I *think* I've found the answer to my own question... In this post...
https://groups.google.com/forum/#!topic/clojure/Cuk_bJrIq-Y I found this link (I changed the line number)... https://github.com/clojure/clojure/blob/master/src/jvm/clojure/lang/Compiler.java#L2589 And if the implementation of eval() within the IfExpr class is anything to go by... public Object eval() { Object t = testExpr.eval(); if(t != null && t != Boolean.FALSE) return thenExpr.eval(); return elseExpr.eval(); } Then I think Rich has implemented it so that nil takes priority over false. ...Which strengthens the case for choosing (if xs b a) over (if (nil? xs) a b). Of course, in practice it probably makes no difference, as either will be very quick compared to other things going on. Cheers, Mark. On Friday, 18 July 2014 14:39:53 UTC+9:30, Mark Phillips wrote: > > Thanks again - that all makes sense. > > One (hopefully) tiny question... an efficiency one... (and feel free not > to answer it if I've already taken up enough of your time) > > If you do that and are careful, the performance of next/nil? is slightly >> better than rest/empty?. >> > > If I use the next/nil? idiom, is there a performance difference between > doing: > > 1. (if (nil? xs) a b) > 2. (if xs b a) > > And a related question... Clojure has two falsey values: nil and false. > I assume that low-level somewhere, when checking for falsey, an (if x a b) > call will do something like "if x is false, short circuit to b, if x is > nil, short circuit to b, otherwise a". Except, I don't know which gets > priority of place with the short circuits - is it false (as I've just > written), or is it nil? > > Actually, if I'm thinking about it correctly... > > If false takes priority then... A branch to "a" will take three tests for > 1. and two tests for 2.... And a branch to "b" will take two tests for 1. > and two tests for 2.. > > If nil takes priority then... A branch to "a" will take three tests for > 1. and one test for 2.... And a branch to "b" will take three tests for 1. > and two tests for 2.. > > So 2. is always as good or better than 1. I think - performance wise. > > Of course, these things are very much minor factors in overall performance > (especially when things like immutable data structures are involved), but I > wouldn't mind knowing. > > Cheers, > > Mark. > > -- 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.
