23_meta.clj

(ns koans.23-meta
  (:require [koan-engine.core :refer :all]))

(def giants
  (with-meta 'Giants
    {:league "National League"}))

(meditations
 "Some objects can be tagged using the with-meta function"
 (= {:league "National League"} (meta giants))

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

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

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

 "And when it doesn't"
 (= nil (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"
 (= \C (#(.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)) inc)
        @(: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 :league "National League" :park "AT&T Park")))

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

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