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

Atoms are un-coordinated so it seems likely that any attempt to do this outside of the swapping function it's self will likely fail. You could write a function that you call instead of swap! which constructs a function that saves the existing value before applying the real function, and then pass this constructed function to swap!.

user> (def foo (atom  []))
#'user/foo
user> (defn save-n-swap! [a f & args] 
         (swap! a (fn [old-val] 
                    (let [new-val (apply f (cons old-val args))] 
                       (println "swapped out: " old-val "\n" "swapped in: " new-val) 
                       new-val)))) 
#'user/save-n-swap!
user> (save-n-swap! foo conj 4) 
swapped out:  [] 
 swapped in:  [4]
[4] 
user> (save-n-swap! foo conj 4) 
swapped out:  [4]
 swapped in:  [4 4]
[4 4] 

This example prints it, It would also make sense to push them to a changelog stored in another atom