Looking at both the implementation for TransformerIterator (sequence), transduce and eduction, it's clear that the 0-arg arity is never invoked on a transducer, while it's ever only used to provide the reducing step function of transduce its init value, if not provided explicitely.
I have to agree that I don't see a point in that 0-arity for transducers. On Thu, Mar 10, 2016 at 2:42 AM, Philos Kim <philo...@gmail.com> wrote: > I wrote the next example to trace the inner workings of transducer. I hope > that this will help. > > The next filter-t(transducer), map-t(transducer) and conj-t(reducer) > functions are excerpted from the filter, map and conj from clojure.core and > then simplified and modified to focus on the understanding of the inner > workings. > > (defn filter-t > [pred] > ;; The first fn is a transducer. It receives the reducer rf and returns > ;; the reducer(the second fn part of this code). > (fn [rf] > (fn > ([] > (let [r (rf)] > (println "filter-t [] post: result =" r) > r)) > ([result] > (println "filter-t [result] pre: result =" result) > (let [r (rf result)] > (println "filter-t [result] post: result =" r) > r)) > ([result input] > (println "filter-t [result input] pre: result =" result ", input =" > input) > (let [r (if (pred input) > (rf result input) > result)] > (println "filter-t [result input] post: result =" r) > r))))) > > (defn map-t > [f] > (fn [rf] > (fn > ([] > (let [r (rf)] > (println "map-t [] post: result =" r) > r)) > ([result] > (println "map-t [result] pre: result =" result) > (let [r (rf result)] > (println "map-t [result] post: result =" r) > r)) > ([result input] > (println "map-t [result input] pre: result =" result ", input =" > input) > (let [r (rf result (f input))] > (println "map-t [result input] post: result =" r) > r))))) > > (defn ^:static conj-t > [] > ;; This is a reducer itself, not a transducer, because it doesn't receive > the reducer > ;; and return a reducer as a transducer. > (fn > ([] > (println "conj-t []: result =" []) > []) > ([result] > (println "conj-t [result]: result =" result) > result) > ([result input] > (println "conj-t [result input] pre: result =" result ", input =" > input) > (let [r (. clojure.lang.RT (conj result input))] > (println "conj-t [result input] post: retrun =" r) > r) ))) > > > The oupput is edited to facilitate the understandings. > > (def xform (comp (filter-t odd?) (map-t #(* % 10)))) > > (transduce xform (conj-t) [1 2 3 4 5]) > ;>> conj-t []: result = [] > ; > ; filter-t [result input] pre: result = [] , input = 1 > ; map-t [result input] pre: result = [] , input = 1 > ; conj-t [result input] pre: result = [] , input = 10 > ; conj-t [result input] post: retrun = [10] > ; map-t [result input] post: result = [10] > ; filter-t [result input] post: result = [10] > ; > ; filter-t [result input] pre: result = [10] , input = 2 > ; filter-t [result input] post: result = [10] > ; > ; filter-t [result input] pre: result = [10] , input = 3 > ; map-t [result input] pre: result = [10] , input = 3 > ; conj-t [result input] pre: result = [10] , input = 30 > ; conj-t [result input] post: retrun = [10 30] > ; map-t [result input] post: result = [10 30] > ; filter-t [result input] post: result = [10 30] > ; > ; filter-t [result input] pre: result = [10 30] , input = 4 > ; filter-t [result input] post: result = [10 30] > ; > ; filter-t [result input] pre: result = [10 30] , input = 5 > ; map-t [result input] pre: result = [10 30] , input = 5 > ; conj-t [result input] pre: result = [10 30] , input = 50 > ; conj-t [result input] post: retrun = [10 30 50] > ; map-t [result input] post: result = [10 30 50] > ; filter-t [result input] post: result = [10 30 50] > ; > ; filter-t [result] pre: result = [10 30 50] > ; map-t [result] pre: result = [10 30 50] > ; conj-t [result]: result = [10 30 50] > ; map-t [result] post: result = [10 30 50] > ; filter-t [result] post: result = [10 30 50] > ;=> [10 30 50] > > From the above output, my conclusion is that the init part(with no argument) > of reducer is