4.11 Exercise 4.11

One of the reasons that the approach using separate lists for variables and values in the environment worked so nicely is that a natural representation containing pointers to these lists fell out of it. If we imagine the bindings instead as a single list of binding pairs (where the value can itself also be a list), our data representation seems more elegant, but we lose that level of indirection that made the mutations easy to write. To deal with this, we can instead define a frame as having a head element in front, such that the cdr of this frame is a reliable pointer to the list of bindings.

(define the-empty-environment-2 '(head))
(define (lookup-variable-value-2 var env)
  (define (env-loop env)
    (define (scan bindings)
      (cond ((null? bindings) (env-loop (enclosing-environment env)))
            ((eq? var (caar bindings)) (cdar bindings))
            (else (scan (cdr bindings)))))
    (if (eq? env the-empty-environment-2)
        (error "Unbound variable" var)
        (let ((frame (first-frame env)))
          (scan (cdr frame)))))
  (env-loop env))
(define (extend-environment-2 bindings base-env)
  (cons (cons 'head bindings) base-env))
(define (add-binding-to-frame-2! var val frame)
  (if (null? (cdr frame))
      (set-cdr! frame (cons var val))
      (begin
        (set-cdr! (cdr frame) (cons (cadr frame) (cddr frame)))
        (set-car! (cdr frame) (cons var val)))))
(define (define-variable-2! var val env)
  (let ((frame (first-frame env)))
    (define (scan bindings)
      (cond ((null? bindings)
             (add-binding-to-frame-2! var val frame))
            ((eq? var (caar bindings))
             (set-car! (car bindings) val))
            (else (scan (cdr bindings)))))
    (scan (cdr frame))))
(define (set-variable-2! var val env)
  (define (env-loop env)
    (define (scan bindings)
      (cond ((null? bindings)
             (env-loop (enclosing-environment env)))
            ((eq? var (caar bindings))
             (set-cdr! (car bindings) val))
            (else (scan (cdr bindings)))))
    (if (eq? env the-empty-environment-2)
        (error "Unbound variable -- SET!" var)
        (let ((frame (first-frame env)))
          (scan (cdr frame)))))
  (env-loop env))

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1	Chapter 1
2	Chapter 2
3	Chapter 3
4	Chapter 4
5	Chapter 5

4.1	Exercise 4.1
4.2	Exercise 4.2
4.3	Exercise 4.3
4.4	Exercise 4.4
4.5	Exercise 4.5
4.6	Exercise 4.6
4.7	Exercise 4.7
4.8	Exercise 4.8
4.9	Exercise 4.9
4.10	Exercise 4.10
4.11	Exercise 4.11
4.12	Exercise 4.12
4.13	Exercise 4.13
4.14	Exercise 4.14
4.15	Exercise 4.15
4.16	Exercise 4.16
4.17	Exercise 4.17
4.18	Exercise 4.18
4.19	Exercise 4.19
4.20	Exercise 4.20
4.21	Exercise 4.21
4.22	Exercise 4.22
4.23	Exercise 4.23
4.24	Exercise 4.24
4.25	Exercise 4.25
4.26	Exercise 4.26
4.27	Exercise 4.27
4.28	Exercise 4.28
4.29	Exercise 4.29
4.30	Exercise 4.30
4.31	Exercise 4.31
4.32	Exercise 4.32
4.33	Exercise 4.33
4.34	Exercise 4.34
4.35	Exercise 4.35
4.36	Exercise 4.36
4.37	Exercise 4.37
4.38	Exercise 4.38
4.39	Exercise 4.39
4.40	Exercise 4.40
4.41	Exercise 4.41
4.42	Exercise 4.42
4.43	Exercise 4.43
4.44	Exercise 4.44
4.45	Exercise 4.45
4.46	Exercise 4.46
4.47	Exercise 4.47
4.48	Exercise 4.48
4.49	Exercise 4.49
4.50	Exercise 4.50
4.51	Exercise 4.51
4.52	Exercise 4.52
4.53	Exercise 4.53
4.54	Exercise 4.54
4.55	Exercise 4.55
4.56	Exercise 4.56
4.57	Exercise 4.57
4.58	Exercise 4.58
4.59	Exercise 4.59
4.60	Exercise 4.60
4.61	Exercise 4.61
4.62	Exercise 4.62
4.63	Exercise 4.63
4.64	Exercise 4.64
4.65	Exercise 4.65
4.66	Exercise 4.66
4.67	Exercise 4.67
4.68	Exercise 4.68
4.69	Exercise 4.69
4.70	Exercise 4.70
4.71	Exercise 4.71