I have a silly question about IORefs and MVars.
When do we have to use MVars if a var is accessed by multiple threads. In fact, I wonder why IORefs updates aren't safe : it seems that preemptive scheduling takes place during memory allocation and I can't see where there could be an allocation (and so a switch) in a read or a write of an IORef.
The problem is how to read the IORef, modify its contents, and write the new value back without being preempted. You can do this easily with an MVar, but not an IORef.
Related question : how less performant is a MVar comparated to simple ref.
reading/writing an MVar is an out-of-line call, whereas the IORef operations are all inline. I don't know the figures though. Recently we added atomicModifyIORef which should allow IORefs to be used in some cases where MVars were required before. We haven't settled on the exact interface yet, though. Cheers, Simon