;; hit-and-run swap!

(defn swap-and-run! [atom f & args]
  (swap! atom (fn [x] (delay (apply f @x args)))))

;;;;;;;;;;;;;;

(defprotocol LazySwap
  (lazyswap [x f]))

(deftype LazyAtom [atom]
  clojure.lang.IDeref
    (deref [x]
      @@atom)
  LazySwap
    (lazyswap [x f]
      (swap! atom f)))

(defn lazy-swap!
 "Atomically swaps the value to be (apply f current-value args). The evaluation
  is suspended until deref. Returns a delay on the value."
 [lazy-atom f & args]
  (lazyswap lazy-atom (fn [d] (delay (apply f @d args)))))

(defn lazy-atom [x]
  (LazyAtom. (atom (delay x))))