SICP Exercise 3.28

sicp

Question

Define an or-gate as a primitive function box. Your or-gate constructor should be similar to and-gate.

Answer

First of all, we need a function to calculate logical or:

(define (logical-or s1 s2)
  (if (or (= s1 1) (= s2 1))
      1
      0))

We can now design our or-gate analogue to and-gate.

(define (or-gate a1 a2 output)
  (define (and-action-procedure)
    (let ((new-value
           (logical-or (get-signal a1) (get-signal a2))))
      (after-delay or-gate-delay
                   (λ ()
                     (set-signal! output new-value)))))
  (add-action! a1 or-action-procedure)
  (add-action! a2 or-action-procedure)
  'ok)