Re: Alist remaining empty

[<tomas@tuxteam.de>]

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On Fri, Sep 27, 2024 at 10:50:21PM +0000, Heime wrote:
> I have alist variable warn-alist, which I set to nil.  I am supposed
> to push some alist examples if the values have not been added yet.
> 
> But I find that with the following function, the warn-alist remains empty.

As it should. Please, please: learn about variables, variable scopes,
values. Otherwise, you're bound to stay in an endless loop.

I intersperse comments into your code:

;; Somewhere "outside", warn-alist has been defined. Or not.
;; We just don't know.

> (defun warn-test ()
> 
>   (interactive)
> 
>   (setq warn-alist nil)

;; Here, its value is set to nil (whatever its value was before).
;; Is it global? Buffer-local? Lexical? We don't know. We don't
;; know either whether it matters.
> 
>   (let ( (qwpanel warn-alist)

;; A new (probably lexical, but possibly dynamic) binding for
;; qwpanel is introduced: its value is set to the value of
;; warn-alist -- which is nil (see above)
> 
>          ;; Init flag to track if values have been added
>          (already-added nil) )
> 
>     ;; If the alist is empty, add the cons data
>     (unless qwpanel
>       (setq already-added t))

;; Things are done in a somewhat roundabout way. In the conditional
;; above, we /know/ that qwpanel is nil, so already-added stays nil
> 
>     (unless already-added

;; this means we could replace here "unless already-added" by
;; "unless qwpanel", and then completely do away with it (a smart
;; compiler is going to do that for you anyway).
>       (push (cons 'ques-string "Warning Message")   qwpanel)
>       (push (cons 'ques-number 42)                  qwpanel)
>       (push (cons 'ques-symbol 'example-symbol)     qwpanel)
>       (push (cons 'ques-list '(1 2 3))              qwpanel)
>       (push (cons 'ques-boolean t)                  qwpanel)
>       (push (cons 'ques-nil nil)                    qwpanel)
>       (push (cons 'ques-cons (cons 'nested "cell")) qwpanel)
>       (push (cons 'ques-function #'print)           qwpanel)) ))

;; and now, when leaving the (let ...) scope, you throw away your
;; qwpanel binding, and all of its value, for the garbage collector
;; to feast on it. Viewed "from the outside", which you haven't
;; shown to us, nothing, except possibly setting warn-alist to nil
;; has happened.

Working as designed, I'd say.

Cheers
-- 
tomás

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