defnkw.clj

(defn uninterleave [xs]
  [(take-nth 2 xs) (take-nth 2 (rest xs))])

(defmacro defnkw [name & handlers]
  (let [handlers (map #(do [(uninterleave (first %)), (rest %)]) handlers)
        selector- (gensym "selector")
        args- (gensym "args")]
    `(defn ~name [& selector-args#]
       (let [[~selector- ~args-] (uninterleave selector-args#)]
         (cond ~@(mapcat (fn [[[test-selector params] body]]
                           `((= ~selector- '~test-selector)
                             (let [~(vec params) ~args-]
                               ~@body)))
                         handlers)
               :else (throw (Exception. "No matching selector found.")))))))

;; Examples

(defnkw please
  ([:multiply x :by y] (* x y))
  ([:add x :to y] (+ x y)))

(please :multiply 2 :by 3) ;; 6
(please :add 2 :to 3)      ;; 5
(please :divide 2 :by 3)   ;; "No matching selector found."