Here are a few discussions about this issue: http://dev.clojure.org/jira/browse/CLJ-1454 <http://dev.clojure.org/jira/browse/CLJ-1454> http://stackoverflow.com/questions/15441638/alternate-version-of-swap-also-returning-swapped-out-value <http://stackoverflow.com/questions/15441638/alternate-version-of-swap-also-returning-swapped-out-value> http://stackoverflow.com/questions/26838348/why-does-clojures-atom-swap-return-the-new-value <http://stackoverflow.com/questions/26838348/why-does-clojures-atom-swap-return-the-new-value>
—Steve > On Feb 1, 2016, at 1:05 PM, Jochen <joc...@riekhof.de> wrote: > > Hi… > > in my little project I do some concurrent stuff using atoms. > Now, during the last days using swap! I encountered the same problem several > times: > Some result value is produced inside the swap! function besides the updated > data that I would like to return. > I first played with two atoms and considered modeling the entire result as > the atom value and even using refs. > The "fat atom" approach is ugly in this case as the enhanced result is just a > local artifact and not of interest outside my calls. Refs/STM looks somewhat > big for this. > But then I saw compare-and-set! and came up with this: > > (defn complex-swap! [atoom get-ref-val f & args] > (loop [last-val @atoom] > (let [result (apply f last-val args)] > (if (compare-and-set! atoom last-val (get-ref-val result)) > result > (recur @atoom))))) > > > It allows to produce an augmented result value, only part of which is the new > atom content passing an additional getter function to extract the new value > part. > > For example: > (def my-atom (atom '(1 2 3 4))) > (complex-swap! my-atom second (partial split-with #(<= % 2))) > => [(1 2) (3 4)] > @my-atom > => (3 4). > > The original swap! would be like this: > (swap! my-atom (comp second (partial split-with #(<= % 2)))) > => (3 4) > @my-atom > => (3 4) > > Now, is there some even easier way to achieve this or is this the best > approach? > > Ciao > > …Jochen > > > > -- > 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 > <http://groups.google.com/group/clojure?hl=en> > --- > You received this message because you are subscribed to the Google Groups > "Clojure" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to clojure+unsubscr...@googlegroups.com > <mailto:clojure+unsubscr...@googlegroups.com>. > For more options, visit https://groups.google.com/d/optout > <https://groups.google.com/d/optout>. -- 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 unsubscribe from this group and stop receiving emails from it, send an email to clojure+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
