promises.clj

(deftype PromiseWrapper [r fun prom]
  clojure.lang.IDeref
  (deref [this]
   (dosync
     (let [current @r]
       (if current (:value current)
         (let [result (fun @prom)]
           (ref-set r {:value result})
           result))))))

(defmacro fmap [fun prom]
  `(PromiseWrapper. (ref nil) ~fun ~prom))

(defmacro then [a b & body]
  `(fmap (fn [r#] (let [~b r#] ~@body)) ~a))