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

[Marcin Borkowski]

On 2022-03-16, at 07:02, Tomas Nordin <tomasn@posteo.net> wrote:

> Marcin Borkowski <mbork@mbork.pl> writes:
>
>> On 2022-03-14, at 14:48, Emanuel Berg via Users list for the GNU Emacs text 
>> editor <help-gnu-emacs@gnu.org> wrote:
>>
>>> Stefan Monnier via Users list for the GNU Emacs text editor wrote:
>>>
>>>>> I assume it would be fairly easy to code such a macro
>>>>> (using `boundp'), but maybe it exists already?
>>>>
>>>> AFAIK it doesn't exist yet. The reason for it is that it is
>>>> not often useful. Typically there are two cases:
>>>>
>>>> - If the var exist, you want to set it and if not you have
>>>>   no fallback. In that case, it is typically harmless to set
>>>>   the var even when it doesn't exist, so the code just uses
>>>>   `setq` without bothering to test `boundp`.
>>>>
>>>> - If the var exist you want to set it, and if it doesn't you
>>>>   want to do something else. In that case, the something
>>>>   else tends to depend on the specifics so (if (boundp 'foo)
>>>>   (setq foo ..) ...) is about s good as it gets.
>>>
>>> If it exists set it with `setq'. If it doesn't exist, create
>>> and set it ... with `setq'?
>>
>> What if it's an internal Emacs variable which might become a user option
>> one day (I submitted a bug report about it) and then my customization
>> silently disappears?  It's the "silently" part I want to guard
>> against...
>
> What about doing an assert of sorts
>
> (message "some--internal-var is %S" some--internal-var)
> (setq some--internal-var 42)
>
> It will crash your init process if some--internal-var is void.

Well, that could work, but is far from elegant;-).

-- 
Marcin Borkowski
http://mbork.pl