問題1.32

この辺から、抽象化しすぎで段々雲行きが怪しくなる。
実際にこのようなコードを書いたとしたら、ちゃんとメンテできるだろうか？

accumulate再帰的手続き

(define (accumulate combiner null-value term a next b)
  (if (> a b)
    null-value
    (combiner (term a)
    	      (accumulate combiner null-value term (next a) next b))))
; sum
(define (sum term a next b)
  (accumulate + 0 term a next b))
(define (sum-cubes a b)
  (sum (lambda (x) (* x x x)) a (lambda (n) (+ n 1)) b))

; product
(define (product term a next b)
  (accumulate * 1 term a next b))
(define (factorial n)
  (product (lambda (x) x) 1 (lambda (n) (+ n 1)) n))

accumulate反復的手続き

(define (accumulate-iter combiner null-value term a next b)
  (define (iter a result)
	(if (> a b)
	  result
	  (iter (next a) (combiner result (term a)))))
  (iter a null-value))

; 検算用(Gaucheのテストモジュールを使えばいいのか)
(define (product-iter term a next b)
  (accumulate-iter * 1 term a next b))
(define (factorial n)
  (product-iter (lambda (x) x) 1 (lambda (n) (+ n 1)) n))