bug#58075: Mapping file types to major modes

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Ikumi Keita [2022-09-28 17:15:43] wrote:
> Hi Stefan,
>>>>>> Stefan Monnier <monnier@iro.umontreal.ca> writes:
>> So instead I suggest consider the current settings as being our
>> implementation of (A), and add (B) as some "mode remapping" layer to
>> decide which major mode to actually for a given "type/mode".
>> So instead of
>
>>     (defalias 'perl-mode #'cperl-mode)
>
>> We could have
>
>>     (add-to-list 'major-mode-remap-alist '(perl-mode . cperl-mode))
>
> In your proposed patch, `major-mode-remap-alist' is a user option
> defined by `defcustom'.
>
> Is it supposed to be legitimate that a site administrator sets up the
> default mapping for the site as
>>     (add-to-list 'major-mode-remap-alist '(perl-mode . cperl-mode))
> in site-start.el?

That's a somewhat general problem we have, indeed: if the
`site-start.el` file uses `add-to-list` this way, then Custom will think
the variable is "changed outside of Custom" and it will complain about
it if the user then tries to configure the var via Custom.

Using `defvar` instead of `defcustom` wouldn't really help tho: the user
still would not be able to configure it via Custom.

To work "properly", `site-start.el` would need to change the *default*
definition of the var (which Custom stores in the `standard-value`
property), and then ask Custom to re-evaluate the var's value.

Emacs *should* provide some convenient way to do that, but Someone™
needs to code it up.

> I briefly skimmed over info documents of Emacs and Elisp, but could not
> find a description whether a site admin is allowed or not to alter the
> value of user customize variable in site-start.el (not default.el, of
> course).

It's allowed and very common in practice (e.g. to
set `send-mail-function` or `smtpmail-smtp-server`).
But doing it will tend to cause the problems mentioned above.


        Stefan