arities.clj

(defn arities 
  "*HACK*
  Returns a seq of arities of f. The seq will be infinite if f uses rest-params,
  starting at the minimum number of args."
  [f]
  (let [invokes (filter #(= "invoke" (.getName %)) (.getDeclaredMethods (class f)))
        arities (sort (map #(count (.getParameterTypes %)) invokes))]
    (if (instance? clojure.lang.RestFn f)
      (concat arities (iterate inc (inc (last arities))))
      arities)))

(comment

user> (take 10 (arities (fn ([a b c])([a b c & d] ))))
(3 4 5 6 7 8 9 10 11 12)
user> (take 10 (arities (fn ([a b c])([a b c d] ))))
(3 4)

)

(defn stack [& exprs]
  (letfn [(do-stack [stk [e & es :as exprs]]
            (if e
              (let [consume (if (fn? e) (first (arities e)) 0)
                    [in remaining] (split-at consume stk)
                    push (if (fn? e) (apply e in) e)]
                (recur (conj remaining push) es))
              stk))]
    (do-stack () exprs)))

(defn add [x y] (+ x y))

(comment

user> (stack 1 2 add 5 6 add 7 add add)
(21)

)