On Saturday, April 25, 2015 at 9:33:18 AM UTC-5, Timur wrote: > > Hi everyone, > > There are situations where I want to dispatch functions using based on > their certain properties. I can also use case statements instead but it > looks more coupled and more change is required if I want to add new types. >
case is useful for the particular situation where you have a finite set of compile-time constants that you are looking for (integers, characters, etc). Then case will leverage a special JVM bytecode that performs a *constant-time* look-up (not linear-time like checking a series of cond conditions, etc). This is one of the two bytecode options (tableswitch, rather than lookupswitch) underlying the Java switch/case feature. What I want to ask is if I need to avoid using multi-methods for > performance reasons? I read somewhere that they are not really fast but the > posts were old and the performance might have been improved in between. > Should I favor case and cond branches instead of defmulti when I need > performance? > multimethods are slower than protocols - they must effectively invoke the dispatch function, perform a linear search for a match across the cases, and then invoke the actual implementation function. Protocols leverage JVM internals to select the right implementation, then invoke it. However, the search part of multimethods is cached, making that step fast after the initial call. The last time I benchmarked multimethods with class dispatch vs protocols on Java 1.8 + Clojure 1.7 alphas (can't remember which one), I found multimethods were about 5x slower. If you want fastest type-based dispatch with open extension, then protocols are the best. If you want any other type of dispatch with open extension, then multimethods are the best. If you want a closed system of constant choices, then case is best (constant-time lookup!) If you want a closed system of arbitrary conditions, then cond is best. I have never compared the performance of cond vs multimethods for something like this. My guess is that multimethods are faster as they evaluate a single condition, then do a table lookup once cached vs evaluating a series of conditions every time. Alex -- 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.
