traits.clj

(defn throw-arity [this n]
  (let [name (-> this .getClass .getSmpleName)
        suffix (.lastIndexOf name "__")
        elided-name (if (= suffix -1) name (.substring name 0 suffix))]
    (throw (clojure.lang.ArityException. n (.replace elided-name \_ \-)))))


(def AFn
  (list
   'java.lang.Callable
   '(call [this]
     (try (-invoke this)
       (catch Exception e
         (throw e))))
   'java.lang.Runnable
   '(run [this] (-invoke this))
   'IFn
   (map (fn [args] (list '-invoke (vec (cons 'this args))
                         (list 'throw-arity 'this (count args))))
        (cons [] (take-while seq (iterate rest (repeat 18 '_)))))
   '(-apply [this arglist]
      (let [arglist-len (count arglist)] 
       (if (< arglist-len 19)
         (eval `(-invoke ~this ~@arglist))
         (eval `(-invoke ~this ~@(take 19 arglist) '~(drop 19 arglist))))))))