foo.clj

(defmacro defpoly [n]
  `(defmulti ~n (let [preds# (atom ())]
                  (with-meta
                    (fn [& args#]
                      (loop [[[p# v#] & ps#] @preds#]
                        (if p#
                          (if (apply p# args#)
                            v#
                            (recur ps#))
                          (throw (Exception.)))))
                    {:preds preds#}))))

(defmacro defpolymethod [n p args body]
  (let [v (gensym)]
    `(do
       (swap! (:preds (meta (.dispatchFn ~n)))
              conj
              [~p '~v])
       (defmethod ~n '~v ~args ~body))))

;; user=> (defpoly foo)
;; nil
;; user=> (defpolymethod foo even? [n] [:even n])
;; #object[clojure.lang.MultiFn 0x29fa6b65 "clojure.lang.MultiFn@29fa6b65"]
;; user=> (defpolymethod foo odd? [n] [:odd n])
;; #object[clojure.lang.MultiFn 0x29fa6b65 "clojure.lang.MultiFn@29fa6b65"]
;; user=> (foo 1)
;; [:odd 1]
;; user=> (foo 2)
;; [:even 2]
;; user=>