[Animats]

I hope not. Then it would be hard to turn async off.

My general position on this is that all-async (like Javascript) is OK, and all-threaded is OK, and all green threads (like Go's goroutines) are OK. But those concepts do not play well together in the same program.

[throwaway81523]

GHC is ok at mixing OS threads and green threads. You launch an OS thread if you want it to make blocking system calls etc. But from the user perspective it's just like a "normal" (green) thread, uses the same mutexes etc.