Thanks, Thats exactly what I needed. Stephen PS Here is another version #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) (let ((x (send event get-x)) (y (send event get-y))) (cond [(and (send event get-left-down) (send event button-changed?)) (make-world (cons (cons (cons x y) '()) (world-lines world)))] [(and (send event get-left-down) (send event moving?) (not (send event button-changed?))) (make-world (cons (cons (cons x y) (car (world-lines world))) (cdr (world-lines world))))] [else world]))) (define (on-paint world dc) (for-each (λ (lines) (send dc draw-lines lines)) (world-lines world))) ;;; ;;; 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) (send (get-dc) set-pen "red" 10 'solid ) (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) <div class="gmail_quote">On Mon, Apr 9, 2012 at 12:12 PM, Jens Axel Søgaard <<a href="mailto:jensaxel@soegaard.net">jensaxel@soegaard.net</a>> wrote: <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Hi Stephen, Here is how I would do it. /Jens Axel #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) <div class="im"> (define diagramframe (new frame% [label "paint"] [width 300] [height 300] [x 1000][y 300])) </div>(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) 2012/4/9 Stephen De Gabrielle <<a href="mailto:stephen.degabrielle@acm.org">stephen.degabrielle@acm.org</a>>: <div><div class="h5">> Hi, > > 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. > > Any suggestons would be appreciated. > > Kind regards, > > Stephen > > > ;;;;---- > #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) > > ;;----- > </div></div>> ____________________ > Racket Users list: > <a href="http://lists.racket-lang.org/users" target="_blank">http://lists.racket-lang.org/users</a> > -- Jens Axel Søgaard </blockquote></div> -- <div> </div><div>--</div><div>Stephen De Gabrielle</div><div><a href="mailto:stephen.degabrielle@acm.org" target="_blank">stephen.degabrielle@acm.org</a></div><div>Telephone +44 (0)20 85670911</div> <div>Mobile +44 (0)79 85189045</div><div><a href="http://www.degabrielle.name/stephen" target="_blank">http://www.degabrielle.name/stephen</a></div><div>----</div><div>Professor: Oh God! I clicked without reading! </div> <div>Cubert: And I slightly modified something I own! </div><div>Professor: We're monsters!</div>