Hi AntC,
Out of interest, is there some reason you don't mention TVar's?
The use case here seemed to be more about having shared data which is only read most of the time and updated occasionally. So the MVar seems to be a quite good fit for this.
I thought they are the (modern) way to "build robust systems" with "different threads using forkIO"(?)
I think it really depends how your threads are accessing the shared data. Sometimes the rollbacks of a STM might hurt you more than waiting for the lock of a MVar. You could use a TVar here, but you have to be aware, that your threads might operate on outdated data - if one thread is currently updating the data - and this might be an issue for your system. If I'm getting it correctly, then a MVar can't be read by mutliple threads at once, even if all threads are only reading from it. That's an advantage of the TVar. So thinking again about it, having only occasionally writes from favorably only one thread and reads from multiple threads, then indeed the TVar might be a quite good fit for this use case. Greetings, Daniel