Moving beyond world/universe - inversion-of-control

Question

I thought I'd try a simple GUI app using the world/universe mutation-free approach,  but trying to implement the 'world/universe' program design myself.
I've got my little sketch below, but I quickly came to conclusion that while I could use the teachpack, I don't know how to achieve the teachpack functionality myself.
I'm guessing I should use continuations, but that doesn't seem to be the approach in the universe.rkt source.
I could always just stuff the program into the canvas class, (as earlier games like slidey and same seem to do), but I really want to get a handle on  how to implement the 'world/universe' style of program control.
;;;;----
#lang racket/gui
; simple drawing program
; mousedown starts recording a list of points
; mousechanged starts recording a new list
; paint callback paints the list of lists as lines.
(define diagramframe (new frame% [label "paint"] [width 300]
[height 300] [x 1000][y 300]))
;(define lines '(((0 . 0) (0 . 300) (250 . 250) (150 . 176))))
(define lines '(((0 . 0) (0 . 300) (250 . 250) (150 . 176))
                ((10 . 4) (280 . 10))))
(define paintcanvas%
  (class canvas%
    (init-field mouse-event-callback)
    (super-new)
    (define dc (send this get-dc))
    (define/override (on-event mouse-event)
      (mouse-event-callback mouse-event))))
(define (paint-cb c dc) 
  (for-each (λ (line) (send dc draw-lines line)) lines))
(define (me-cb mouse-event)
(let ((x (send mouse-event get-x))
        (y (send mouse-event get-y)))
    (when (and (send mouse-event get-left-down)
               (send mouse-event moving?))
      (if (send mouse-event button-changed?)
          ; if true append as new list
          '() 
          ; if false append existing list
          '()))))
(define Paintcanvas (new paintcanvas%
                         [parent diagramframe]
[paint-callback paint-cb]
[mouse-event-callback me-cb]))
(define (main world)
 (when world (main (??? world)))
  (send diagramframe show #t))
 
(main lines)
(send diagramframe show #t)
;;-----

Answer 1

Here is how I would do it.
Note that the GUI event loop acts as the program main.
Use timer events to implement on-tick.
#lang racket/gui
;;;
;;; WORLD
;;;
(define-struct world (lines))
(define the-world (make-world '((0 . 0) (0 . 300) (250 . 250) (150 . 176) (10 . 4) (280 . 10))))
;;;
;;; USER LAND
;;;
(define (on-mouse-event world event)
(if (and (send event get-left-down)
(send event moving?)
#; (send event button-changed?))
(let ((x (send event get-x))
(y (send event get-y)))
(make-world (cons (cons x y) (world-lines world))))
world))
(define (on-paint world dc)
(send dc draw-lines
(map pair->point (world-lines world))))
(define (pair->point p)
(make-object point% (car p) (cdr p)))
;;;
;;; SYSTEM
;;;
(define user:on-paint on-paint)
(define diagramframe (new frame% [label "paint"] [width 300] [height 300] [x 1000][y 300]))
(define paintcanvas%
(class canvas%
(inherit get-dc refresh)
(super-new)
(define/override (on-paint)
(send (get-dc) suspend-flush)
(user:on-paint the-world (get-dc))
(send (get-dc) resume-flush))
(define/override (on-event mouse-event)
(let* ([old-world the-world]
[new-world (on-mouse-event the-world mouse-event)])
(if (eq? old-world new-world)
(super on-event mouse-event)
(begin
(set! the-world new-world)
(refresh)))))))
(define paintcanvas (new paintcanvas% [parent diagramframe]))
(send diagramframe show #t)