Actaully caching makes a difference even with inc and a sequence of just 1000 elements:
Clojure 1.7.0-alpha5 user=> (def a (iterate inc 0)) #'user/a user=> (time (reduce (fn [_ x]) nil (take 1000 a))) "Elapsed time: 4.170778 msecs" nil user=> (time (reduce (fn [_ x]) nil (take 1000 a))) "Elapsed time: 1.589569 msecs" nil Clojure 1.7.0-master-SNAPSHOT user=> (def a (iterate inc 0)) #'user/a user=> (time (reduce (fn [_ x]) nil (take 1000 a))) "Elapsed time: 4.831726 msecs" nil user=> (time (reduce (fn [_ x]) nil (take 1000 a))) "Elapsed time: 4.058695 msecs" nil Nicola Mometto writes: > The recent changes to iterate come with an interesting consequence: > reducing an iterate multiple times will cause the entire chain of x, (f > x), (f (f x)) .. to be recalculated every time. > > I'd argue that this is not desiderable and a regression (even though > probably one considered by design), and that this change in behaviour is > going to produce a degradation in performance when the function to > iterate actually does some computation rather than being a trivial > function like the one used in the benchmarks in the CLJ-1603 ticket > (inc) since its result won't be cached. > > If changing the new behaviour is out of the question I'd suggest to at > least document this new behaviour. The docstring of iterate talks about > returning a lazy-seq while when used with reduce it is actually a > generator. > > To show what I mean here's a silly example using Threa/sleep to simulate > computation: > > Clojure 1.7.0-alpha5 > user=> (def a (iterate #(do (Thread/sleep 2) (inc %)) 0)) > #'user/a > user=> (time (reduce (fn [_ x] (if (= 50 x) (reduced nil))) nil a)) > "Elapsed time: 106.385891 msecs" > nil > user=> (time (reduce (fn [_ x] (if (= 50 x) (reduced nil))) nil a)) > "Elapsed time: 0.560275 msecs" > nil > > Clojure 1.7.0-master-SNAPSHOT > user=> (def a (iterate #(do (Thread/sleep 2) (inc %)) 0)) > #'user/a > user=> (time (reduce (fn [_ x] (if (= 50 x) (reduced nil))) nil a)) > "Elapsed time: 109.088477 msecs" > nil > user=> (time (reduce (fn [_ x] (if (= 50 x) (reduced nil))) nil a)) > "Elapsed time: 109.51494 msecs" > nil -- -- 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.
