On Fri, 2021-09-03 at 00:00 +0800, YueCompl wrote:
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case right, but if the "mutation" instead of the "mutated result" can be (might non-trivially) computed from a possibly outdated state, and the "mutation" can be trivially applied, I think `modifyTVar'` is the way to go.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.
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