default-map.clj

(comment
  (:b (DefaultMap. :foo {:a 1}))
  ; => :foo
  (:a (DefaultMap. :foo {:a 1}))
  ; => 1
  (merge-with conj (DefaultMap. [] {}) {:a 1} {:a 2} {:a 3})
  ; => {:a [1 2 3]}
  )

;;; method implementations basically taken from clojure.core/emit-defrecord
(deftype DefaultMap [default __map]
  Object
  (hashCode [this] (reduce hash-combine
                           (keyword (str *ns*) "DefaultMap")
                           default
                           __map))
  (equals [this other]
    (boolean (or (identical? this other)
                 (when (identical? (class this) (class other))
                   (every? true? (map = [default __map]
                                        [(.default other)
                                         (.__map other)]))))))
  clojure.lang.IObj
  (meta [this] (.meta __map))
  (withMeta [this m]
    (DefaultMap. default (.withMeta __map m)))
  clojure.lang.ILookup
  (valAt [this k] (.valAt __map k default))
  (valAt [this k else] (.valAt __map k else))
  clojure.lang.IKeywordLookup
  (getLookupThunk [this k]
    (reify clojure.lang.ILookupThunk
      (get [thunk target]
        (if (identical? (class target) (class this))
          (.valAt this k)))))
  clojure.lang.IPersistentMap
  (count [this] (.count __map))
  (empty [this] (DefaultMap. default (.empty __map)))
  (cons [this e] (DefaultMap. default (.cons __map e)))
  (equiv [this o] (.equals this o))
  (containsKey [this k] true)
  (entryAt [this k] (.entryAt __map k))
  (seq [this] (.seq __map))
  (assoc [this k v]
    (DefaultMap. default (.assoc __map k v)))
  (without [this k]
    (DefaultMap. default (.without __map k))))