Is it possible to write a reducing function that mutates a list in Clojure? Sure. But I think it is absurdly unlikely that it would happen by accident. My 1% chance wasn't hedging against that case -- I was hedging against a bug in reduce itself.
I don't really see even a 1% likelihood of either of those, but I played D&D as a kid and learned that even the most unlikely things happen one time in twenty. :-) Stu On Tue, Dec 11, 2012 at 3:30 PM, Marko Topolnik <marko.topol...@gmail.com>wrote: > Just curious, how did you immediately eliminate the possibility that the > reducing function was mutating the list that is being reduced? No > concurrency involved. In regular Java the 95% leading cause of CME is > precisely that. > > Anyway, this applies to immutable structures per se, whether combined with > atoms, refs, or none. But a full wartime story must also cover how the > solution avoids the pitfalls of retryable transactions. This is the real > sore point in my experience, and the one which makes STM an all-or-nothing > enterprise. > > > On Tuesday, December 11, 2012 8:53:40 PM UTC+1, stuart....@gmail.comwrote: >> >> Hi Paul, >> >> Here is a real-world, production software example of the advantage of >> values+refs over mutable objects and locks. A Datomic user reported the >> following stack trace as a potential bug: >> >> 12:45:43.480 [qtp517338136-84] WARN c.v.a.s.p.e.UnknownExceptionH**andler >> - UnknownExceptionHandler: null >> java.util.ConcurrentModificati**onException: null >> at >> java.util.ArrayList$Itr.checkF**orComodification(ArrayList.**java:819) >> ~[na:1.7.0_07] >> at java.util.ArrayList$Itr.next(A**rrayList.java:791) >> ~[na:1.7.0_07] >> at clojure.core.protocols$fn_**_5871.invoke(protocols.clj:76) >> ~[clojure-1.4.0.jar:na] >> at >> clojure.core.protocols$fn_**_5828$G__5823__5841.invoke(pro**tocols.clj:13) >> ~[clojure-1.4.0.jar:na] >> at clojure.core$reduce.invoke(cor**e.clj:6030) >> ~[clojure-1.4.0.jar:na] >> >> I immediately had 99% confidence that the bug was in user code, and even >> a pretty good idea what went wrong. A call to "reduce" is a functional >> transformation, and it expects to be passed values. The exception clearly >> indicates a violation of that contract, and is caused by cross-thread >> aliasing and mutation in the calling code. >> >> Regards, >> Stu >> > -- > 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 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
