(define (sum term a next b)
(define (iter a result)
(if (> a b)
result
(iter (next a)
(+ (term a) result))))
(iter a 0))
;test
(sum (lambda (x) x)
1
(lambda (i) (+ i 1))
10)
为什么要把iter塞到sum里面去:
(define (sum term a next b)
(iter term a next b 0))
(define (iter term a next b result)
(if (> a b)
result
(iter term
(next a)
next
b
(+ (term a) result))))
嗯,为了减少通信开销.
回头看递归写法:
(define (sum term a next b)
(if (> a b)
0
(+ (term a)
(sum term (next a) next b))))
"我们要以困难的方式搞定它!"
原文：http://www.cnblogs.com/R4mble/p/7892764.html