togglecounter.clj

;; Toggle a counter that tracks how manny times a function is called 

(defn instrument-fn [f]
  (let [counter (atom 0)]
    (-> (fn [& args] ;; NOTE could unroll for 1-n args for speed
          (swap! counter inc)
          (apply f args))
        (with-meta
          {:uninstrumented f
           :counter counter}))))

(defn toggle-fn-instrumentation
  [f]
  (assert (fn? f) "Only fns can be instrumented")
  (let [{:keys [counter uninstrumented]} (meta f)]
    (if uninstrumented
      uninstrumented
      (instrument-fn f))))

(defn toggle-counter [fn-var]
  (alter-var-root fn-var toggle-fn-instrumentation))

(defn foo [] (println "FOO"))

(foo)

(some-> foo meta :counter deref)

(toggle-counter #'foo)

(some-> foo meta :counter deref)

(dotimes [_ 5]
  (foo))

(some-> foo meta :counter deref)