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 <[email protected] <javascript:>> on
> behalf of Stephen Nelson <[email protected] <javascript:>>
> *Reply-To: *Clojure Mailing List <[email protected] <javascript:>>
> *Date: *Wednesday, March 9, 2016 at 12:23 PM
> *To: *Clojure Mailing List <[email protected] <javascript:>>
> *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 <[email protected]
> <javascript:>> 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
> <http://reactivex.io/documentation/operators/scan.html>.
>
> (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
>
>
>
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