Guowei Lv

3 minute read

Do you think Computer Science equals building websites and mobile apps?

Are you feeling that you are doing repetitive and not so intelligent work?

Are you feeling a bit sick about reading manuals and copy-pasting code and keep poking around until it works all day long?

Do you want to understand the soul of Computer Science?

If yes, read SICP!!!


Agenda is a data structure, that contains a schedule of things to do.

This is best explained using an example.

The Agenda is essentially just a list.

The first element is the current time in the simulation. In this example, 5.

Then the cdr of the list represents things to do at different time in the future. For example in this example, at time 6, 10 and 13 there are queues of things to be done.


We can now define operations for the agenda.

Make a new empty agenda

(define (make-agenda) (list 0))

Test empty agenda

(define (empty-agenda? agenda)
  (null? (segments agenda)))
(define (segments agenda) (cdr agenda))

Get the first item on the agenda

(define (first-agenda-item agenda)
  (if (empty-agenda? agenda)
      (error "Agenda is empty -- FIRST-AGENDA_ITEM")
      (let ((first-seg (first-segment agenda)))
        (set-current-time! agenda (segment-time first-seg))
        (front-queue (segment-queue first-seg)))))
(define (first-segment agenda)
  (car (segments agenda)))
(define (set-current-time! agenda time)
  (set-car! agenda time))
(define (segment-queue s) (cdr s))

Remove the first item on the agenda

(define (remove-first-agenda-item! agenda)
  (let ((q (segment-queue (first-segment agenda))))
    (delete-queue! q)
    (if (empty-queue? q)
        (set-segments! agenda (rest-segments agenda)))))
(define (set-segments! agenda segments)
  (set-cdr! agenda segments))
(define (rest-segments agenda)
  (cdr (segments agenda)))

Adding the given action procedure to be run at the specified time

(define (add-to-agenda! time action agenda)
  (define (belongs-before? segments)
    (or (null? segments)
        (< time
           (segment-time (car segments)))))
  (define (make-new-time-segment time action)
    (let ((q (make-queue)))
      (insert-queue! q action)
      (make-time-segment time q)))
  (define (add-to-segments! segments)
    (if (= (segment-time (car segments)) time)
         (segment-queue (car segments))
        (let ((rest (cdr segments)))
          (if (belongs-before? rest)
               (cons (make-new-time-segment
                     (cdr segments)))
              (add-to-segments! rest)))))
  (let ((segments (segments agenda)))
    (if (belongs-before? segments)
         (cons (make-new-time-segment
        (add-to-segments! segments))))

Get the current simulation time

(define (current-time agenda) (car agenda))

It may look like a lot of stuff here, but if you have the structure of the agenda in mind and follow the code slowly, it is actually quite straightforward.


The simulation is driven by the procedure propagate, which executing each procedure on the agenda in sequence.

(define (propagate)
  (if (empty-agenda? the-agenda)
      (let ((first-item
             (first-agenda-item the-agenda)))
        (remove-first-agenda-item! the-agenda)

Test everything works

;; Define a global the-agenda
(define the-agenda (make-agenda))

;; Test that the-agenda is empty
(empty-agenda? the-agenda)
;Value: #t

;; The current time should be 0
(current-time the-agenda)
;Value: 0

;; Add a procedure to execute at time 1
(add-to-agenda! 1 (lambda() (display "Wake up!")) the-agenda)

;; Add another procedure to execute at time 1
(add-to-agenda! 1 (lambda() (display "Wake up again!")) the-agenda)

;; Add a procedure to execute at time 2
(add-to-agenda! 2 (lambda() (display "Go to school!")) the-agenda)

;; At this point, there should be two queues in the-agenda, one for time 1 and one for time 2, let's examine the queue for time 1
(print-queue (cdr (first-segment the-agenda)))
;(#[compound-procedure 14] #[compound-procedure 17])

;; Time to propagate!
;Wake up! Wake up again! Go to school!

;; After propagate, let's check the current time

(current-time the-agenda)
;Value: 2
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