Re: Alist remaining empty

[Heime <heimeborgia@protonmail.com>]

On Saturday, September 28th, 2024 at 5:26 PM, tomas@tuxteam.de <tomas@tuxteam.de> wrote:

> 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)

I set the global to nil so that the push upon qwpanel will execute on
first call.  
 
> ;; 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).

Right, agreed
 
> > (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.

Inside the let scope I call the following function with qwpanel
 
(defun alcons (alist &optional bfrn)
  "Print Associateed List ALIST to the buffer BFRN."

  (let* ( (bfname (or bfrn "ALIST Cons"))
          (dbuffer (get-buffer-create (concat "𒆳 " bfname))) )

    (with-current-buffer dbuffer
      (face-remap-add-relative 'default :background "blue")

      (insert "\n ALIST Cons \n\n")

      (dolist (entry alist)
        (insert (format " %S\n" entry)))) ))

;; --------------------------------------------------------------------

(defun warn-test ()

  (setq warn-alist nil)

  (let ( (qwpanel warn-alist) )
    (unless qwpanel
      (push (cons 'ques-string "Warning Message") qwpanel)
      (push (cons 'ques-number 42) qwpanel))
    (alcons qwpanel "Alist Cons Test")) )


> Working as designed, I'd say.
> 
> Cheers
> --
> tomás