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. -- 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
invariants.patch
Description: Binary data
