Extending Clojure Map with `deftype` and `defprotocol`

extmap.clj

(ns extmap
  (:import
   [clojure.lang ILookup Associative]))

;; ----------------------------------------------------------------
;; TrackableMap is a persistent map which keeps track of the number of
;; times `get` was called on it.

(defprotocol Trackable
  (getStats [self]))

(deftype TrackableMap [data stats]
  ILookup
  (valAt [this key]
    (swap! stats inc)
    (get @data key))

  Associative
  (assoc [this key val]
    (swap! data assoc key val)
    this)

  Trackable
  (getStats [this] @stats))

(defn make-trackable-map [& [init-map]]
  (TrackableMap. (atom (or init-map {})) (atom 0)))

(let [m (assoc (make-trackable-map {:foo 1}) :bar 2)]
  [(get m :foo) (get m :foo) (get m :bar) (getStats m)])

;; => [1 1 2 3]

;; ----------------------------------------------------------------

;; RandomValueMap is a persistent map which creates new associations
;; for keys which miss, with random integer values

(deftype RandomValueMap [data]
  ILookup
  (valAt [this key]
    (or (get @data key)
        (let [val (Math/round (* (Math/random) 100))]
          (swap! data assoc key val)
          val)))

  Associative
  (assoc [this key val]
    (swap! data assoc key val)
    this))

(defn make-random-value-map [& [init-map]]
  (RandomValueMap. (atom (or init-map {}))))

(let [m (assoc (make-random-value-map {:foo 1}) :baz 2)]
  [(get m :foo)
   (get m :bar)                         ; create new assoc
   (get m :bar)                         ; return the assoc created above
   (get m :baz)])

;; => [1 38 38 2]