4.11 add-binding-to-frame!

[wdanxna]

您好
练习4.11 的add-binding-to-frame! 我觉得你实现错了
(define (add-binding-to-frame! var val frame) (cons (cons var val) frame))

这里不应该是简单的返回一个新的frame，而是要修改原来的frame

(define (add-binding-to-frame! var val frame)
  (let ((old-car (car frame)))
    (set-car! frame (cons var val))
    (set-cdr! frame (cons old-car (cdr frame)))))

;;test
(let ((frame (make-frame '(a b c) '(1 2 3))))
  (add-binding-to-frame! 'd 4 frame)
  (display frame))