1.30 (递归的sum变迭代)

(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!"