arity.clj

(ns strangeness.fn-arity
  (:import java.lang.reflect.Method))

(defn declared-methods [^Class class]
  (filter #(= class (.getDeclaringClass ^Method %))
          (.getMethods class)))

(defn has-arity? [fun arity]
  (let [cls (class fun)
        methods (declared-methods cls)]
    (or (when (instance? clojure.lang.RestFn fun)
          (<= (.getRequiredArity ^clojure.lang.RestFn fun) arity))
        (boolean (some (fn [^Method m]
                         (and (= "invoke" (.getName m))
                              (= (count (.getParameterTypes m)) arity)))
                       methods)))))

user> (map #(has-arity? (fn
([])
([a b])
([a b c d & e]))
%)
[0 1 2 3 4 5 6 7 8 9 10])

(true false true false true true true true true true true)