Indeed you're right, the drop lazy-seq prevents the lookahead. Thanks Meikel for the thoughtful response!
On Tuesday, October 25, 2016 at 2:36:26 AM UTC-4, Meikel Brandmeyer (kotarak) wrote: > > In fact, (doall (take (count t) t)) actually realises t completely. But > not because of the doall or take, but because of the count. The problem > is not take, it's the take-while of split-with, which is the trouble. You > know that the input is 50 items long. take-while does not. It has to > check the 51st item to identify the end of the sequence. So it has to hold > onto the head of the tail sequence. No matter what your (take 50 t) does. > This combined with the fact, that you hold onto the head of t with the (count > t) after the (count d). > > drop does obviously not look ahead due to the wrapping lazy-seq. This can > be easily verified in the repl. Please consider, that upon realisation via > seq, the first element of the remaining output has to be realised anyway! > > user> (def s (drop 2 (map #(doto % prn) (list 1 2 3 4 5)))) > #'user/s > user> (def t (seq s)) > 1 > 2 > 3 > #'user/t > user> > > Again that notwithstanding drop could be optimised to save a wrapping > lazy-seq object in the case we call drop with 0. But that again is an > optimisation, not a bug. An implementation could look like this: > > (defn drop+ > [n coll] > (if (pos? n) > (lazy-seq (drop+ (dec n) (rest coll))) > coll)) > > In general lookahead is not debatable. A sequence function which looks > further ahead then absolutely necessary to perform its function is broken. > > -- 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.
