arities.clj

(defn arities
  "Returns a vector of the arities of function f, or of the dispatch
   function if f is a multifn.

   Example:
   => (arities (fn ([]) ([x]) ([x y & z])))
   [0 1 2 :more]"
  [f]
  (let [f            (if (instance? clojure.lang.MultiFn f) (.dispatchFn f) f)
        methods      (.getDeclaredMethods (class f))
        count-params (fn [m] (map #(count (.getParameterTypes %))
                                  (filter #(= m (.getName %)) methods)))
        invokes      (count-params "invoke")
        do-invokes   (map dec (count-params "doInvoke"))
        arities      (vec (sort (distinct (concat invokes do-invokes))))]
    (if (seq do-invokes)
      (conj arities :more)
      arities)))