共享和相等

参考这里

;3.16(define (count-pairs x)  (if (not (pair? x))      0      (+ (count-pairs (car x))         (count-pairs (cdr x))         1)));;4个(define x '(a b))(define y (cons (cdr x) x))(count-pairs y);;7个(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个检测过程都是破坏性的;这是错误的，因为用的是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)))));;下面的两个版本都创建了一个真正唯一的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))