ref.clj

(defmacro futures
  [n & exprs]
  (vec (for [_ (range n)
             expr exprs]
         `(future ~expr))))

(defmacro wait-futures
  [& args]
  `(doseq [f# (futures ~@args)]
     @f#))

(defn character
  [name & {:as opts}]
  (ref (merge {:name name :items #{} :health 500}
         opts)))

(def smaug (character "Smaug" :health 500 :strength 400 :items (set (range 50))))
(def bilbo (character "Bilbo" :health 100 :strength 100))
(def gandalf (character "Gandalf" :health 75 :mana 750))

(defn loot
  [from to]
  (dosync
    (when-let [item (first (:items @from))]
      (alter to update-in [:items] conj item)
      (alter from update-in [:items] disj item))))

(wait-futures 1
  (while (loot smaug bilbo))
  (while (loot smaug gandalf)))