On Tue, 2011-11-08 at 15:43 +0000, Simon Marlow wrote:
Hmm, but there is something you could do. Suppose a thread could be in a mode in which instead of blocking on a BLACKHOLE it would just throw an asynchronous exception WouldBlock. Any computation in progress would be safely abandoned via the usual asynchronous exception mechanism, and you could catch the exception to implement your evaluateNonBlocking operation.
I'm not sure this would actually be useful in practice, but it's certainly doable.
The linux kernel folks have been discussing a similar idea on and off for the last few years. The idea is to "return" in another thread if the initial system call blocks. Perhaps there's an equivalent here. We have an evaluateThingy function and when the scheduler notices that thread is going to block for some reason (either any reason or some specific reason) we return from evaluateThingy with some info about the blocked thread. The thing that the kernel folks could never decide on was to do with thread identity: if it was the original thread that blocked and we return in a new thread, or if the original thread returns and a clone is the one that blocks. Or perhaps it's a crazy idea and it would never work at all :-) Duncan