a new dispatch mechanism similar to condp

defmulti.clj

(defmacro defmulti-m [multi-fn-name dispatch-fn ]
  `(let [dfn# ~dispatch-fn
         dvalmap# (atom {})]
     (defn ~(symbol (str "add-method-to-" (name multi-fn-name)))
       [key# method-fn#]
       (swap! dvalmap# assoc key# method-fn#))
     (defn ~multi-fn-name [& args#]
       (let [ks# (keys @dvalmap#)
             k# (or (some #(when (dfn# % args#) %) ks#) :default)
             func-to-call# (@dvalmap# k#)]
         (if func-to-call#
           (try (apply func-to-call# args#)
                (catch Exception _#
                  (throw (Exception. (str "application of " ~(name multi-fn-name)
                                          " method corresponding to key "
                                          k# " with args " args#
                                          "failed with an exception...")))))
           (throw (Exception.
                   (str "could not find an approriate method for the args"
                        args#))))))))

(defmacro defmethod-m [multi-fn-name dispatch-val args & body]
  `(~(symbol (str "add-method-to-" (name multi-fn-name)))
    ~dispatch-val (fn ~args ~@body)))

(defmulti-m hello #(even? (apply - %1 %2)))
(defmethod-m hello 3 [& d] :hello)
(defmethod-m hello 4 [& d] :heeeeellllllooooooooooo)