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 Clojure

Answer 1

The arity of a function is stored in the metadata of the var.

(:arglists (meta #'str)) ;([] [x] [x & ys]) 

This requires that the function was either defined using defn, or the :arglists metadata supplied explicitly.

Answer 2

Sneaky reflection:

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

Or :

(defn arg-count [f]   {:pre [(instance? clojure.lang.AFunction f)]}   (-> f class .getDeclaredMethods first .getParameterTypes alength))