Alternate version of swap! also returning swapped out value

Question

I talked about this a bit on IRC\'s #clojure channel today but would like to go more in detail here. Basically, in order to better understand atoms, swap!

Answer 1

You could rely on a promise to store the current value inside the swap! operation. Then you return the new and old value in a vector, as follows:

(defn- swap-and-return-old-value!
  [^clojure.lang.IAtom atom f & args]
  (let [old-value-promise (promise)
        new-value (swap! atom
                     (fn [old-value]
                       (deliver old-value-promise old-value)
                       (apply f old-value args)))]
    [new-value @old-value-promise]))