Hi, I expect the cost of calling `identity` to be negligible. Not for sure, but the JVM might even inline it at run-time, or there might be optimizations for it in clojure.core during compilation... I cannot comment on that. But even with a full virtual call, it should be faster than iterating the whole map again.
Also, that `map-vals` is still indeed clunkier ;) Different usages, but for me whenever I use `group-by` I very often find I prefer to map the values too (to get a nice streamlined data structure to be passed around for further processing). Just my experience. It was very handy in .NET, and I think it was there for this reason. Regards, Daniel On Monday, December 17, 2012 8:21:44 AM UTC, Alex Baranosky wrote: > > I haven't run into this issue (yet). My first devil's advocate thought > was to suggest that you could map over the data after calling the group-by. > > (->> (group-by :type animals) > (map-vals #(map :name %))) > > There are two problems with this. One, it uses a custom util function > `map-vals` so it is a bit of a cheat. Two, even with that it still looks > pretty clunky. > > How does the `identity` effect performance? I wouldn't think much. > > Alex > > On Fri, Dec 14, 2012 at 9:58 AM, Daniel Dinnyes > <dinn...@gmail.com<javascript:> > > wrote: > >> Hi, >> >> I would like to suggest an enhancement to the clojure.core/group-by >> function. The idea came from using Enumerable.GroupBy >> <http://msdn.microsoft.com/en-us/library/bb534304.aspx>extension method >> in .NET quite much. It is really handy to have an optional value-mapper >> function which transforms the elements before adding them to the collection >> under the key. It is backward compatible, because calling the overload with >> 2 parameters can call the 3 parameter one with clojure.corj/identity as >> value-mapper function. >> >> The implementation is easy-peasy (almost the same as the original): >> >> (defn group-by >> ([f g coll] >> (persistent! >> (reduce >> (fn [ret x] >> (let [k (f x)] >> (assoc! ret k (conj (get ret k []) (g x))))) >> (transient {}) coll))) >> ([f coll] >> (group-by f identity coll))) >> >> Without the value-mapper argument it is very awkward to achieve the same >> structure after the group-by call. Also, doing the transformation before >> the group-by is often impossible, because the key function depends on some >> property of the source element, which would be removed after the >> transformation. >> >> To demonstrate the usage, check out the below calls: >> >> (def animals [{:name "Betsy" :type :cow} >> {:name "Murmur" :type :cat} >> {:name "Lessie" :type :dog} >> {:name "Dingo" :type :dog} >> {:name "Rosie" :type :cat} >> {:name "Rex" :type :dog} >> {:name "Alf" :type :cat}]) >> >> (group-by :type animals) ; old usage >> > ... ugly stuff >> >> (group-by :type :name animals) ; new usage >> > {:cow ["Betsy"], :cat ["Murmur" "Rosie" "Alf"], :dog ["Lessie" "Dingo" >> "Rex"]} >> >> (group-by :type #(.toUpperCase (:name %)) animals) ; hell yeah! >> > {:cow ["BETSY"], :cat ["MURMUR" "ROSIE" "ALF"], :dog ["LESSIE" "DINGO" >> "REX"]} >> >> It would be so cool to have this in the core. What do you guys think? >> >> Regards, >> Daniel Dinnyes >> >> -- >> You received this message because you are subscribed to the Google >> Groups "Clojure" group. >> To post to this group, send email to clo...@googlegroups.com<javascript:> >> Note that posts from new members are moderated - please be patient with >> your first post. >> To unsubscribe from this group, send email to >> clojure+u...@googlegroups.com <javascript:> >> 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 at http://groups.google.com/group/clojure?hl=en
