clojure-koans/23_meta.clj

  ; "Some objects can be tagged using the with-meta function"
  (= __ (meta giants))

  ; "Or more succinctly with a reader macro"
  (= __ (meta '^{:division "West"} Giants))

  ; "While others can't"
  (= __ (try
          (with-meta
            2
            {:prime true})
          (catch ClassCastException e
            "This doesn't implement the IObj interface")))

  ; "Notice when metadata carries over"
  (= __ (meta (merge '^{:foo :bar} {:a 1 :b 2}
                     {:b 3 :c 4})))

  ; "And when it doesn't"
  (= __ (meta (merge {:a 1 :b 2}
                     '^{:foo :bar} {:b 3 :c 4})))

  ; "Metadata can be used as a type hint to avoid reflection during runtime"
  (= __ (#(.charAt ^String % 0) "Cast me"))

  ; "You can directly update an object's metadata"
  (= 8 (let [giants
             (with-meta
               'Giants
               {:world-series-titles (atom 7)})]
         (swap! (:world-series-titles (meta giants)) __)
         @(:world-series-titles (meta giants))))

  ; "You can also create a new object from another object with metadata"
  (= {:league "National League" :park "AT&T Park"}
     (meta (vary-meta giants
                      assoc __ __)))

  ; "But it won't affect behavior like equality"
  (= __ (vary-meta giants dissoc :league))

  ; "Or the object's printed representation"
  (= __ (pr-str (vary-meta giants dissoc :league)))