SICP q2.38

gistfile1.scm

(define nil '())

(define (accumulate op initial sequence)
  (if (null? sequence)
    initial
    (op (car sequence)
        (accumulate op initial (cdr sequence)))))
(define fold-right accumulate)

(define (fold-left op initial sequence)
  (define (iter result rest)
    (if (null? rest)
      result
      (iter (op result (car rest))
            (cdr rest))))
  (iter initial sequence))


(print (fold-right / 1 (list 1 2 3))) ; 3/2 = 1/(2/(3/1))
(print (fold-left / 1 (list 1 2 3))) ; 1/6 = ((1/1)/2)/3
(print (fold-right list nil (list 1 2 3))) ; (1 (2 (3 ())))
(print (fold-left list nil (list 1 2 3))) ; (((() 1) 2) 3)

; 交換法則が成り立てば同じ値になる

initialが単位元なら結合だけでいいよね