closure_in_lisp.clj

;;;; cf. http://riktor.hatenablog.com/entry/2012/07/22/223937

(def counter
  (let [cnt (atom 0)]
    (fn [] (swap! cnt inc))))

(counter)
(counter)
(counter)

(defn make-counter []
  (let [cnt (atom 0)]
    (fn [] (swap! cnt inc))))

(def counter1 (make-counter))
(def counter2 (make-counter))
(counter1)
(counter1)
(counter1)
(counter2)
(counter2)
(counter1)

(def counter
  (let [cnt (atom 0)]
    (fn [msg & [arg]]
      (let [delta (or arg 1)]
        (case msg
          :inc (swap! cnt + delta)
          :dec (swap! cnt - delta))))))

(counter :inc)
(counter :inc 2)
(counter :dec)
(counter :dec 2)

(sort (let [cnt (atom 0)]
        (fn [a b]
          (println (swap! cnt inc))
          (< a b)))
      '(1 33 4 -2 3 1 432))


(defmacro defn-memoized [fn-name args & body]
  `(let [memo# (atom {})]
     (defn ~fn-name [~@args]
       (if-let [n# (get @memo# [~@args])]
         n#
         (let [m# (do ~@body)]
           (swap! memo# assoc [~@args] m#)
           m#)))))

(defn-memoized fib-memoized [n]
  (if (or (= n 0) (= n 1))
    1
    (+ (fib-memoized (- n 1))
       (fib-memoized (- n 2)))))