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 use a macro like:

(defmacro swap!-> [atom & args]
  `(let [old-val# (atom nil)
         new-val# (swap! ~atom #(do
                                  (swap! old-val# (constantly %))
                                  (-> % ~args)))]
     {:old @old-val# :new new-val#}))

(def data (atom {}))

(swap!-> data assoc :a 3001)
=> {:new {:a 3001} :old {}}