The docs cover this information under "Creating Transducers", but it doesn't make clear that the same pattern applies to reducing functions as well.
- James On 5 December 2014 at 14:15, Ivan Mikushin <i.mikus...@gmail.com> wrote: > Thanks for the answers! > > BTW there isn't a word in the docs (http://clojure.org/transducers and > http://clojure.github.io/clojure/branch-master/clojure.core-api.html#clojure.core/transduce) > about the intended use of arity-1 of the reducing function f. > > @Gary, thanks for suggesting to watch the talk: it's currently the only > informative source on the topic. > > I was curious if we could transduce sequences without initial values with > semigroup functions (like min or max) as is. Turns out, we can't. > That's okay: the world can't be perfect :) > > > On Friday, December 5, 2014 5:50:07 PM UTC+5, vve...@gmail.com wrote: >> >> I find these examples very memorable. Despite the doc strings clearly >> stating the differences between transduce and reduce, one can still hastily >> assume that transducing [0 1 2] will have 0 as the init argument. >> >> I will add that the culprit is in defining the +ten's arguments with [& >> args] form, which allows the function to be called without args (or with >> any number of args). >> (apply + 10 []) => 10 >> (+ten) => 10 >> Which means these are equivalent: >> (transduce (map (partial * 2)) +ten [0 1 2]) >> (transduce (map (partial * 2)) +ten 10 [0 1 2]) >> >> Reducing [0 2 4] yields 26. With 10 as init value it yields 46. After >> consuming all elements of coll, the 1-arity fn is run to provide the return >> value. Due to the [& args] form the completion stage evaluates (+ten 46) >> and returns 56. >> >> On Friday, December 5, 2014 10:51:49 AM UTC+1, Ivan Mikushin 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 > 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.
