On Sun, Aug 2, 2009 at 5:45 PM, Rich Hickey<richhic...@gmail.com> wrote: > > On Sun, Aug 2, 2009 at 3:20 PM, Mark Volkmann<r.mark.volkm...@gmail.com> > wrote: >> >> The doc for commute says "At the commit point of the transaction, sets >> the value of ref to be: >> (apply fun most-recently-committed-value-of-ref args)". >> >> Looking at the source code in LockingTransaction.java, that doesn't >> seem to always be the case. I see this: >> f.fn.applyTo(RT.cons(vals.get(ref), f.args)) >> >> vals is a HashMap of in-transaction values, not most recently committed >> values. >> >> Inside a transaction, Refs get in-transaction values if you do a >> ref-set or alter on them. >> Within a transaction you can ref-set or alter a Ref and then do a >> commute on the same Ref. >> You can't reverse the order. Once you do a commute on a Ref, you can't >> do a ref-set or alter on it in the same transaction. >> >> During transaction commit and immediately before the commute functions >> for the Ref are called, it determines whether the Ref has an >> in-transaction value, meaning that ref-set or alter was called on it. >> If it does not then the most recently committed value for the Ref is >> used for the in-transaction value. >> >> So I think it is only partially true to say that when commute >> functions are rerun during a commit, they use the most recently >> committed values of Refs. That only happens if ref-set and alter were >> not used on the Ref before the call to commute. >> >> Am I understanding this correctly? >> > > Not really. If this transaction is succeeding, and it has written to a > ref prior to its commute, then those writes *are* the most recently > committed value, committed now as a prior part of the same > transaction.
Thanks for explaining that! I see what you're saying, but I also see how someone, like me ;-), could be confused by the doc string. When the code in question is running, the commit hasn't completed yet. It doesn't seem correct to say "those writes *are* the most recently committed value", but rather "those writes *will become* the most recently committed value. I hope I'm not just splitting hairs, but it seems that "most-recently-committed-value-of-ref" implies that its a value that can be seen now by all threads. Maybe this is splitting hairs too, but if the code in a commute function throws an exception when it is called during the commit then values set before the commutes in the transaction body really will not become the most recently committed value. That said, I can't think of a concise way to explain this in the doc string. -- R. Mark Volkmann Object Computing, Inc. --~--~---------~--~----~------------~-------~--~----~ 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 -~----------~----~----~----~------~----~------~--~---
