
broadcast, but using STM within StateT actions still can.

On Wed, 1 Sep 2021, 17.45 Olaf Klinke, <olf at aatal-apotheke.de> wrote:

> 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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