called only in the last reducer(conj-t in this case) and never > called in the reducers within the transducers(filter-t and map-t). > > If you give the init value to the transduce function as follows, > > (transduce xform (conj-t) [] [1 2 3 4 5]) > ;>> filter-t [result input] pre: result = [] , input = 1 > ; map-t [result input] pre: result = [] , input = 1 > ; conj-t [result input] pre: result = [] , input = 10 > ; conj-t [result input] post: retrun = [10] > ; map-t [result input] post: result = [10] > ; filter-t [result input] post: result = [10] > ; > ; filter-t [result input] pre: result = [10] , input = 2 > ; filter-t [result input] post: result = [10] > ; > ; filter-t [result input] pre: result = [10] , input = 3 > ; map-t [result input] pre: result = [10] , input = 3 > ; conj-t [result input] pre: result = [10] , input = 30 > ; conj-t [result input] post: retrun = [10 30] > ; map-t [result input] post: result = [10 30] > ; filter-t [result input] post: result = [10 30] > ; > ; filter-t [result input] pre: result = [10 30] , input = 4 > ; filter-t [result input] post: result = [10 30] > ; > ; filter-t [result input] pre: result = [10 30] , input = 5 > ; map-t [result input] pre: result = [10 30] , input = 5 > ; conj-t [result input] pre: result = [10 30] , input = 50 > ; conj-t [result input] post: retrun = [10 30 50] > ; map-t [result input] post: result = [10 30 50] > ; filter-t [result input] post: result = [10 30 50] > ; > ; filter-t [result] pre: result = [10 30 50] > ; map-t [result] pre: result = [10 30 50] > ; conj-t [result]: result = [10 30 50] > ; map-t [result] post: result = [10 30 50] > ; filter-t [result] post: result = [10 30 50] > ;=> [10 30 50] > > even the init part(with no argument) of reducer(conj-t in this case) is not > called as above. > > Wiithin into and sequence functions, the init part(with no argument) of > reducer are never called as follows. > > (into () xform [1 2 3 4 5]) > ;>> filter-t [result input] pre: result = () , input = 1 > ; map-t [result input] pre: result = () , input = 1 > ; map-t [result input] post: result = (10) > ; filter-t [result input] post: result = (10) > ; > ; filter-t [result input] pre: result = (10) , input = 2 > ; filter-t [result input] post: result = (10) > ; > ; filter-t [result input] pre: result = (10) , input = 3 > ; map-t [result input] pre: result = (10) , input = 3 > ; map-t [result input] post: result = (30 10) > ; filter-t [result input] post: result = (30 10) > ; > ; filter-t [result input] pre: result = (30 10) , input = 4 > ; filter-t [result input] post: result = (30 10) > ; > ; filter-t [result input] pre: result = (30 10) , input = 5 > ; map-t [result input] pre: result = (30 10) , input = 5 > ; map-t [result input] post: result = (50 30 10) > ; filter-t [result input] post: result = (50 30 10) > ; > ; filter-t [result] pre: result = (50 30 10) > ; map-t [result] pre: result = (50 30 10) > ; map-t [result] post: result = (50 30 10) > ; filter-t [result] post: result = (50 30 10) > ;=> (50 30 10) > > (sequence xform [1 2 3 4 5]) > ;>> filter-t [result input] pre: result = nil , input = 1 > ; map-t [result input] pre: result = nil , input = 1 > ; map-t [result input] post: result = nil > ; filter-t [result input] post: result = nil > > ; filter-t [result input] pre: result = nil , input = 2 > ; filter-t [result input] post: result = nil > > ; filter-t [result input] pre: result = nil , input = 3 > ; map-t [result input] pre: result = nil , input = 3 > ; map-t [result input] post: result = nil > ; filter-t [result input] post: result = nil > > ; filter-t [result input] pre: result = nil , input = 4 > ; filter-t [result input] post: result = nil > > ; filter-t [result input] pre: result = nil , input = 5 > ; map-t [result input] pre: result = nil , input = 5 > ; map-t [result input] post: result = nil > ; filter-t [result input] post: result = nil > > ; filter-t [result] pre: result = nil > ; map-t [result] pre: result = nil > ; map-t [result] post: result = nil > ; filter-t [result] post: