Re: Is there a way of setting a variable only when it exists?

[Emanuel Berg via Users list for the GNU Emacs text editor <help-gnu-emacs@gnu.org>]

Marcin Borkowski wrote:

>>> so that `hello' is set to `"world"' if it is an existing
>>> variable (e.g., defined by `defvar') and an error is
>>> raised if `hello' does not exist.
>>
>> Byte compiling can help with things like name changes.
>
> Not necessarily. My use-case is an internal Emacs variable
> I'm setting in my `init.el'; at the same time I submitted
> a bug report/feature request to make it a user option, so
> I assume that it might change its name in a future
> Emacs version.

If that variable change its name the byte-compiler will warn
you since you will be shooting blanks.

Here, try type:

  (setq no-such-var 'oh-la-la)

then byte-compile, the byte-compiler will say:

  In toplevel form:
  geh.el:19:7: Warning: assignment to free variable ‘no-such-var’

BTW I wrote a program ages ago [1] and it tells me in 152
Elisp files just a small bunch of very few variables that I've
ever used `setf' or `setq' on

;; .emacs.el, 204: show-paren-delay
;; erc-incal.el, 34: erc-user-full-name
;; erc-incal.el, 77: erc-header-line-format
;; spell.el, 17: ispell-program-name
;; w3m-incal.el, 26: w3m-tab-width

are actually options.

I don't remember anymore why that was important to know?

I didn't like custom or options back then (and still don't) so
maybe an attitude/political thing but ... yeah, what?

[1] https://dataswamp.org/~incal/emacs-init/custom-vars.el

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