On Tue, Nov 24, 2009 at 3:14 AM, Christophe Grand <christo...@cgrand.net> wrote: > On Mon, Nov 23, 2009 at 11:56 PM, John Harrop <jharrop...@gmail.com> wrote: >> >> I'm starting to think that for some tasks Clojure could use a concept of >> "row locking" with maps. It would mean having a map-of-refs type that was >> integrated with the STM, so multiple updates whose keys didn't collide could >> occur concurrently. >> It *might* be possible to do this using commute and update-in, with a ref >> wrapping the whole map. The tricky thing is you want the update-ins to >> commute if the keys are not the same, but not if they are. Perhaps we need a >> "conditional commute" that takes two extra arguments, a value to test and a >> binary predicate. Then it could be done with (conditional-commute key = map >> update-in [key] val-transform-fn). >> The idea here being, in this case, that if two of these were done in >> overlapping transactions, the first arguments would be compared using the >> second argument. If the result was true one transaction would be retried, if >> false the operations would commute. (If the second arguments to the two >> conditional-commutes differed the transaction would be retried.) > > I had a similar idea but more general: being able to specify invariants > inside a transaction. Commit will procede only if the invariant still holds. > Your proposed conditional-commute could be rewritten: > > ;; (conditional-commute key = map update-in [key] val-transform-fn) > (invariant (@map key)) > (commute map update-in [key] val-transform-fn) > > See the attached patch for a prototype. >
I had forgotten about this, could you please make an issue for it? I'd like to look into it for a future feature. Thanks, Rich -- 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
