"Simon Marlow"
The portable interface could be Control.Concurrent.MVar, perhaps.
I don't really understand the problem, maybe I'm missing something. I thought the idea would be that a thread-safe library would simply use MVar instead of IORef.
I was misled by several people's hand-waving assertion that, provided you used MVars "correctly", there would be no synchronisation problems. But no-one had yet defined what "correct" meant. I kind of assumed they meant you could write concurrent threaded code (with only some minor restrictions) and have it work in a single-threaded implementation without change. This seemed like a pretty strong (and dubious) claim to me. But now I see you are actually saying something quite different. (And I recall some discussion on these points from a few months ago.) * IORef is inherently thread-unsafe, and so we should eliminate IORefs from the language. * One can write single-threaded code using MVars instead of IORefs, and it will be safe on a multi-threaded implementation. The latter point is quite the opposite of what I thought was being proposed. Regards, Malcolm