Count is going to be slow for seqs or cons's, but I think a generalized function could be implemented via protocol, falling back to seq. Things that extend https://github.com/clojure/clojure/blob/master/src/jvm/clojure/lang/Counted.java are going to have a fast 'count' method.
On Thu, Oct 1, 2015 at 6:40 PM Nathan Davis <nda...@positronic-solutions.com> wrote: > On Thursday, October 1, 2015 at 2:31:46 PM UTC-5, Dave Tenny wrote: >> >> So I understand that 'seq' is the idiomatic way to see if a >> collection/sequence is empty. >> >> > I'm not sure where you got this from. I personally use empty? to check > whether a collection is empty. It is true that (not (empty c)) is not > encouraged. I believe the main rationale for this this is that (empty c) > is (not (seq c)), so (not (empty c)) is (not (not (seq c)). > > >> Logically I'm looking for an O(1) predicate with which I can determine if >> a seq/collection is empty, and a well behaved >> one that is idempotent and side effect free (for general performance >> reasons). >> > > I believe all the implementations of seq in Clojure core are O(1), > although some (most?) allocate objects. I'm not sure if it's explicitly > spelled out anywhere, but I would consider it a bug it was anything other > than O(1) (or perhaps O(log n) at most). > > In what ways is the current implementation of empty not well behaved and > idempotent? > > With regards to side effects, if you can find a completely generic, > side-effect-free way of determining whether a lazy sequence is empty > without potentially realizing its head, please let the Clojure community > know! > > I'm not saying having an explicit 'empty' method is a bad idea, but I'm > not sure the current situation is as bad as you think. > > > Nathan Davis > > -- > 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.
