Yes, that does.
Thanks to both of you.
Tim
On Oct 25, 10:16 am, Chris Perkins <chrisperkin...@gmail.com> wrote:
> On Tuesday, October 25, 2011 12:00:04 PM UTC-4, Tim Robinson wrote:
>
> > This code probably will not make a whole lotta sense since I reduced
> > it down to show only the problem at hand, but I'm hoping someone can
> > explain why this doesn't work the way I expected it would:
>
> > => (def data (atom {:k1 "v1" :k2 "v2" :k3 "v3"}))
> > #'user/data
>
> > => (def flag (atom nil))
> > #'user/flag
>
> > => (defn oops! []
> > (let [x1 (atom (hash-map))
> > v1 (filter
> > #(let [xv1 (@data %)]
> > (if (= xv1 "v1")
> > (swap! x1 assoc :k1 "other")))
> > (keys @data))
> > rxv (reset! flag @x1)]
> > (println v1)))
>
> > => (oops!)
> > (:k1)
> > nil
>
> > => @flag
> > {}
>
> > I had expected this flag would now hold the value from x1, but instead
> > it's empty.
>
> > No, it's not empty - it holds the value that x1 had at the time you swapped
>
> it, which is an empty map.
>
> Later, the x1 atom had its value changed, by then it was too late to affect
> flag. Specifically, x1 changed only when you printed v1, thus forcing
> realization of the lazy sequence that filter returned.
>
> Make sense?
>
> - Chris
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