test.clj

(defstruct item :title :current-price)
(defstruct bid :user :amount)
(def ^:dynamic history ())
(def ^:dynamic droid (struct item "Droid X" 0))
(defn place-offer [offer]
  (binding [history (cons offer history)
            droid (assoc droid :current-price (get offer :amount))]
    (println droid history)))
(place-offer {:user "Anthony" :amount 10})
(println droid)

(def droid (atom (struct item "Droid X" 0)))
(def history (atom ()))
(defn place-offer [offer]
  (reset! droid (assoc @droid :current-price (:amount offer))))
(place-offer {:user "Anthony" :amount 10})
(println @droid)

;; transactional refs
(def droid (ref (struct item "Droid X" 0)))
(def history (ref ()))
(defn place-offer [offer]
  (dosync
   (ref-set droid (assoc @droid :current-price (get offer :amount)))
   (ref-set history (cons offer @history))))
(place-offer {:user "Tony" :amount 22})
(println @droid @history)

;; agents
(def history (agent ()))
(def droid (agent (struct item "Droid X" 0)))
(defn place-offer [offer]
  (send droid #(assoc % :current-price (get offer :amount))))
(place-offer {:user "Tony" :amount 34})
(await droid)
(println @droid)