I was afraid that directly setting the default may break many people's muscle
memory, or hypothetically maybe someday "#" won't be the most common unknown
case anymore. Between a new custom variable and the default I have no strong
opinion otherwise.

Daanturo

On Mar 3 2025, at 8:59 am, Stefan Kangas <stefankangas@gmail.com> wrote:
> Daan Ro <daanturo@gmail.com> writes:
>
> > "#" is the most common comment syntax in text files whose major mode
> > couldn't be recognized automatically.
> >
> > I hope this can be customized in Emacs 30.
>
> Thanks for the patch, and sorry for the long delay.
> Maybe we should just offer "#" as the default instead, if it's the most
> common one? My reasoning here is that there is no need to add a
> defcustom, if no one has asked for it.
>
> > From f0a011fdc888f41eb49cb44e1be577261f8b2a9b Mon Sep 17 00:00:00 2001
> > From: Daanturo <daanturo@gmail.com>
> > Date: Mon, 6 May 2024 23:16:56 +0700
> > Subject: [PATCH] Add `comment-start-default-suggestion' user option
> >
> > * lisp/newcomment.el: Define `comment-start-default-suggestion' Lisp
> > variable and use it in `comment-normalize-vars'.
> >
> > * etc/NEWS: Announce the option.
> > ---
> > etc/NEWS | 5 +++++
> > lisp/newcomment.el | 9 ++++++++-
> > 2 files changed, 13 insertions(+), 1 deletion(-)
> >
> > diff --git a/etc/NEWS b/etc/NEWS
> > index 456f9b8f8b8..fa9b7c82a97 100644
> > --- a/etc/NEWS
> > +++ b/etc/NEWS
> > @@ -541,6 +541,11 @@ whereas if the mouse pointer is in the left half of a glyph, point
> > will be put in front the buffer position corresponding to that glyph.
> > By default this is disabled.
> >
> > +---
> > +** New user option 'comment-start-default-suggestion'.
> > +When 'comment-normalize-vars' asks for nil 'comment-start', use this as
> > +the default suggested string in the prompt.
> > +
> > ** Internationalization
> >
> > ---
> > diff --git a/lisp/newcomment.el b/lisp/newcomment.el
> > index ee7b2ea34d8..86529668067 100644
> > --- a/lisp/newcomment.el
> > +++ b/lisp/newcomment.el
> > @@ -349,6 +349,12 @@ terminated by the end of line (i.e., `comment-end' is empty)."
> > "Return the mirror image of string S, without any trailing space."
> > (comment-string-strip (concat (nreverse (string-to-list s))) nil t))
> >
> > +(defcustom comment-start-default-suggestion nil
> > + "The default suggestion when unset `comment-start' is asked."
> > + :type 'string
> > + :version "30.1"
> > + :group 'comment)
> > +
> > ;;;###autoload
> > (defun comment-normalize-vars (&optional noerror)
> > "Check and set up variables needed by other commenting functions.
> > @@ -358,7 +364,8 @@ functions work correctly. Lisp callers of any other `comment-*'
> > function should first call this function explicitly."
> > (unless (and (not comment-start) noerror)
> > (unless comment-start
> > - (let ((cs (read-string "No comment syntax is defined. Use: ")))
> > + (let ((cs (read-string "No comment syntax is defined. Use: "
> > + comment-start-default-suggestion)))
> > (if (zerop (length cs))
> > (error "No comment syntax defined")
> > (setq-local comment-start cs)
>