Update state with a pure function

atomic_state_update.clj

;; fixed version of 'state' at http://mishadoff.com/blog/clojure-design-patterns/#episode-3-state
;; takeaway: if you call 'swap!' twice on the same atom, you are probably making a mistake

(def user {:name "Jackie Brown"
           :balance 0
           :subscription? false})
				 
(def ^:const SUBSCRIPTION_COST 30)

(defn pay
  [{:keys [balance name subscription?]} amount]
  (let [balance (+ balance amount)]
    (if (and (> balance SUBSCRIPTION_COST) (not subscription?))
      {:name name :subscription? true :balance (- balance SUBSCRIPTION_COST)}
      {:name name :subscription? subscription? :balance balance})))

(reductions pay user [10 25 25])

;; now you can put user into any reference type you like: atom, ref, agent...

Excellent purification :)

One small fix: > should be >= so that (reductions pay user [10 20]) ends up with :subscription? true.