<一>
(define (new-if predicate then-clause else-clause) (cond (predicate then-clause) (else else-clause)))
(new-if (= 2 3) 0 5)(new-if (= 1 1) 0 5)
new-if在两种环境下均运行正常.
(if (= 2 3) 0 5)(if (= 1 1) 0 5)
if 在R5RS下输出正常,在Lazy Racket下为(delay ...)
<二>
(define (new-if predicate then-clause else-clause) (cond (predicate then-clause) (else else-clause)))
(define (sqrt-iter guess x) (new-if (good-enough? guess x) guess (sqrt-iter (improve guess x) x)))
(define (improve guess x) (average guess (/ x guess)))
(define (average x y) (/ (+ x y) 2))
(define (good-enough? guess x) (< (abs (- (square guess) x)) 0.001))
(define (sqrta x) (sqrt-iter 1.0 x))
(define (square x) (* x x))
(sqrta 9)
在R5RS下内存被耗尽,而在Lazy Racket下正常输出.
原因是new-if在R5RS,也就是应用序下,传入的每一个分支都会被求值.
<三>
(define (new-if predicate then-clause else-clause) (cond (predicate then-clause) (else else-clause)))
(new-if #t (display "good") (display "bad"))
R5RS输出:goodbad
Lazy Racket输出:good
转载于:https://www.cnblogs.com/R4mble/p/7878755.html