result = nil > ;=> (10 30 50) > > However, I don't understand the last output in which every 'result' prints > nil. > > > 2016년 3월 10일 목요일 오전 10시 15분 43초 UTC+9, Sean Corfield 님의 말: >> >> Can we at least get an example of situation where the zero-arity version >> would be called? >> >> >> >> Right now it seems that all the transducer literature out there says there >> must be three arities – and that’s how map etc are defined – but it doesn’t >> seem, based on various people’s simple tests, that the zero-arity version is >> ever called… so why would we define it? >> >> >> >> Sean Corfield -- (904) 302-SEAN >> An Architect's View -- http://corfield.org/ >> >> "If you're not annoying somebody, you're not really alive." >> -- Margaret Atwood >> >> >> >> From: Clojure Mailing List <clo...@googlegroups.com> on behalf of Stephen >> Nelson <ste...@montoux.com> >> Reply-To: Clojure Mailing List <clo...@googlegroups.com> >> Date: Wednesday, March 9, 2016 at 12:23 PM >> To: Clojure Mailing List <clo...@googlegroups.com> >> Subject: Re: Understanding init (the zero arity function) for transducers. >> >> >> >> This was discussed further in http://dev.clojure.org/jira/browse/CLJ-1569 >> >> >> >> On Wed, Mar 9, 2016 at 3:43 PM Patrick Curran <patrick...@gmail.com> >> wrote: >> >> Thanks Alex, >> >> If you ever do get a chance, I'd be curious to know what it was. The more >> I think about it the more I think Dan is correct. Also "scan" seems like a >> natural thing that one should be able to do without having to jump through >> hoops. >> >> >> >> On Monday, February 29, 2016 at 5:10:53 PM UTC-5, Alex Miller wrote: >> >> I think that Rich had an objection to this, however in the haziness of >> time I don't recall specifically what it was. If I get a chance, I will ask >> him this week. >> >> On Monday, February 29, 2016 at 3:27:15 PM UTC-6, Patrick Curran wrote: >> >> Hi, >> >> I was trying to write a transducer and the 0-arity part of it never got >> called, which was unexpected. I did some searching and found this post: >> https://groups.google.com/forum/#!msg/clojure/uVKP4_0KMwQ/-oUJahvUarIJ. What >> Dan is proposing in that post would essentially solve my problem, but it >> doesn't look like his proposal has gotten much traction... >> >> Specifically I was trying to implement scan. >> >> (defn scan >> ([f] (scan f (f))) >> ([f init] >> (fn [xf] >> (let [state (volatile! init)] >> (fn >> ([] (xf (xf) init)) >> ([result] (xf result)) >> ([result input] >> (let [next-state (f @state input)] >> (vreset! state next-state) >> (xf result next-state)))))))) >> >> >> Which results in the following: >> >> (require '[clojure.core.reducers :as r]) >> (r/reduce ((scan + 3) conj) [1 2 3]) >> => [3 4 6 9] >> (transduce (scan + 3) conj [1 2 3]) >> => [4 6 9] >> (transduce (scan + 3) conj (((scan + 3) conj)) [1 2 3]) >> => [3 4 6 9] >> >> My expectation would be that we'd always get the 3 at the front of the >> vector. >> >> I'm actually using core.async and I'm expecting that the initial value be >> available to be taken from the channel. >> >> (require '[clojure.core.async :as a :include-macros true]) >> (def c (a/chan 1 (scan + 3))) >> (a/go (println (a/<! c))) >> ; expecting 3 to immediately be printed. >> (a/>!! c 1) >> => 4 >> >> So this is more of a conceptual thing rather than just how transduce is >> implemented. >> >> I'd love to hear other people's thoughts on this. I'm quite new, but Dan's >> proposal definitely feels "correct" and the current implementation >> definitely feels "wrong". >> >> --Patrick >> >> >> >> -- >> 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 unsubscribe from this group and stop receiving emails from it, send an >> email to clojure+u...@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 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 unsubscribe from this group and stop receiving emails from it, send an >> email to clojure+u...@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. -- 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.
