Obviously, my brain wasn't working when I typed the last part of that post.  It should be: The last two do exactly what I want.  The first two give "get-field: expected an object, got #f" when the Test button is clicked.  What is the difference? Sorry for the typos. <div class="gmail_quote">On Tue, Oct 28, 2008 at 7:01 PM, Doug Williams <<a href="mailto:m.douglas.williams@gmail.com">m.douglas.williams@gmail.com</a>> wrote: <blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">Ok, I got that working, but don't understand something.  Here is the code with two different instantiations of my-frame% - two with the instantiate within a parameterize and two using a let and current-frame call.  [I have two to test that it works for multiple frame instances.]<div class="Ih2E3d"> #lang scheme/gui (define current-frame   (make-parameter #f)) (define my-frame%   (class frame%     (super-instantiate ())     (field (text-field             (instantiate text-field%               ("Press test button to update:" this))))     (instantiate button%       ("Clear" this)       (callback        (lambda (b e)          (send text-field set-value ""))))     (instantiate button%       ("Test" this)       (callback        (lambda (b e)          (update-text-field)))))) (define (update-text-field)   (send (get-field text-field (current-frame))         set-value (send (current-frame) get-label))) </div><div class="Ih2E3d">(define (call-in-other-eventspace e thunk)   (let ([ch (make-channel)])     (parameterize ([current-eventspace e])       (queue-callback (lambda ()                         (channel-put ch (thunk)))))     (channel-get ch))) (call-in-other-eventspace  (make-eventspace)  (lambda ()    (parameterize ((current-frame (instantiate my-frame%                                    ("Frame 1")))) </div>     (send (current-frame) show #t))))<div class="Ih2E3d"> (call-in-other-eventspace  (make-eventspace)  (lambda ()    (parameterize ((current-frame (instantiate my-frame%                                    ("Frame 2")))) </div>     (send (current-frame) show #t))))<div class="Ih2E3d"> (call-in-other-eventspace  (make-eventspace)  (lambda () </div>   (let ((frame (instantiate my-frame%                   ("Frame 3"))))      (current-frame frame)      (send (current-frame) show #t))))<div class="Ih2E3d"> (call-in-other-eventspace  (make-eventspace)  (lambda () </div>   (let ((frame (instantiate my-frame%                   ("Frame 4"))))      (current-frame frame)      (send (current-frame) show #t)))) The last two do exactly what I want.  The first two give the effect I was looking for.  The first wo give "get-field: expected an object, got #f" when the Test button is clicked.  What is the difference? Doug P.S.  I'm not sure I would ever have come up with the solution.  Thanks for the answer.<div><div></div><div class="Wj3C7c"> <div class="gmail_quote">On Tue, Oct 28, 2008 at 6:20 PM, Matthew Flatt <<a href="mailto:mflatt@cs.utah.edu" target="_blank">mflatt@cs.utah.edu</a>> wrote: <blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"><div>At Tue, 28 Oct 2008 12:41:08 -0600, "Doug Williams" wrote: > I assume the problem is with my mental model of parameters and eventspaces. > I had assumed that each eventspace would have, in addition to its event > queues, etc,  its own execution thread, and copies of any parameters. </div>That's correct. <div> > Since > it allows me to parameterize or set the current-frame in a new eventspace, I > assume the eventspace 'knows about' my current-frame parameter.  What I > don't understand is why a call to update-text-field by a frame that I think > is running under 'new' eventspace doesn't get a value for the current-frame > parameter. </div>In  (parameterize ([current-eventspace E])    B) the `parameterize' sets the `current-eventspace' parameter to `E' while evaluating `B', it still evaluates `B' in the current thread --- not the handler thread of `E'. So if you mutate a parameter within `B', then it still mutates the parameter for the current thread, not for the thread of `E'. Use `queue-callback' to jump to another eventspace's handler thread. The `queue-callback' function uses the `current-eventspace' parameter (of the current thread) to determine where to queue the callback, and the callback is invoked in that eventspace's handler thread.  (define (call-in-other-eventspace e thunk)   (let ([ch (make-channel)])     (parameterize ([current-eventspace e])       (queue-callback (lambda ()                         (channel-put ch (thunk)))))     (channel-get ch)))  (call-in-other-eventspace  (make-eventspace)  (lambda () <div>    (parameterize ((current-frame (instantiate my-frame%                                               ("Frame 2"))))      (send (current-frame) show #t) </div>      (update-text-field))))  (call-in-other-eventspace  (make-eventspace)  (lambda () <div>    (let ((frame-3 (instantiate my-frame%                                ("Frame 3"))))      (current-frame frame-3)      (send (current-frame) show #t) </div>      (update-text-field)))) </blockquote></div> </div></div></blockquote></div>