Re: [SOLVED with `eval']: Why I cannot use this variable in macro call from function?

[Jean Louis <bugs@gnu.support>]

* tomas@tuxteam.de <tomas@tuxteam.de> [2021-06-09 09:11]:
> You'll see "43" in your minibuffer. We managed to set the dynamically
> "defined" variable "that-other-var" to 43. Yay! Now enter
> 
>   that-other-var
> 
> ...and again C-x e. Minibuffer says... 43. The variable escaped the
> scope. Bad variable, but hey, that's how setq goes, no?

Yes. I did it.

> * Experiment 2
> 
> Make new lisp interaction buffer, as above. Make sure you have
> lexical binding (when experimenting, always change at most one
> thing at a time, right?). Now:
> 
>   (let ((some-var 42)
>         (that-other-var 44)) ; bind that-other-var locally
>     (eval '(setq that-other-var 43))
>     (message "%S\n" that-other-var))
> 
> Again, minibuffer says 43. Now...
> 
>   that-other-var
> 
> ...and C-x e. What does minibuffer say? Is that right or wrong?
> Explain.

The one bound locally got 44, it was protected from a global
variable. That is so far expected. If I wish for example avoid case
sensitive entries, I do:

(let ((completion-ignore-case t)
      (collection '("One" "Two")))
  (completing-read "Choice: " collection nil t))
  
In that case the global variable `completion-ignore-case' is nil,
but I bind it temporarily globally to T, for better user
experience.

> I guess you'll need a more differentiated model than just "works".
> Works how? Do you want it to "work" like that? If yes, then fine.

Here is the function that just works, it asks for global
variable. If I would know better way, I would do it and if you
know some other way to dynamically assign global variable let me
know:

(defun rcd-db-completing-table-history (table)
  "Return symbol of history variable for TABLE.

If TABLE is \"businesses\" the symbol will become
`rcd-db-completing-table-history-businesses' and it will be used
for functions `completing-read' and `read-from-minibuffer'
history.

If a dynamically generated variable does not exist, it will be
generated on the fly."
  (let ((rcd-symbol (intern (concat "rcd-db-completing-table-history-" table)))
	(description (format "History for table `%s'" table)))
    (if (boundp rcd-symbol)
	rcd-symbol
      (eval (list 'defvar rcd-symbol nil description)))))

Just like any code, the above (eval (list 'defvar rcd-symbol nil
description)) is understandable only to people who know what
eval, list, defvar and variables mean. We are now in
collaboration and I am asking if there is better way to
dynamically assign a global variable if it does not exist and
return a symbol of it.

What I don't know is why the function work as I am somehow
expecting that new evaluation of (defvar ex-sting-variable nil)
would make it nil, but it does not and history is remembered. 

> A very wise person [1] once said that he doesn't write programs
> to get the computer to do what he wants, but to convince his
> colleagues that the computer is doing the right thing.

Fine, but that way I don't get my dynamically assigned global
variables. (づ｡◕‿‿◕｡)づ

-- 
Jean

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