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)
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