+1 that answer Also if it served your needs, watches give you the old and new values http://clojure.github.io/clojure/clojure.core-api.html#clojure.core/add-watch
On Tuesday, April 8, 2014 11:00:20 AM UTC-5, A. Webb wrote: > > See https://groups.google.com/d/topic/clojure/2dHvX7bf7nA/discussion, > http://stackoverflow.com/a/22409846/1756702, where the old and new state > of an atom is returned using the lower-level compare-and-set! operation. > > On Tuesday, April 8, 2014 10:41:50 AM UTC-5, John Hume wrote: >> >> I sometimes find that after mutating an atom, I want to create some >> side-effect that depends on the old and new state as well as the context in >> which the change was made. Because of the dependence on context, a watch >> doesn't work (unless there's something I'm not thinking of). So I add >> things to the new atom state (returned by swap!) purely to tell the calling >> code what side-effect to have (or give it the data it needs to decide what >> side-effect to have). That additional state isn't used anywhere other than >> the fn that called swap!. >> >> One gotcha to this approach is that one must be careful not to leave some >> old side-effect causing state in place to cause another side-effect based >> on stale data. >> >> Is there a name for this pattern? A standard way of implementing it? A >> better alternative? >> >> One alternative I'm aware of is using mutable locals (provided by >> https://github.com/ztellman/proteus) as a side-channel of communication >> from swap!. Both approaches strike me as messy, though a let-mutable >> probably makes it more obvious that something funny is going on, and it >> doesn't pollute the atom. >> >> Thanks. >> -hume. >> > -- 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 unsubscribe from this group and stop receiving emails from it, send an email to clojure+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
