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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