Hi gfb
Thank you for replying to my question.
Your reply has saved me from writing unnecessary boiler plate code for my
click events. I see the difference between mentioning a function and
evaluating it now.
What I ended up going for looks like this:
; Make a button in the frame
(new button% [parent work-panel]
[label "Work"]
; Callback procedure for a button click:
[callback (lambda (button event)
(send work-msg set-label "Work timer started")
(make-timer 25 (lambda ()
(timer-callback work-msg "Not
working" "25 Minutes have passed, take a break."])
Regards
Philip
On Tuesday, 12 May 2020 20:37:32 UTC+2, gfb wrote:
>
> The difference is whether you are evaluating an expression, versus
> putting it inside a function to be evaluated when the function is called.
>
> (require racket/gui)
>
> ; Call message-box, get an immediate effect ...
> (message-box "What do you think?" "hi, would you like a tomato" #f
> '(yes-no))
> ; The result ('yes or 'no) is printed in the repl/Interactions.
>
> ; Create a function/procedure, that takes no arguments, and its body calls
> message-box ...
> (lambda () (message-box "Greeting" "hi, would you like a tomatillo" #f
> '(yes-no)))
> ; That which prints as # since it's a function, and we didn't
> call it.
>
> ; To call a function we put parentheses around it ...
> ((lambda () (message-box "Greeting" "hi, would you like a pomodoro" #f
> '(yes-no)))
> )
> ; That evaluated the body.
>
> ; Name a function ...
> (define greeter (lambda () (message-box "Greeting" "hi, would you like a
> green tomato" #f '(yes-no
> ; That didn't call it, and neither do ...
> greeter
> greeter
> ; Those just mention the function, which prints as #.
> ; Call it to evaluate the body ..
> (greeter)
>
> By the way, your examples that don't wait 30 minutes actually do,
> and if you wait 30 minutes they will try to call the callback, which
> is not a function but the 'ok result from the message-box.
>
> Try setting the timer to something shorter, and then wait after
> you click the immediate message box, then when the timer
> goes off there will be an error something like ...
> application: not a procedure;
> expected a procedure that can be applied to arguments
> given: 'ok
> arguments...: [none]
>
>
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