;;; Using plain maps.

(let [id-generator-value (atom 0)]
  (defn id-generator-next []
    (swap! id-generator-value inc)))


(defn make-ID
  ([] (make-ID {}))
  ([m] (assoc m :id (id-generator-next))))


(defn make-Widget [& {:keys [element-type] :or {element-type "div"}}]
  (let [widget (make-ID)]
    (assoc widget :element-type element-type)))




> (make-Widget)
{:element-type "div", :id 21}
> (make-Widget :element-type "p")
{:element-type "p", :id 22}
>




;;; Using defrecord.

(defrecord IDMixin
  [id])

(defn make-IDMixin
  ([] (IDMixin. (id-generator-next)))
  ([obj] (assoc obj :id (id-generator-next))))


(defrecord Widget
  [element-type])

(defn make-Widget [& {:keys [element-type] :or {element-type "div"}}]
  (let [widget (make-IDMixin)]
    (assoc widget :element-type element-type)))


> (make-Widget)
#user.IDMixin{:id 15, :element-type "div"}
> (make-Widget :element-type "p")
#user.IDMixin{:id 16, :element-type "p"}
> (.id (make-Widget :element-type "p"))
17
>