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+* Lecture 2B: Computational Objects
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+https://youtu.be/yedzRWhi-9E
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+
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+we can inherit the modularity of the world into our code
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+
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+circuits are the best models in physics, according to gjs
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+
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+: (define a (make-wire))
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+: (or-gate a b d)
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+: etc.
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+
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+#+BEGIN_SRC scheme
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+ (define (half-adder a b s c)
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+ (let ((d (make-wire)) (e (make-wire)))
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+ (or-gate a b d)
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+ (and-gate a b c)
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+ (inverter c e)
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+ (and-gate d e s)))
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+#+END_SRC
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+
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+"if a compound object doesn't look like a primitive, there's something
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+wrong with the language"
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+
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+we only have to implement the primitives. means of combination are
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+gifted by lisp
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+
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+this simulator is a real world model that was used to manufacture a
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+large computer
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+
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+AGENDAS
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+
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+#+BEGIN_SRC scheme
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+ (make-agenda)
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+ (current-time agenda)
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+ (empty-agenda? agenda)
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+ (add-to-agenda! time action agenda)
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+ (first-item agenda)
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+ (remove-first-item! agenda)
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+#+END_SRC
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+
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+"if im going to change things, i need a place to make the change"
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+
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+QUEUE
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+
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+can delete from the beg and add to the end
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+
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+#+BEGIN_SRC scheme
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+ (make-queue)
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+ (insert-queue! queue item)
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+ (delete-queue! queue)
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+ (front-queue queue)
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+ (empty-queue? queue)
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+#+END_SRC
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+
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+--> break
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+
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+what makes things the same. what makes things different?
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+{recall ship of theseus}
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+
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+#+BEGIN_SRC scheme
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+ (define a (cons 1 2))
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+ (define b (cons a a))
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+ (set-car! (car b) 3)
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+#+END_SRC
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+
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+"inadverted sharing is the source of most of the bugs in complicated
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+programs. introducing the possibility of things having identity and
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+sharing and having multiple names for the same thing, we get a lot of
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+power but we have to pay for it with lots of complexity and bugs."
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+
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+"once you let the camel's nose in the tent, the rest of him follows."
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+
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+#+BEGIN_SRC scheme
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+ (define (cons x y) ;; alonzo
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+ (λ (m) (m x y))) ;; church's
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+ (define (car x) ;; hack
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+ (x (λ (a d) a)))
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+ (define (cdr x)
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+ (x (λ (a d) d)))
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+#+END_SRC
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+
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+-- invented by alonzo church "the greatest programmer of the 20th
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+ century even though he never saw a computer"
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