gistfile1.clj

(defn arities
  "Returns the possible arities of a function implementing ifn. For want of
  anything better to return for a variadic 'rest function', returns
  nil."
  [ifn]
  {:pre [(ifn? ifn)]}
  (let [invokes (filter #(= (.getName %) "invoke") (.getDeclaredMethods (class ifn)))
        arity   #(count (.getParameterTypes %))]
    (if (instance? clojure.lang.RestFn ifn)
      '(nil)
      (map arity invokes))))

(defn has-arity?
  [ifn arity]
  (when (some #(= % arity) (arities ifn))
    true))