Sorta off-topic from the main discussion, but in reference to the error you
pointed out, one clever fix for this is to add a delay around the future:
(defn cache-image [icache url]
(let [task (delay (future (download-image url)))]
(doto (swap! icache update-in [url] #(or % task))
(-> (get url) (force)))))
(defn wait-for-image [icache url]
(deref (force (get icache url))))
As long as your swap! never overwrites an existing delay, only one will get
added, and thus only one gets forced.
On Wednesday, July 18, 2012 4:48:17 AM UTC-7, tbc++ wrote:
>
> (def icache (atom {}))
>
> (defn cache-image [url]
> (let [f (future (download-image url))]
> (swap! icache assoc url f)))
>
> I'll admit, we'll still try to download the image twice if two people
> try to cache the same image at the exact same time but we could get
> around that using promise or agents. So in general, your attitude when
> using atoms, refs, or agents should be "get in and get out". Don't do
> your work inside of swap!, send or alter....
>
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