1 665c255d 2023-08-04 jrmu (define (cons x y)
 2 665c255d 2023-08-04 jrmu   (define (set-x! v) (set! x v))
 3 665c255d 2023-08-04 jrmu   (define (set-y! v) (set! y v))
 4 665c255d 2023-08-04 jrmu   (define (dispatch m)
 5 665c255d 2023-08-04 jrmu     (cond ((eq? m 'car) x)
 6 665c255d 2023-08-04 jrmu 	  ((eq? m 'cdr) y)
 7 665c255d 2023-08-04 jrmu 	  ((eq? m 'set-car!) set-x!)
 8 665c255d 2023-08-04 jrmu 	  ((eq? m 'set-cdr!) set-y!)
 9 665c255d 2023-08-04 jrmu 	  (else (error "Undefined operation -- CONS" m))))
10 665c255d 2023-08-04 jrmu   dispatch)
11 665c255d 2023-08-04 jrmu (define (car z) (z 'car))
12 665c255d 2023-08-04 jrmu (define (cdr z) (z 'cdr))
13 665c255d 2023-08-04 jrmu (define (set-car! z new-value)
14 665c255d 2023-08-04 jrmu   ((z 'set-car!) new-value)
15 665c255d 2023-08-04 jrmu   z)
16 665c255d 2023-08-04 jrmu (define (set-cdr! z new-value)
17 665c255d 2023-08-04 jrmu   ((z 'set-cdr!) new-value)
18 665c255d 2023-08-04 jrmu   z)
19 665c255d 2023-08-04 jrmu 
20 665c255d 2023-08-04 jrmu ;; Exercise 3.20.  Draw environment diagrams to illustrate the evaluation of the sequence of expressions
21 665c255d 2023-08-04 jrmu 
22 665c255d 2023-08-04 jrmu (define x (cons 1 2))
23 665c255d 2023-08-04 jrmu (define z (cons x x))
24 665c255d 2023-08-04 jrmu (set-car! (cdr z) 17)
25 665c255d 2023-08-04 jrmu (car x)
26 665c255d 2023-08-04 jrmu 17
27 665c255d 2023-08-04 jrmu