练习2.47
这道题重点就是要理清楚list和cons的关系了。我们依次来写出过程函数。
(define (make-frame origin edge1 edge2)
(list origin edge1 edge2))
(define (origin-frame f) (car f))
(define (edge1-frame f) (cadr f))
(define (edge2-frame f) (caddr f))
(define (make-frame origin edge1 edge2)
(cons origin (cons edge1 edge2)))
(define (origin-frame f) (car f))
(define (edge1-frame f) (cadr f))
(define (edge2-frame f) (cddr f))
