Something like the following would be fine for me (def cache (atom {})) (defn lookup [cache k] (let [v (get @cache k ::nil)] (if (= v ::nil) (let [v (calc-value k)] (swap! cache assoc k v) v) v))) (let [value (lookup cache k)] ; use value and @cache here )
суббота, 30 августа 2014 г., 9:27:05 UTC+4 пользователь Colin Fleming написал: > > Hi all, > > I want to use a map to cache values based on a key. I'm planning to use an > atom for this. My basic operation is "give me the value for this key" - if > the value exists in the map then that value should be returned, otherwise a > new value should be calculated, inserted in the map and then returned. My > plan is to implement something like the following: > > > (defn ensure [cache key] (if (contains? cache key) cache (assoc cache > key (calc-value key))))(let [value (get (swap! cache ensure key) key)] ... > do my thing with value ...) > > > So 'ensure' ensures that the cache contains the value for key, the swap! > operation returns the cache with the value and then I get it out. This > works but feels a little clumsy, is there a better way to do this? > > Also, looking at the Atom source code, I see that this will cause a CAS > operation even if the value returned from swap! is identical to the > original value. It seems like a reasonable optimisation would be to check > if the values are identical and not update if so - is there a reason this > might not be a good idea? > > Thanks, > Colin > -- 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.