On Thu, Aug 6, 2009 at 4:09 PM, Stuart Halloway<stuart.hallo...@gmail.com> wrote: > > Is the following an improvement on clojure.contrib.seq-utils/ > reductions, or a step backwards? > > (defn my-reductions > ([f coll] > (if (seq coll) > (cons (first coll) (my-reductions f (first coll) (rest coll))) > (cons (f) nil))) > ([f acc coll] > (if (seq coll) > (let [nextval (f acc (first coll))] > (lazy-seq (cons nextval (my-reductions f nextval (rest > coll)))))))) > > On the plus side, it appears to be faster (as collections grow large), > and it doesn't "cheat" by introducing an atom. On the minus side it > isn't as pretty as the one in contrib. >
Good call, the version in contrib predates lazy-seq and full laziness. You can see the challenges faced in its implementation (back in the lazy-cons days) here: http://groups.google.com/group/clojure/browse_thread/thread/3edf6e82617e18e0/58d9e319ad92aa5f?#58d9e319ad92aa5f But, the version you have isn't fully lazy either. As a general rule, lazy-seq should wrap the entire body: (defn reductions ([f coll] (lazy-seq (if (seq coll) (cons (first coll) (reductions f (first coll) (rest coll))) (cons (f) nil)))) ([f acc coll] (lazy-seq (if (seq coll) (let [nextval (f acc (first coll))] (cons nextval (reductions f nextval (rest coll)))))))) Rich --~--~---------~--~----~------------~-------~--~----~ 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 -~----------~----~----~----~------~----~------~--~---
