I’m trying to wrap my head around how to correctly communicate between threads in Haskell.

I have multiple threads that read some state, and whenever it changes they need to do some update. Then I have multiple threads that can change that state.

I first looked at MVar, but that won’t do because it blocks when writing and only one thread can read.

So best thing I found is a TVar, I can write to it and read the current state from it. So I could just have the threads that read poll the state for changes every second or so (that small delay doesn’t really matter, nor does it matter that it’s possible a thread will miss in-between states). That would work.

However, I was wondering, is there a way that doesn’t need to polling? Some other primitive I could use? Or a way I can “listen” to a TVar?