Realized I forgot to include an example of how the code I linked to can be used to solve the original problem, which will make it clearer how it is used.
https://gist.github.com/4349972 But when writing that example, I remembered why I created factory-style functions. The group-by from Daniel is assuming we want our result with vectors as values, which is a case that I didn't want to be limited to, even though the existing group-by results in values that are a vector of maps. Merging the two implementations yields: https://gist.github.com/4350075 Basically, just giving more responsibility/power to the g fn, which makes producing the same result as the original look like this instead (a lot busier): (x-group-by :type #(conj (or % []) (.toUpperCase (:name %2))) animals) => {:cow ["BETSY"], :cat ["MURMUR" "ROSIE" "ALF"], :dog ["LESSIE" "DINGO" "REX"]} On Thursday, December 20, 2012 11:19:01 AM UTC-6, Alex Walker wrote: > > I like the idea of it being built-in and might prefer that approach, > however, I wanted to share an alternative. > > https://gist.github.com/4346395 > > I needed to take a denormalized table of config data and create a nested > lookup map, so that I wouldn't need to repeatedly filter the dataset while > using it to process a stream of data. > > On Monday, December 17, 2012 3:20:32 PM UTC-6, Alex Baranosky wrote: >> >> I think it sounds like a nice addition, after mulling it over a little. >> >> On Mon, Dec 17, 2012 at 4:47 AM, László Török <ltor...@gmail.com> wrote: >> >>> Hi, >>> >>> I have come across use cases in the past where an additional >>> transformation step was indeed very handy and I wrote my own version of >>> group-by, one identical to Daniel's. >>> >>> Maybe a function worthwhile for c.c.incubator. >>> >>> Las >>> >>> >>> 2012/12/17 Daniel Dinnyes <dinn...@gmail.com> >>> >>>> Also note that I wrote in my first post that "Without the value-mapper >>>> argument it is very awkward to achieve the same structure after the >>>> group-by call". The `map-vals` function is almost the closest you can get >>>> to map values after a group-by in a streamlined and clean manner. There is >>>> ` >>>> fmap<http://clojuredocs.org/clojure_contrib/clojure.contrib.generic.functor/fmap>` >>>> >>>> in the contrib which does a similar thing already though. >>>> >>>> An even cleaner mapper would be something like a `map-multi-vals`, so >>>> that you can do something like this: >>>> >>>> (->> (group-by :type animals) >>>> (map-multi-vals :name)) >>>> >>>> That's the cleanest one can get with a separate value-mapper. In my >>>> opinion that has little added benefit though, and possibly the performance >>>> is worse too. The only benefit would be separation of concern: you can map >>>> values of a multi-map without knowing how it was created. Now think about >>>> it: how often would you use `map-multi-vals` separately, not right after a >>>> group-by? My impression is that whenever an multi-map is created, it >>>> almost >>>> always involves in some way a `group-by` - which itself is a special case >>>> of `reduce`. There is always a `reduce` somewhere, whether an `into`, a >>>> `for`, or some imperative iteration. Only `group-by` is the simplest for >>>> this specific purpose of creating a multi-map. >>>> >>>> My argument therefore is that whenever you need a multi-value mapping, >>>> it is always preceded by a group-by, and therefore I feel the right place >>>> for the value-mapper is as an optional parameter for `group-by` itself. >>>> >>>> What do you think? >>>> >>>> Cheers, >>>> Daniel >>>> >>>> >>>> On Monday, December 17, 2012 10:13:17 AM UTC, Daniel Dinnyes wrote: >>>>> >>>>> 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>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 >>>>>>> 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 >>>>>>> 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 post to this group, send email to clo...@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+u...@googlegroups.com >>>> For more options, visit this group at >>>> http://groups.google.com/group/clojure?hl=en >>>> >>> >>> >>> >>> -- >>> László Török >>> >>> -- >>> 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 >>> 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 >>> 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
