Using Clojure add-watch to track the previous state of an atom

add-watch.cljc

(def current (atom 1))
(def previous (atom nil))

(add-watch current :dontcare
           (fn [_ _ old-state _] 
               (reset! previous old-state)))
         
(repeatedly 10 #(do (swap! current * 2)
                    (println "Current:" @current "Previous:" @previous)))

;Current: 2 Previous: 1
;Current: 4 Previous: 2
;Current: 8 Previous: 4
;Current: 16 Previous: 8
;Current: 32 Previous: 16
;Current: 64 Previous: 32
;Current: 128 Previous: 64
;Current: 256 Previous: 128
;Current: 512 Previous: 256
;Current: 1024 Previous: 512