prevent infinite print recursion for IDeref

gistfile1.clj

;; As originally posted, circa July 2009: http://web.archive.org/web/20110919081924/http://paste.lisp.org/display/83647

(def *iderefs* #{})

(defmethod print-method clojure.lang.IDeref [o #^Writer w]
  (if (*iderefs* o)
    (.write w (format "#<%s@%x>"
                      (.getSimpleName (class o))
                      (System/identityHashCode o)))
    (binding [*iderefs* (conj *iderefs* o)]
      (print-sequential (format "#<%s@%x: "
                                (.getSimpleName (class o))
                                (System/identityHashCode o))
                        pr-on, "", ">"
                        (list (if (and (future? o) (not (future-done? o)))
                                :pending
                                @o))
                        w))))

;; === without 'future' support ===

(def *iderefs* #{})

(defmethod print-method clojure.lang.IDeref [o #^Writer w]
  (if (*iderefs* o)
    (.write w (format "#<%s@%x>"
                      (.getSimpleName (class o))
                      (System/identityHashCode o)))
    (binding [*iderefs* (conj *iderefs* o)]
      (print-sequential (format "#<%s@%x: "
                                (.getSimpleName (class o))
                                (System/identityHashCode o))
                        pr-on, "", ">", (list @o), w))))

;; === manual binding scope, without future ===

(def *iderefs* #{})

(defmethod print-method clojure.lang.IDeref [o #^Writer w]
  (if (*iderefs* o)
    (.write w (format "#<%s@%x>"
                      (.getSimpleName (class o))
                      (System/identityHashCode o)))
    (do
      (try
        (set! *iderefs* (conj *iderefs* o))
        (catch IllegalStateException e))
      (print-sequential (format "#<%s@%x: "
                                (.getSimpleName (class o))
                                (System/identityHashCode o))
                        pr-on, "", ">", (list @o), w))))

user=> (binding [*iderefs* #{}]
         (let [a (atom :a)
               b (atom :b)]
           (reset! b a)
           (reset! a b)
           (prn [a b])))
[#<Atom@78467991: #<Atom@5ad75c47: #<Atom@78467991>>> #<Atom@5ad75c47>]
nil