►zthomae’s SICP solutions
▼3 Chapter 3
3.1 Exercise 3.1
3.2 Exercise 3.2
3.3 Exercise 3.3
3.4 Exercise 3.4
3.5 Exercise 3.5
3.6 Exercise 3.6
3.7 Exercise 3.7
3.8 Exercise 3.8
3.9 Exercise 3.9
3.10 Exercise 3.10
3.11 Exercise 3.11
3.12 Exercise 3.12
3.13 Exercise 3.13
3.14 Exercise 3.14
3.15 Exercise 3.15
3.16 Exercise 3.16
3.17 Exercise 3.17
3.18 Exercise 3.18
3.19 Exercise 3.19
3.20 Exercise 3.20
3.21 Exercise 3.21
3.22 Exercise 3.22
3.23 Exercise 3.23
3.24 Exercise 3.24
3.25 Exercise 3.25
3.26 Exercise 3.26
3.27 Exercise 3.27
3.28 Exercise 3.28
3.29 Exercise 3.29
3.30 Exercise 3.30
3.31 Exercise 3.31
3.32 Exercise 3.32
3.33 Exercise 3.33
3.34 Exercise 3.34
3.35 Exercise 3.35
3.36 Exercise 3.36
3.37 Exercise 3.37
3.38 Exercise 3.38
3.39 Exercise 3.39
3.40 Exercise 3.40
3.41 Exercise 3.41
3.42 Exercise 3.42
3.43 Exercise 3.43
3.44 Exercise 3.44
3.45 Exercise 3.45
3.46 Exercise 3.46
3.47 Exercise 3.47
3.48 Exercise 3.48
3.49 Exercise 3.49
3.50 Exercise 3.50
3.51 Exercise 3.51
3.52 Exercise 3.52
3.53 Exercise 3.53
3.54 Exercise 3.54
3.55 Exercise 3.55
3.56 Exercise 3.56
3.57 Exercise 3.57
3.58 Exercise 3.58
3.59 Exercise 3.59
3.60 Exercise 3.60
3.61 Exercise 3.61
3.62 Exercise 3.62
3.63 Exercise 3.63
3.64 Exercise 3.64
3.65 Exercise 3.65
3.66 Exercise 3.66
3.67 Exercise 3.67
3.68 Exercise 3.68
3.69 Exercise 3.69
3.70 Exercise 3.70
3.71 Exercise 3.71
3.72 Exercise 3.72
3.73 Exercise 3.73
3.74 Exercise 3.74
3.75 Exercise 3.75
3.76 Exercise 3.76
3.77 Exercise 3.77
3.78 Exercise 3.78
3.79 Exercise 3.79
3.80 Exercise 3.80
3.81 Exercise 3.81
3.82 Exercise 3.82
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3.28 Exercise 3.28

Implementing or-gate by copying and-gate is fairly trivial:

(define or-gate-delay 5)
(define (or-gate o1 o2 output)
  (define (or-action-procedure)
    (let ((new-value (logical-or (get-signal o1) (get-signal o2))))
      (after-delay or-gate-delay
                   (lambda () (set-signal! output new-value)))))
  (add-action! o1 or-action-procedure)
  (add-action! o2 or-action-procedure)
  'ok)
(define (logical-or a b)
  (cond ((and (= a 0) (= b 0)) 0)
        ((and (= a 1) (= b 0)) 1)
        ((and (= a 0) (= b 1)) 1)
        ((and (= a 1) (= b 1)) 1)
        (else (error "Invalid signals" a b))))

The fact that this is so similar to and-gate is a clue that a more general function is hiding – in this case, that of the binary operation:

(define (binary-op op delay)
  (lambda (x y output)
    (define (binary-action-procedure)
      (let ((new-value (op (get-signal x) (get-signal y))))
        (after-delay delay
                     (lambda () (set-signal! output new-value)))))
    (add-action! x binary-action-procedure)
    (add-action! y binary-action-procedure)
    'ok))
(define and-gate (binary-op logical-and and-gate-delay))
(define or-gate (binary-op logical-or or-gate-delay))
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