I was wondering the same thing, Jörg. This thread <https://groups.google.com/forum/#!topic/clojure/VQj0E9TJWYY> talks about it as well. I posted a note there which I will reproduce here for your convenience:
I think it's safe to assume that since `ArrayList` uses unsynchronized mutability internally (a quick review of the GrepCode entry for `ArrayList` confirms this <http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/8u40-b25/java/util/ArrayList.java>), then we can rest assured that a `volatile` box as opposed to a totally unsynchronized mutable variable is unnecessary, even in the context of `fold`. After all, `reduce` (and by extension, `transduce`) is only ever going to be single-threaded unless the data structure in question unexpectedly implements a multithreaded reduce, which should never happen (and if it does, you likely have bigger problems). To be honest, I'm not sure why `volatile` is used in transducers instead of e.g. an `unsynchronized-mutable` box. There may be a good reason, but I'm not seeing it immediately. I'd love to learn. On Friday, January 2, 2015 at 10:06:25 AM UTC-5, Jörg Winter wrote: > > So if I'd need that extra performance, say in a private library of > transducers not intended to be shared with other Clojure developers, it is > perfectly Ok to use a java.lang.Object instead of a volatile, right ? > > J > > 2015-01-02 15:59 GMT+01:00 Timothy Baldridge <tbald...@gmail.com > <javascript:>>: > >> "As far as I understand, the step-function of a transducer is never(?) >> accessed concurrently by more than 1 thread." >> >> It's actually "one thread at a time". And you're right, stuff like >> Core.async may bounce a transducer between several different threads, but >> only 1 thread "owns" it at a given time. >> >> Timothy >> >> On Fri, Jan 2, 2015 at 7:11 AM, Jörg Winter <jwin...@gmail.com >> <javascript:>> wrote: >> >>> Hi, >>> >>> As seen in this example of a stateful transducer... >>> >>> http://crossclj.info/ns/org.clojure/clojure/latest/clojure.core.html#_partition-by >>> >>> ... I am wondering what is the concrete motivation behind using >>> 'volatile!' instead of say a simple (mutable) Java-Object wrapper ? >>> In the partition-all example, an ArrayList is used for aggregating the >>> 'temporary' results for the step-function, so this mutable state is not >>> concerned with threading at all. >>> Why then is there a threading-concern with pv (the volatile!) ? >>> >>> As far as I understand, the step-function of a transducer is never(?) >>> accessed concurrently by more than 1 thread. >>> >>> Is volatile! necessary because transducers should be usable with >>> core.async ? >>> Or is it just an easy way to get a mutable object in Clojure ? >>> >>> >>> Best, >>> Joerg >>> >>> -- >>> You received this message because you are subscribed to the Google >>> Groups "Clojure" group. >>> To post to this group, send email to clo...@googlegroups.com >>> <javascript:> >>> Note that posts from new members are moderated - please be patient with >>> your first post. >>> To unsubscribe from this group, send email to >>> clojure+u...@googlegroups.com <javascript:> >>> 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+u...@googlegroups.com <javascript:>. >>> For more options, visit https://groups.google.com/d/optout. >>> >> >> >> >> -- >> “One of the main causes of the fall of the Roman Empire was that–lacking >> zero–they had no way to indicate successful termination of their C >> programs.” >> (Robert Firth) >> >> -- >> You received this message because you are subscribed to the Google >> Groups "Clojure" group. >> To post to this group, send email to clo...@googlegroups.com >> <javascript:> >> Note that posts from new members are moderated - please be patient with >> your first post. >> To unsubscribe from this group, send email to >> clojure+u...@googlegroups.com <javascript:> >> For more options, visit this group at >> http://groups.google.com/group/clojure?hl=en >> --- >> You received this message because you are subscribed to a topic in the >> Google Groups "Clojure" group. >> To unsubscribe from this topic, visit >> https://groups.google.com/d/topic/clojure/CjxK7xEsOKQ/unsubscribe. >> To unsubscribe from this group and all its topics, send an email to >> clojure+u...@googlegroups.com <javascript:>. >> For more options, visit https://groups.google.com/d/optout. >> > > -- You received this message because you are subscribed to the Google Groups "Clojure" group. 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