On Nov 8, 2008, at 10:23 PM, Stuart Halloway wrote:
>
> How about:
>
> (defn runonce
> "Create a function that will only run its argument once."
> [function]
> (let [call-count (ref 0)]
> (fn [& args]
> (when (= 1 (dosync (alter call-count inc)))
> (apply function args)))))
The counter occurred to me too. I don't think it's necessary though.
What's important is that one thread "know" that it was the one that
cause the "false" to "true" transition.
Regarding the value returned by dosync above, I'd believe it if
someone told me that it could only return 1 to one thread, one time,
guaranteed, but I've been wrong before in my reasoning about alter vs.
commute regarding what's possible, so I'll have to give it more
thought on correctness.
Here's perhaps a more basic "one shot" building block:
(defn once
"Returns a function that returns v the first time it's called and
nil every time after that"
[v]
(let [returned (ref false)]
(fn []
(dosync
(when-not (ensure returned)
(ref-set returned true)
v)))))
--Steve
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