Using comment characters for specific major modes

[martin-kemp@brusseler.com]

I have found it easier to use a condition depending on the major mode of the buffer to set the comment

prefix directly.






From: Stefan Monnier via Users list for the GNU Emacs text editor <help-gnu-emacs@gnu.org>
To: help-gnu-emacs@gnu.org
Subject: Re: Using comment characters for specific major modes
Date: 06/06/2021 05:24:13 Europe/Paris

> That uses the command (comment-dwim ARG), but I got to get the comment characters
> to append to the string "s".
>
>   (setq-local s (concat ";; " (make-string lena ?\;)))
>
> At least that was my plan.  You may know the proper way to do it though.

Here's how you could go about finding the answer:

Seeing that `comment-dwim` manages to find the ";;" you're looking for,
you could look into it code: `C-h o comment-dwim RET` then click on the
link to jump to the definition. There you'll see:

[...]
(comment-normalize-vars)
(if (use-region-p)
(comment-or-uncomment-region (region-beginning) (region-end) arg)
(if (save-excursion (beginning-of-line) (not (looking-at "\\s-*$")))
;; FIXME: If there's no comment to kill on this line and ARG is
;; specified, calling comment-kill is not very clever.
(if arg (comment-kill (and (integerp arg) arg)) (comment-indent))
;; Inserting a comment on a blank line. comment-indent calls
;; c-i-c-f if needed in the non-blank case.
(if comment-insert-comment-function
(funcall comment-insert-comment-function)
(let ((add (comment-add arg)))
;; Some modes insist on keeping column 0 comment in column 0
;; so we need to move away from it before inserting the comment.
(indent-according-to-mode)
(insert (comment-padright comment-start add))
(save-excursion
(unless (string= "" comment-end)
(insert (comment-padleft comment-end add)))
(indent-according-to-mode)))))))

And here you already see some functions and variable which might give
you some answers, such as `comment-add`,
`comment-insert-comment-function`, `comment-start`, ...
Using `C-h o` on those should fairly quickly lead you to:

comment-start is a variable defined in ‘newcomment.el’.
Its value is ";"
Local in buffer newcomment.el.gz; global value is nil

This variable is safe as a file local variable if its value
satisfies the predicate ‘string-or-null-p’.
Probably introduced at or before Emacs version 21.1.

Documentation:
String to insert to start a new comment, or nil if no comment syntax.

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-- Stefan