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timmc/gist:3218623

Created July 31, 2012 17:13
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variant on swap! that returns the old value
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gistfile1.clj
(defn swap2! [a f & args]
(let [oldp (promise)
f* (fn [oldv] (deliver oldp oldv) (apply f oldv args))]
(swap! a f*) @oldp))
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