There was a stackoverflow question recently that requested a solution for a similar problem:
https://stackoverflow.com/questions/22409638/remove-first-item-from-clojure-vector-atom-and-return-it One solution there is similar to this: (defn swap*! "Like swap! but returns a vector of [old-value new-value]" [atom f & args] (loop [old-value @atom] (let [new-value (apply f old-value args)] (if (compare-and-set! atom old-value new-value) [old-value new-value] (recur @atom))))) This will return the correct old-value and new-value which you can diff. Another note: > (swap! state #(update-in % [:teams] make-team)) can be written more succinctly: (swap! state update-in [:teams] make-team) --Steve On Mar 20, 2014, at 6:28 PM, Jakub Holy <jakub.h...@iterate.no> wrote: > I have couple of times run into a situation where I want to update a state > map held in an atom > and return the change, not the new value. I haven't found a good way to do it > so either I am missing > something obvious or there are more idiomatic ways to achieve what I need. > Could you advise me? > > A concrete example: In ma webapp I want to assign a unique random ID to each > user. Creating that ID is simple: > > (def state (atom {:teams {}})) > > ;; Remove already used IDs from a lazy seq of random IDs (=> unique), take > the 1st one > (defn unique-rand-id [id-set] > (first (remove id-set (repeatedly #(rand-int Integer/MAX_VALUE)))))) > > ;; Add a new team with a unique random ID to the teams map > (defn make-team [teams] > (let [id (unique-rand-id (set (keys teams)))] > (assoc teams id {}))) > > ;; Create a new team; TODO: How to get the new team's ID?! > (swap! state #(update-in % [:teams] make-team)) > > So I can generate and remember a new unique random ID but there is no way to > find out > what ID it was (I cannot just take diff of state before and after since other > threads could > have also added new IDs to it in the meanwhile.) > > Any advice is appreciated. Thank you! > > -- > 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. -- 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.
