BTW I understood the most about reducers (still not quite there yet, though :) ) from Rich Hickey's talk at EuroClojure:
https://vimeo.com/45561411 On Friday, December 7, 2012 10:21:59 AM UTC+1, Balint Erdi wrote: > > Hey, > > Reducers is fascinating and quite complex at the same time. I feel like > "best practices" around it has not quite solidified yet. > > Here is how I made your example work: > > (ns group-by-reducers.core > (:require [clojure.core.reducers :as r :only [fold reduce map]]) > (:require [criterium.core :as c])) > > (defn group-by-naive [f coll] > (reduce > (fn [groups a] > (assoc groups (f a) (conj (get groups a []) a))) > {} > coll)) > > (defn group-by-red [f coll] > (letfn [(reduce-groups > ([] {}) > ([g1 g2] > (merge-with concat g1 g2)))] > (r/fold reduce-groups (r/map #(hash-map (f %) [%]) coll)))) > > (defn -main > [& args] > (c/quick-bench (group-by #(mod % 29) (range 10000))) > (c/quick-bench (group-by-naive #(mod % 29) (range 10000))) > (c/quick-bench (group-by-red #(mod % 29) (range 10000)))) > > (available as a gist here: https://gist.github.com/4232044) > > The core and naive versions perform roughly equally (~4ms) while the > reducers version takes 10x as much time (~40ms) on my two-core laptop. > > I'm absolutely sure I'm doing something wrong (there is probably too much > memory allocated by the map function) and would like to hear the opinion of > someone more knowledgeable with reducers. I just did not want the subject > to be buried. > > Hope that still helps a bit, > Balint > > ps. There are some tips and tricks in this post: > http://www.thebusby.com/2012/07/tips-tricks-with-clojure-reducers.html > > On Monday, December 3, 2012 5:04:17 PM UTC+1, Las wrote: >> >> Hi, >> >> As I was trying to wrap my head around the reducers library[1], I thought >> implementing group-by would be a good exercise to gain some insight. >> >> After spending a few hours with it, I'm still pretty much clueless, so >> hope to find someone here to help me out: >> >> So if I understood the reducer lingo introduced in [2],[3] and group-by >> correctly, it reduces the following reducing function on a collection >> >> (fn group-by-reducef [keyfn ret x] >> (let [k (keyfn x)] >> (assoc ret k (conj (get ret k []) x)))) >> >> where keyfn is provided by a partial function application. >> >> fold needs a combining function that takes two result maps that have >> already been grouped and merges them. >> A naive implementation could look like >> >> (defn group-by-combinef >> ([] {}) >> ([g1 g2] >> (persistent! >> (reduce (fn [res k v] >> (assoc! res k (into (get res k []) v))) >> (transient g1) g2)))) >> >> (defn group-by [f coll] >> (fold (partial gr-by-reducef f) gr-by-combinef coll)) >> >> Now couple of questions: >> 1) I expected fold to actually perform the operation, how can I force it >> to give me the result? >> 2) Can somehow the actual reducing at the leaf nodes still take advantage >> of transient collections? >> 3) I took a look at flatten as it seems the "closest" match. Again, if I >> call (flatten [[1 2] [2 4]]), I don't actually get the result. How do I get >> to the result? >> >> >> Thanks! >> >> [1] >> https://github.com/clojure/clojure/blob/master/src/clj/clojure/core/reducers.clj >> [2] >> http://clojure.com/blog/2012/05/08/reducers-a-library-and-model-for-collection-processing.html >> [3] http://clojure.com/blog/2012/05/15/anatomy-of-reducer.html >> -- >> László Török >> >> -- 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
