macrostuff.clj

(set! *warn-on-reflection* true)

(defn apply-fn1 [x f arg]
  ;; apply f
  x)

(defmacro apply-fn2 [x f arg type]
  (vary-meta (do
               ;; apply f
               x)
             assoc :tag type))

(defn foo1 [x]
  (-> x
      (apply-fn1 nil nil)
      (.length))) ;; Reflection warning!

(defn foo2 [x]
  (-> x
      (apply-fn2 nil nil String)
      (.length))) ;; No reflection warning