Hello Per, thats very cool! Many thanks! Completely solves my problem. (As far as I can see :-)
I'm not sure whether I completely understand the implications of the nondeterminism you described; will think about it. You say "When each promise can only be written by a single writer.." - I assume you mean this as "Even if ..". Or am I on a wrong track? Background: What I (still) try to do is to implement a blocking lazy list. I'm not sure whether that should be used in the end, but while experimenting with it, I want to print these lists, or, rather, a state of such beast at a certain moment - and so the printer has to stop at the current end of the list, and not block. Thanks again, and kind regards, alux On 17 Apr., 16:46, Per Vognsen <per.vogn...@gmail.com> wrote: > Not currently. I added the capability to some code I posted last month > for fixing print-method for promises so they aren't auto-dereferenced > when undelivered: > > (defn promise? > [p] > (isa? (type p) ::Promise)) > > (defn promise-delivered? > [p] > (zero? (.getCount (:d p)))) > > (defmethod print-method ::Promise > [writer p] > ((get-method print-method clojure.lang.IDeref) writer p)) > > (defn promise > "Alpha - subject to change. > Returns a promise object that can be read with deref/@, and set, > once only, with deliver. Calls to deref/@ prior to delivery will > block. All subsequent derefs will return the same delivered value > without blocking." > [] > (Promise (java.util.concurrent.CountDownLatch. 1) (atom nil))) > > But there are good reasons for why you might want to discourage this > kind of checking in user code. When each promise can only be written > by a single writer (i.e. there is not a race among several potential > writers to first deliver the promise) then the input-output behavior > of readers is deterministic only if such checking is disallowed. Under > such conditions it doesn't matter if the readers try to dereference > the promise before or after it is ultimately delivered by the writer. > If readers can check the status of the promise, then you can get races > between readers and the writer. > > -Per > > > > On Sat, Apr 17, 2010 at 7:00 PM, alux <alu...@googlemail.com> wrote: > > Hi, > > > is there any nonblocking way I kind find out whether a promise has > > been delivered? > > > Thanky you, alux > > > -- > > You received this message because you are subscribed to the Google > > Groups "Clojure" group. > > To post to this group, send email to clojure@googlegroups.com > > Note that posts from new members are moderated - please be patient with > > your first post. > > To unsubscribe from this group, send email to > > clojure+unsubscr...@googlegroups.com > > For more options, visit this group at > >http://groups.google.com/group/clojure?hl=en > > -- > You received this message because you are subscribed to the Google > Groups "Clojure" group. > To post to this group, send email to clojure@googlegroups.com > Note that posts from new members are moderated - please be patient with your > first post. > To unsubscribe from this group, send email to > clojure+unsubscr...@googlegroups.com > For more options, visit this group > athttp://groups.google.com/group/clojure?hl=en -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en
