SICPで同じような関数を定義、その度に名前を付け変えるのは結構面倒なんだけど、clojureだとdynamic varsを使うとその場で関数の定義を変えれて便利だなーと思った例

example_of_dynamic_vars_in_sqrt.clj

(defn average [x y]
  (/ (+ x y) 2))

(defn improve [guess x]
  (average guess (/ x guess)))

(defn square [x] (* x x))

(defn good-enough? [guess x]
  (< (Math/abs (- (square guess) x)) 0.001))

(defn ^:dynamic sqrt-iter [guess x]
  (if (good-enough? guess x)
    guess
    (sqrt-iter (improve guess x) x)))

(defn sqrt [x]
  (sqrt-iter 1.0 x))

(println (sqrt 9)) ; 3.00009155413138

(defn new-if [predicate then-clause else-clause]
  (cond predicate then-clause
	:else else-clause))

(new-if (= 2 3) 0 5) ; 5
(new-if (= 1 1) 0 5) ; 0

(binding [sqrt-iter (fn [guess x]
		      (new-if (good-enough? guess x)
			      guess
			      (sqrt-iter (improve guess x) x)))]
  (try
    (sqrt 9)
    (catch StackOverflowError e (.toString e))))

example_of_dynamic_vars_in_sqrt_using_with-redefs.clj

(defn average [x y]
  (/ (+ x y) 2))

(defn improve [guess x]
  (average guess (/ x guess)))

(defn square [x] (* x x))

(defn good-enough? [guess x]
  (< (Math/abs (- (square guess) x)) 0.001))

(defn sqrt-iter [guess x]
  (if (good-enough? guess x)
    guess
    (sqrt-iter (improve guess x) x)))

(defn sqrt [x]
  (sqrt-iter 1.0 x))

(println (sqrt 9)) ; 3.00009155413138

(defn new-if [predicate then-clause else-clause]
  (cond predicate then-clause
	:else else-clause))

(new-if (= 2 3) 0 5) ; 5
(new-if (= 1 1) 0 5) ; 0

(with-redefs [sqrt-iter (fn [guess x]
			  (new-if (good-enough? guess x)
				  guess
				  (sqrt-iter (improve guess x) x)))]
  (try
    (sqrt 9)
    (catch StackOverflowError e (.toString e))))

ifを置き変えれるとよかったんだけど、ifは特殊形式だからか怒られてしまいました。。。

このアドバイスを使って書き変えてみます。

• http://twitter.com/#!/fatrow/status/168291001289543680