I believe the short answer is: There are existing implementers of the IAtom interface out there. If the interface grew, those implementers would become broken with Clojure 1.9.0.
There is no deep discussion of the details here, but a mention of wanting to avoid breaking changes in the IAtom interface here: https://groups.google.com/forum/#!searchin/clojure-dev/iatom%7Csort:date/clojure-dev/_ixXNiIyiuQ/p1HrCRGJDAAJ Andy On Sun, Dec 17, 2017 at 2:16 PM, dimitris <jimpil1...@gmail.com> wrote: > Hi all, > > This is mainly a question for the Clojure core dev team. I'm trying really > hard to understand the thinking behind the new IAtom2 interface. My train > of thought is detailed below, but you don't have to read the full thing. > The core question is "How come the existing IAtom didn't grow, given the > fact that you guys *own* the interface?" > > OK so here is the full story: > > I recently found out about https://dev.clojure.org/jira/browse/CLJ-1454, > and it's true that often you want a version of `swap!` that returns the > value that was swapped *out*. As indicated on the ticket various people > have worked around the lack of such a fn in various ways. The solution that > I'm using/maintaining looks very much like the approach taken with IAtom2, > only it's goes through a protocol and it's written in clojure. Basically > I've defined a new abstraction and extending it to Atom. > > (defprotocol IAtomic > ... ;; more methods > (trade! [this f] > [this f x] > [this f x y] > [this f x y more] > "Like `clojure.core/swap!`, but returns the value that was swapped out.")) > > (defmacro ^:private trade* > [ref f & args] > `(let [validate# (.getValidator ~ref)] ;; extract the validator-fn once > (loop [] > (let [oldv# (.deref ~ref) > newv# (~f oldv# ~@args)] > (try (when-not (validate# newv#) > (throw (IllegalStateException. "Invalid reference state"))) > (catch Exception e# > (IllegalStateException. "Invalid reference state" ) e#)) > > (if (.compareAndSet ~ref oldv# newv#) > (do (.notifyWatches ~ref oldv# newv#) > oldv#) > (recur)))))) > > (extend-protocol IAtomic > ... ;; more types > > Atom > (trade! ([this f] > (trade* this f)) > ([this f x] > (trade* this f x)) > ([this f x y] > (trade* this f x y)) > ([this f x y more] > (trade* this #(apply f % x y more))) > ) > ) > > > Ok, so at this point I want to stress out that the way I see it this > solution seems to me like the second best option. I say 'second', because > IMO the best option would be to add `trade!` (or however you want to call > it) to the original IAtom.java and implement it straight in Atom.java. But > obviously I can't do that - only clojure.core can. Since clojure.core > didn't, I'd say that it's safe to assume that either a) growing IAtom is > not desirable, and/or b) having IAtom2 is a superior solution . Personally, > I wouldn't be able to explain/defend any of those in a conversation. I'm > probably missing something here, and that's exactly why I'm sending this > email. Enlighten me please... :) what am i missing? How come IAtom was, in > some sense, cloned, rather than grown? Many thanks in advance... > > Jim > > ps: oh and btw congrats on the 1.9 release :) > > > -- > 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. > -- 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.
