Extend IFn.clj

(defn gen-symbols [n] (map #(symbol (str "arg_" %)) (range 0 n))) 

(def *invoke-max* 20)          

(defn invoke-count [sym n] 
  (let [over-max? (> n *invoke-max*)
        n (min n *invoke-max*)
        in-args (gen-symbols n)]            
    (list 'invoke (vec (concat in-args (if over-max? '[& args] [])))  
          (apply list (concat (if over-max? '[apply]) [sym]  in-args (if over-max? ['args]))))))

(defn gen-invoke [sym]
  (concat
   ['clojure.lang.IFn (list 'applyTo '[args] (list 'apply sym '(seq args)))]
    (map #(invoke-count sym %) (range 0 (+ 2 *invoke-max*)))))

(defmacro definvokable [sym & body] 
  "Has a 20 argument limit, supports vairadic arguments"
  `(deftype ~@(concat body (gen-invoke sym))))

;;Usage
(definvokable f TestFn [f])

((TestFn +) 1) => 1
((TestFn +) 1 2) => 3
(apply (TestFn +) [1 2]) => 3