How about using a read-write lock?
data Locked a = Locked { lockVar :: TVar Bool, valueVar :: TVar a }
new :: a -> IO (Locked a)
new val = Locked <$> newTVarIO False <*> newTVarIO val
-- lock blocks until unlocked
lock, unlock :: Locked a -> STM ()
-- never blocks
read :: Locked a -> STM a
-- blocks until unlocked
write :: Locked a -> a -> STM ()
When you want to apply your effectful state trasition `f :: s -> m s`:
l <- ask
s <- liftIO . atomically $ lock l *> read l
s' <- f s
liftIO . atomically $ write l s' *> unlock l
On Fri, 3 Sep 2021, 3:46 am Olaf Klinke,
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case right, but if the "mutation" instead of the "mutated result" can be (might non-trivially) computed from a possibly outdated state, and the "mutation" can be trivially applied, I
On Fri, 2021-09-03 at 00:00 +0800, YueCompl wrote: think `modifyTVar'` is the way to go. `readTVar` can be used to obtain an almost up-to-date state on demand, at low frequency.
To be concrete, my state is a collection of time stamped values, where the monoid operation overwrites old values with new ones. But I need to know the current state (x,t) to determine the "mutation", because I'll be asking questions like "server, tell me if there is a value of x newer than t." Any observer whose initial state is synchronized with the worker thread can in principle re-construct the worker's internal state by observing the stream of emitted "mutations".
The most general abstraction would be that of a monoid action on a type, but in my case the monoid (mutations) and the mutated type are identical.
act :: m -> a -> a act memtpy = id act (x <> y) = act x . act y -- monoid homomorphism act (x <> x) = act x -- idempotent
Olaf
_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.