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."))))])


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 #<procedure> 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 #<procedure>.
> ; 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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