[Haskell-cafe] broadcasting stateful computations

YueCompl compl.yue at icloud.com
Thu Sep 2 16:00:10 UTC 2021

Um, I'm not sure I understand your case right, but if the "mutation" instead of the "mutated result" can be (might non-trivially) computed from a possibly outdated state, and the "mutation" can be trivially applied, I think `modifyTVar'` is the way to go. `readTVar` can be used to obtain an almost up-to-date state on demand, at low frequency.

> On 2021-09-02, at 23:25, Olaf Klinke <olf at aatal-apotheke.de> wrote:
> Thanks everyone for the helpful suggestions!
> Bryan Richer wrote:
>> 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.
> I don't understand this, but it may go into the same direction as:
> Chris Smith wrote:
>> I had the same suggestion.  However, you then no longer need StateT, as
>> ReaderT is enough.  Using a `ReaderT (TVar s) IO a` will allow atomic
>> changes to the state s, along with interleaved IO when it's done safely
>> rather than in the middle of a transaction.
> ReaderT (TVar s) IO is interesting! It does not solve the problem of
> atomic modifications with side-effects, though. Currently I am
> implicity using ReaderT (TVar s), by explicitly passing the TVar
> around. 
> In principle, for atomic modifications I have to get hold of a pure
> version (s -> s) of the modification. Maybe within a monadic context.
> But if determining this modification function is side-effectful, then
> it can not depend on the initial contents of the TVar! 
> This leads me to the conclusion that either 
> (a) atomic modification by a Kleisli map is impossible and another
> mechanism needs to ensure thread safety, or
> (b) we need to keep two versions of the state: one private, pure s and
> one public TVar s which we synchronize from time to time. The latter
> can be accomplished with atomic writes. More on that below.  
> Isaac Elliott wrote:
>> What about creating a MonadState instance for the TVar?
> Very neat, but it would not solve the atomicity problem, since the
> update I have is not expressible as stateTVar. 
> YueCompl wrote:
>> I believe this is the use case for TMVar, use `takeTMVar` / `putTMVar` instead of `readTVar` / `writeTVar` will do.
> TMVars appear non-empty for one thread only, as far as I understand. 
> So any observer thread would be blocked while the worker thread is
> updating the state? 
> Not good for my use case. The observer thread will be a webserver. 
> I'd rather have the webserver report old states than block. 
> I have neglected to mention one aspect of my problem that might be
> essential: All my state updates are monoidal. By that I mean that s is
> a monoid with 
> 	s' <> s <> s = s' <> s 
> and any effectful modification will be of the form 
> f = \s -> (k s) >>= (\s' -> s' <> s)
> for some k :: s -> m s.
> This entails that instead of emitting modifications as functions I can
> emit modifications as values. 
> emitModification :: (Monad m, Monoid s) => 
>  StateT s m a -> WriterT s (StateT s m) a
> emitModification (StateT f) = WriterT $ StateT $ \s -> do 
>   (a,s') <- f s 
>   return ((a,s'),s')
> broadcastModification :: (MonadIO m, Semigroup s) => 
>  TVar s -> (WriterT s m) a -> m a
> broadcastModification ref (WriterT m) = do 
>   (a,s') <- m
>   (liftIO.atomically) (modifyTVar ref (\s -> s' <> s))
>   return a
> Then 
> \ref -> broadcastModification ref . emitModification
>  :: (MonadIO m, Monoid s) => 
>    TVar s -> StateT s m a -> StateT s m a
> This has the additional charm that I can explicitly control when writes
> to the TVar happen, by choosing which (StateT s m) blocks to enclose in
> this wrapper. I might even set up my communication as a TChan, if the
> observing thread maintains an own copy of state itself to merge the
> updates s' into. 
> Olaf
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