Hi Beau, I've not used the STM stuff at all, but my understanding is that dosync is intended for co-ordinated updates to refs, and has no effect on atoms. It's really intended for co-ordinating updates to multiple refs rather than as a synchronisation primitive on a single entity. I might be wrong though, as I say I've never used it.
Cheers, Colin On 31 August 2014 00:58, Beau Fabry <imf...@gmail.com> wrote: > I must be missing something, because this is too simple? > > (defn get-maybe-cached [cache key] > (dosync > (if-let [v (get @cache key)] > v > (do > (reset! cache (assoc @cache key (calculate key))) > (get @cache key))))) > > > On Saturday, August 30, 2014 3:27:05 PM UTC+10, Colin Fleming wrote: >> >> 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. > -- 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.
