Clojure: How to find out the arity of function at runtime?

Question

Given a function object or name, how can I determine its arity? Something like (arity func-name) .

I hope there is a way, since arity is pretty central in C

Answer 1

Actually it also works on macros:

(defn arg-count [f]
  (let [m (first (.getDeclaredMethods (class f)))
        p (.getParameterTypes m)]
    (alength p)))

(defmacro my-macro [])

(arg-count @#'my-macro)
; 2

Why 2? Because every macro has two implicit arguments &form and &env respectively.