clojure-recur-loop.clj

; NON-tail recursive version of size
; because the recursive call must be returned to the caller
(defn non-tail-size [v]
  (if (empty? v)
    0
    (inc (non-tail-size (rest v)))))
user=> (non-tail-size [1 2 3])
3
user=> (non-tail-size (range 10000))
StackOverflowError   clojure.core/range/fn--4269 (core.clj:2664)

; now for the tail recursive version (excuse the crappiness)
(defn tail-size [v]
  (let [helper (fn itsame [xs acc]
               (if (empty? xs)
                 acc
                 (itsame (rest xs) (inc acc))))]
    (helper v 0)))
user=> (tail-size (range 10000)) ; STILL stack overflows because there's no automatic TCO
StackOverflowError   clojure.core/range/fn--4269 (core.clj:2664)

; now let's actually optimize using recur and loop
(defn size [v]
  (loop [xs v, acc 0]
    (if (empty? xs)
      acc
      (recur (rest xs) (inc acc)))))
user=> (size (range 100000)) ; WOOT! success :)
100000