SICP 問題 3.77

ループのあるシステムで使えるようにintegrandをdelayed-integrandにする．
integralを呼び出す時の第一引数にはdelayをつける

(define (integral integrand initial-value dt)
  (cons-stream initial-value
               (if (stream-null? integrand)
                   the-empty-stream
                   (integral (stream-cdr integrand)
                             (+ (* dt (stream-car integrand))
                                initial-value)
                             dt))))

(define (integral delayed-integrand initial-value dt)
  (cons-stream initial-value
               (let ((integrand (force delayed-integrand)))
                 (if (stream-null? integrand)
                     the-empty-stream
                     (integral (delay (stream-cdr integrand))
                               (+ (* dt (stream-car integrand))
                                  initial-value)
                               dt)))))