Thanks, I hadn't seen those issues. https://dev.clojure.org/jira/browse/CLJ-1218 talks about mapcat, and https://dev.clojure.org/jira/browse/CLJ-1583 talks about apply. Those are aspects of the problem, for sure, but fixing those two issues would not solve the problem with (apply concat ...), I don't think, because another facet of the problem is that concat's args are of the form [x y & zs].
Gary Fredericks recognizes this problem in the comments for CLJ-1218: "I realized that concat could actually be made lazier without changing its semantics, if it had a single [& args] clause that was then implemented similarly to join above." But for the most part, the comments are focused on fixing mapcat by implementing a new function, join, rather than fixing the problem with concat directly. Seems better to fix concat, if possible. I'm truly astonished I haven't gotten bitten by this before. This is a pattern I use frequently in my code; I guess I just never had deep enough recursion for it to slow things down enough for me to realize what was happening. On Wed, Jul 18, 2018 at 12:53 AM, Nicola Mometto <brobro...@gmail.com> wrote: > This behaviour is known and there are a couple of tickets about it : > > https://dev.clojure.org/jira/browse/CLJ-1583 > https://dev.clojure.org/jira/browse/CLJ-1218 > > On Wed, 18 Jul 2018, 08:28 Mark Engelberg, <mark.engelb...@gmail.com> > wrote: > >> I'm kind of surprised I haven't run across this before, but tonight I was >> debugging a function that was doing an explosion of computation to return >> the first value of a lazy sequence, and I was able to reduce the problem >> down to this example: >> >> > (first (apply concat (map #(do (println %) [%]) (list 1 2 3 4 5)))) >> 1 >> 2 >> 3 >> 4 >> 1 >> >> The last 1 is the return value, but notice that it realized 4 values in >> order to return the 1. This has nothing to do with chunked sequences, by >> the way -- a list is an unchunked sequence. It appears to be that the way >> concat is written, it realizes the first two elements, and then another two >> elements in a recursive call before the lazy-seq kicks in. >> >> In the function in question, the "apply concat" was part of a recursion, >> causing that explosion of realizing values (four at each level of the >> recursion, it would seem) to get at the first element. >> >> Note that this affects mapcat as well, which relies on concat under the >> hood: >> > (first (mapcat #(do (println %) [%]) (list 1 2 3 4 5))) >> 1 >> 2 >> 3 >> 4 >> 1 >> >> I don't see a quick fix other than writing my own improved >> concat/mapcat. Do you? >> >> -- >> 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. > -- 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.
