Maybe watching Rich Hickey's talk at the conj will make what is happening clearer?
It's basically what James said, but with a lot more details and a few examples. On Friday, 5 December 2014, James Reeves <ja...@booleanknot.com> wrote: > Reducing functions have three arities: > > 0-arity: returns the initial state > 1-arity: handles completion > 2-arity: reduce step function > > Using arities to denote different functionality is a little odd, but it > does make sense for functions like addition and conjoin: > > (+) => 0 > (+ 1) => 1 > (+ 1 2) => 3 > > (conj) => [] > (conj [1]) => [1] > (conj [1] 2) => [1 2] > > The 1-arity state is often just the identity function, but for channels > and I/O processes it's useful for closing resources, flushing buffers and > generally cleaning up after itself. > > You're getting strange results because your reducing function is strange. > You're not thinking about how the different arities are used. > > Look at it another way: > > (defn +ten > ([] 10) ;; initial state is 10 > ([x] (+ 10 x)) ;; on completion, add 10 > ([x y] (+ 10 x y)) ;; step function adds x and y and 10 > > So let's take a look at your initial transduce again: > > (transduce (map (partial * 2)) +ten [0 1 2]) > > You're essentially saying: > > "Set the initial value to 10. Double the values in the array and add then > together in pairs. Every time you add a pair of numbers, add 10 to the > result. On completion, add 10 again." > > - James > > On 5 December 2014 at 09:51, Ivan Mikushin <i.mikus...@gmail.com > <javascript:_e(%7B%7D,'cvml','i.mikus...@gmail.com');>> wrote: > >> First: transducers are a very cool idea, and I'm really looking forward >> to clojure 1.7 release when we'll be able to use them in production code. >> >> Now, clojure 1.7 is still alpha, so here's my feedback while it's not >> (too) late. >> >> transduce has a pretty unintuitive behaviour. Suppose, you've got some >> transformation, which you are going to apply to your reducing function and >> finally use it to reduce a collection. Currently, you have to know the >> implementation detail of transduce to approach this task. >> >> Just a couple examples. >> >> Example 1: >> (defn +ten [& args] >> (println (str "+ten " args)) >> (apply + 10 args)) >> >> (transduce (map (partial * 2)) +ten [0 1 2]) ;; Should equal *26*, right? >> +ten >> +ten (10 0) >> +ten (20 2) >> +ten (32 4) >> +ten (46) >> => *56* >> >> (transduce (map (partial * 2)) +ten 0 [1 2]) ;; OK, now this SHOULD equal >> *26*... >> +ten (0 2) >> +ten (12 4) >> +ten (26) >> => *36* >> >> (transduce (map (partial * 2)) +ten []) ;; Let's see: just no-args (+ten) >> *once* here... >> +ten >> +ten (10) >> => *20* >> WAT? >> >> Example 2: >> (transduce (map (partial * 2)) max 0 [1 2]) >> => *4* >> (transduce (map (partial * 2)) max [0 1 2]) ;; Should give *4*... >> *ArityException Wrong number of args (0) passed to: core/max >> clojure.lang.AFn.throwArity (AFn.java:429)* >> WAT? >> >> I know transduce requires f to accept arity 1 (as well as 0 and 2). >> Looking at the current implementation ( >> https://github.com/clojure/clojure/blob/clojure-1.7.0-alpha4/src/clj/clojure/core.clj#L6518), >> we can see this arity is applied just before returning result. Why? >> >> Why not use it (okay, (xform f)) when there's no init AND coll has just >> one element. >> >> -- >> 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 >> <javascript:_e(%7B%7D,'cvml','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 >> <javascript:_e(%7B%7D,'cvml','clojure%2bunsubscr...@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 >> <javascript:_e(%7B%7D,'cvml','clojure%2bunsubscr...@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 > <javascript:_e(%7B%7D,'cvml','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 > <javascript:_e(%7B%7D,'cvml','clojure%2bunsubscr...@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 > <javascript:_e(%7B%7D,'cvml','clojure%2bunsubscr...@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. 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