In general the problem of whether a local can be cleared is undecidable. However a very advanced compiler might be able to find out that the closure of a future will only be called once, and clear the reference to the closure. It is much simpler to just special case futures. One problem that might come up is that the closure of a future might be called multiple times when a transaction rolls back. Or is this not possible?
On Aug 11, 9:06 pm, Stuart Halloway <[email protected]> wrote: > Chouser, > > There is now a ticket and roadmap for fixing this: > Seehttps://www.assembla.com/spaces/clojure/tickets/423-make-sure-future-.... > > Stu > > > > > On Wed, Aug 11, 2010 at 1:25 AM, timcharper <[email protected]> wrote: > > >> I've distilled the issue down to a series of two tests: in one case, > >> the head is properly released, and in the other, the head is retained > >> even though it seems it shouldn't be. > > >>http://gist.github.com/510601 > > > Those are nice tests. Please excuse my poorer ones below... > > > ...but I think this isn't specifically about lazy seqs or > > futures, though those clearly are affected. It's actually about > > gc and closures. > > > Let's start with a helper function that retains a reference to > > a closure while trying to GC a possibly related object: > > > (defn attempt-gc [closure weak-ref n] > > (loop [i 0] > > (System/gc) > > (when (and (< i n) (.get weak-ref)) > > (Thread/sleep 1000) > > (recur (inc i)))) > > closure) > > > Now if we call this with a closure that does *not* close over the > > object the weak-ref refers to, the object gets GC'ed promptly and > > control returns immediately: > > > (let [x (Object.), r (java.lang.ref.WeakReference. x)] > > (attempt-gc #(do nil) r 5)) > > > Note that the Clojure compiler's local-clearing is helping us > > here -- it's noticing nothing in the body of the let needs x, so > > it's getting cleared and GC'ed before the attempt-gc loop has to > > run more than once. > > > But if the closure *does* close over the object, the closure > > itself retains a reference to it and local-clearing in the 'let' > > doesn't help. So attempt-gc tries for several seconds before > > giving up: > > > (let [x (Object.), r (java.lang.ref.WeakReference. x)] > > (attempt-gc #(do x nil) r 5)) > > > It's not clear to me whether or not the Compiler could do > > something about this. > > > Note there's no lazy seq or future (or indeed thread boundaries > > of any kind) in either of my examples, though of course retaining > > the head of a lazy seq retains the entire seq, and creating > > a future does create a closure. > > > --Chouser > >http://joyofclojure.com/ > > > -- > > You received this message because you are subscribed to the Google > > Groups "Clojure" group. > > To post to this group, send email to [email protected] > > Note that posts from new members are moderated - please be patient with > > your first post. > > To unsubscribe from this group, send email to > > [email protected] > > 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 post to this group, send email to [email protected] Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/clojure?hl=en
