2010/11/16 Meikel Brandmeyer <m...@kotka.de> > Hi, > > On 16 Nov., 02:31, Ken Wesson <kwess...@gmail.com> wrote: > > > Eh. I'd heard first and rest had replaced next. No? > > No. This was a misinformation. To elaborate a bit more on the > differences pointed out by Sean: > > next returns nil if there are no more items in the sequence, which is > nice to use in if and when statements to test for emptiness. However > in order to be able to return nil the sequence has to be realised. > Otherwise you can't know whether to return nil or not. So next is "non- > lazy of order 1" so to say. To allow full laziness lazy-seq was > introduced. It returns an object which doesn't know directly whether > it's empty, but it knows how to find out, when you ask for it. When > you know call empty? on the lazy-seq you realise the sequence and the > seq call hidden in empty will return most likely a Cons, where first > and rest are just field references, or nil of there are no elements in > the sequence. But you throw this away. Then you call first on the lazy- > seq. first itself calls seq and gets again the now cached Cons. Then > you call rest, which calls seq, which returns the cached Cons. A lot > of indirection going on. If you handle non-seqs like vectors or maps, > you have to replace that with multiple creations of very short-lived > objects. This is even more expensive. > > So you can see, that by capturing the return value of the seq call you > can save quite a bit of indirection. Plus seq becomes the identity, > which should be fast. > > I did some quick'n'dirty benchmarking and the improvement seems to be > 12% for me. Not thaaaat much, but quite a bit for such trivial > changes. >
Hello, Agreed with the explanation, but ... 12% of what, exactly ? -- 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
