共享和相等

參考這裏

;3.16
(define (count-pairs x)
  (if (not (pair? x))
      0
      (+ (count-pairs (car x))
         (count-pairs (cdr x))
         1)))
;;4個
(define x '(a b))
(define y (cons (cdr x) x))
(count-pairs y)
;;7個
(define x '(a))
(define y (cons x x))
(define z (cons y y))
;3.17
(define (safe-count-pairs x)
  (define (rec-trace x trace)
    (if (or (not (pair? x)) (memq x trace))
        trace
        (rec-trace (cdr x) (rec-trace (car x) (cons x trace)))))
  (define (length seq len)
    (if (null? seq)
        len
        (length (cdr seq) (+ len 1))))
  (length (rec-trace x '()) 0))
;3.18
(define (last-pair x)
  (if (null? (cdr x))
      x
      (last-pair (cdr x))))
;下面3個檢測過程都是破壞性的
;這是錯誤的，因爲用的是equal?,不唯一
(define (loop? x)
  (define identity (cons '() '()))
  (cond ((null? x) #f)
        ((equal? identity (car x)) #t)
        (else (set-car! x identity)
              (loop? (cdr x)))))
;;下面的兩個版本都創建了一個真正唯一的identity,使用的是共享
(define (loop?-1 x)
  (define identity (cons '() '()))
  (define (loop?-iter x)
    (cond ((null? x) #f)
          ((eq? identity (car x)) #t)
          (else (set-car! x identity)
                (loop?-iter (cdr x)))))
  (loop?-iter x))
(define (loop?-2 x)
  (let ((identity '(())))
    (define (loop?-iter x)
      (cond ((null? x) #f)
            ((eq? identity (car x)) #t)
            (else (set-car! x identity)
                  (loop?-iter (cdr x)))))
    (loop?-iter x)))
;3.19
(define (loop?-3 x)
  (define (loop?-iter x y)
    (cond
     ((eq? x y) #t)
     ((or (null? (cdr x))
          (null? (cdr y))
          (null? (cddr y))) #f)
     ((loop?-iter (cdr x) (cddr y)))))
  (cond ((or (null? x)
             (null? (cdr x)))
         #f)
        (else (loop?-iter x (cdr x)))))
(define x '(1 (2 3) 1))
(define loop (begin
               (set-cdr! (last-pair x) x)
               x))