I think you could get this behavior by creating a manager thread when you create the new kind of box. If threads are too heavyweight, though, you can get the effect of a primitive by using `ffi/unsafe/atomic'.
At Sat, 22 Oct 2011 10:24:27 -0400, Tony Garnock-Jones wrote: > On 2011-10-22 9:43 AM, Tony Garnock-Jones wrote: > > Nothing like the 20 seconds or so after a post to make one question > > oneself. Could it be that semaphore-peek-evt could be used to get what I > > need? I'll experiment. > > The answer is "almost", i.e. "no". But scheme_sema_post_all doesn't do > what I want either. And I don't think having a thread issue an infinite > sequence of (channel-put)s can be used either. I think I need something > else. Something primitive, maybe. > > - If I use semaphore-peek-evt or scheme_sema_post_all, I still have a > problem with kill safety, because I have to do something like: > (when (semaphore-try-wait? (blocking-box-used b)) > (set-blocking-box-cell! b the-value) > (semaphore-post (blocking-box-ready b))) > ...which might be killed between the try-wait and the post. > > - If I use a thread issuing an infinite sequence of channel-puts, > (thread (lambda () > (when (semaphore-try-wait? (blocking-box-used b)) > (let loop () > (channel-put c v) > (loop))))) > ...the custodian could be shut down at some point. Trying the same > trick as the buffered async channels doesn't work here, because I'd > need to know which thread to thread-resume when I checked the box's > value, and to do that I'd need a kill-safe box that can be written > into only once, which is an infinite regress. > > It looks like I need something like a cross between CAS and a semaphore. > > Perhaps I'm having imagination failure here. Is there something I'm > overlooking that would get me an event to wait on until a value arrives, > and that enforces that second and subsequent value-setting attempts do > not succeed? > > (This is closely related to E's Promises and less closely related to > Scheme's delay/force.) > > Regards, > Tony _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/dev
