(defn gen-id-dispatch
  [& args]
  (let [n (count args)]
    (cond
      (= n 1) (class (first args))
      (= n 2) :pair
      :else nil)))

(defmulti gen-id #'gen-id-dispatch :default nil)

(defmethod gen-id :pair
  [x y]
  {:a x :b y})

(defmethod gen-id String
  [x]
  {:a 0 :b x})

(defmethod gen-id nil [x] nil)