bug#36227: 26.1; sgml-mode indents as comment inside of -- text blocks

bug#36227: 26.1; sgml-mode indents as comment inside of -- text blocks

[Akkana Peck]

In emacs -Q, edit a file in html-mode (derived from sgml-mode),
e.g. visit /tmp/foo.html.

M-x auto-fill-mode <RET>

Type some nonsense words with ' -- ' in the middle, like this:

asd shdf ladshjkl sjdk -- asdfh jklsdfh

and keep typing until it wraps.

sgml-mode will wrap adding unwanted extra dashes and indentations,
like this:

asd shdf ladshjkl sjdk -- asdfh jklsdfh jasdklf hjsdkl hjsdaklf sdf --
		       -- jkldsfhj kalshfjkasdlfh jaksd

This happens because sgml-mode is confused about what constitutes a
comment in sgml/html.

At some point, someone clearly agreed that this was a bug and tried to
fix it. If you look at the sgml.el source and search for --,
you'll see comments like:

> (defvar sgml-specials '(?\")
>   "List of characters that have a special meaning for SGML mode.
> This list is used when first loading the `sgml-mode' library.
> The supported characters and potential disadvantages are:
>
>   ?\\\"	Makes \" in text start a string.
>   ?\\='	Makes \\=' in text start a string.
>   ?-	Makes -- in text start a comment.
>
> When only one of ?\\\" or ?\\=' are included, \"\\='\" or \\='\"\\=', as can be found in
> DTDs, start a string.  To partially avoid this problem this also makes these
> self insert as named entities depending on `sgml-quick-keys'.
>
> Including ?- has the problem of affecting dashes that have nothing to do
> with comments, so we normally turn it off.")

The dash is not included in sgml-specials by default, but the problem
affecting dashes that have nothing to do with comments happens anyway.
Setting sgml-specials to nil in the mode hook doesn't help either.

The problematic behavior seems to come from comment-line-break-function.
One workaround I've found is to add this in my html-mode and sgml-mode
hooks to prevent the special processing of line breaks inside comments:

    (kill-local-variable 'comment-line-break-function)

Maybe comment-line-break-function should check sgml-specials and return
without doing anything if - isn't one of the specials?



In GNU Emacs 26.1 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.4)
 of 2019-02-03, modified by Debian built on zam904
Windowing system distributor 'The X.Org Foundation', version 11.0.12004000
System Description:	Debian GNU/Linux 10 (buster)

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

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bug#36227: 26.1; sgml-mode indents as comment inside of -- text blocks

[Noam Postavsky]

severity 36227 minor
tags 36227 + confirmed
quit

Akkana Peck <akkana@shallowsky.com> writes:

> In emacs -Q, edit a file in html-mode (derived from sgml-mode),
> e.g. visit /tmp/foo.html.
>
> M-x auto-fill-mode <RET>
>
> Type some nonsense words with ' -- ' in the middle, like this:
>
> asd shdf ladshjkl sjdk -- asdfh jklsdfh
>
> and keep typing until it wraps.
>
> sgml-mode will wrap adding unwanted extra dashes and indentations,
> like this:
>
> asd shdf ladshjkl sjdk -- asdfh jklsdfh jasdklf hjsdkl hjsdaklf sdf --
> 		       -- jkldsfhj kalshfjkasdlfh jaksd

> The problematic behavior seems to come from comment-line-break-function.
> One workaround I've found is to add this in my html-mode and sgml-mode
> hooks to prevent the special processing of line breaks inside comments:
>
>     (kill-local-variable 'comment-line-break-function)
>
> Maybe comment-line-break-function should check sgml-specials and return
> without doing anything if - isn't one of the specials?

It's not to do with the sgml-specials, the problem is that the standard
auto-fill-function, `do-auto-fill', calls `default-indent-new-line'
which calls `comment-line-break-function' regardless of whether point is
inside a comment or not.  As far as I can tell, it works in most modes
because they disable auto-filling outside of comments in the first
place.  sgml-mode doesn't, because most of its non-comment text is just
plain text that should be filled normally.

So I think there should be a sgml-mode specific
comment-line-break-function, which checks whether it's in a comment or
not (presumably using syntax-ppss).

bug#36227: 26.1; sgml-mode indents as comment inside of -- text blocks

[Lars Ingebrigtsen]

Noam Postavsky <npostavs@gmail.com> writes:

> So I think there should be a sgml-mode specific
> comment-line-break-function, which checks whether it's in a comment or
> not (presumably using syntax-ppss).

I've now done this in Emacs 28, and it seems to work correctly in the
scenario described.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no