;; A class is a named dyn map with a shape tag. A slot with no type holds
;; any value, and its constructor is the class's own name, positional.
(defclass point [x y])
(defclass circle [r])

;; CLOS-style: the generic states the return type once, and each method
;; dispatches on the class of its first argument.
(defgeneric area [self] dyn)
(defmethod area point  [p] (* (get p :x) (get p :y)))
(defmethod area circle [c] (* 3 (get c :r) (get c :r)))

;; Clojure-style: the generic's body is the dispatch value, and a method
;; names the value it answers for. :else is the arm everything falls to.
(defmulti describe [x] dyn (get x :kind))
(defmethod describe :square [s] (get s :side))
(defmethod describe :else  [s] "something else")

(defn main [] ()
  (let [p (point 3 4)]
    (println (area p))
    (println (area (circle 2)))
    ;; A slot is a map key: get, put and has-key are how one is read.
    (println (get p :y))
    ;; class-of answers the tag, and nil for anything that is not an instance.
    (println (class-of p))
    (println (class-of 7))
    (println p)
    (println (describe {:kind :square :side 9}))
    (println (describe {:kind :blob}))))