這兩天公司的項目告一段落,學校的作業不算太難。因此我頗有點閑暇,於是重看SICP,同時學學JavaScript(看了google suggest, google maps, 還有CanyonBridge的cbconnect, 對JavaScript的興趣一下濃厚起來)。 Douglas Crockford 說得不錯,JavaScript的確和Scheme有很多相似之處。JavaScript的作者一定諳熟函數編程,不然JavaScript的函數是lambda運算元也太過巧合了。比如SICP1.1裡計算平方根的Newton-Raphson演算法:
(define (sqrt x)
(define (good-enough? guess)
(< (abs (- (square guess) x)) 0.001))
(define (improve guess)
(average guess (/ x guess)))
(define (sqrt-iter guess)
(if (good-enough? guess)
guess
(sqrt-iter (improve guess))))
(sqrt-iter 1.0))
換成JavaScript版本近乎機械:
//Newton's method to find /sqrt{x}
function sqrt(x){
function sqrt_iter(guess){
if(good_enough(guess)){
return guess;
}
return sqrt_iter(improve(guess));
}
function improve(guess){
return average(guess, x/guess);
}
function average(x, y){
return (x + y)/2;
}
function good_enough(guess){
return Math.abs(guess * guess - x) < 0.001;
}
return sqrt_iter(1.0);
}
呵呵,看來把SICP裡的程式用JavaScript重寫也是件有趣的事。
呵呵,看來把SICP裡的程式用JavaScript重寫也是件有趣的事。