bug#35898: 26.1; line-beginning-position sometimes does not yield line beginning position

bug#35898: 26.1; line-beginning-position sometimes does not yield line beginning position

[Markus Triska]

Please download hello.png with:

    $ wget https://www.metalevel.at/ei/hello.png

and then, starting Emacs with "emacs -Q", evaluate the following form:

    (progn
      (goto-char (point-min))
      (save-excursion (insert "\n"))
      (goto-char (+ (point) (cadr (insert-image-file "hello.png"))))
      (line-beginning-position))

This yields 592, whereas I expect it to yield 1.

For comparison, when I do C-a to move to the beginning of the line, it
moves to position 1, as expected.

Is there a way to make line-beginning-position yield the line beginning
position in cases like this? Would you please consider adding this feature?

Thank you and all the best,
Markus

In GNU Emacs 26.1 (build 3, x86_64-pc-linux-gnu, X toolkit, Xaw scroll bars)
 of 2019-04-09 built on mt-laptop
Windowing system distributor 'The X.Org Foundation', version 11.0.11906000

bug#35898: 26.1; line-beginning-position sometimes does not yield line beginning position

[Eli Zaretskii]

> From: Markus Triska <triska@metalevel.at>
> Date: Sat, 25 May 2019 15:23:48 +0200
> 
>     $ wget https://www.metalevel.at/ei/hello.png
> 
> and then, starting Emacs with "emacs -Q", evaluate the following form:
> 
>     (progn
>       (goto-char (point-min))
>       (save-excursion (insert "\n"))
>       (goto-char (+ (point) (cadr (insert-image-file "hello.png"))))
>       (line-beginning-position))
> 
> This yields 592, whereas I expect it to yield 1.

Your expectations are unjustified, because line-beginning-position is
not supposed to work on visual lines, it works on logical/physical
lines, i.e. it scans the buffer for newline characters.

For visual-line operation, you can use either beginning-of-visual-line
or vertical-motion (the former actually calls the latter internally).

> Is there a way to make line-beginning-position yield the line beginning
> position in cases like this? Would you please consider adding this feature?

We already have features to query about the visual lines, so I don't
see why we would need line-beginning-position work in terms of visual
lines.  It will fail Lisp programs that want to disregard the visual
appearance of the buffer.

bug#35898: 26.1; line-beginning-position sometimes does not yield line beginning position

[Stefan Kangas]

tags 35898 + notabug
close 35898
thanks

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Markus Triska <triska@metalevel.at>
>> Date: Sat, 25 May 2019 15:23:48 +0200
>>
>>     $ wget https://www.metalevel.at/ei/hello.png
>>
>> and then, starting Emacs with "emacs -Q", evaluate the following form:
>>
>>     (progn
>>       (goto-char (point-min))
>>       (save-excursion (insert "\n"))
>>       (goto-char (+ (point) (cadr (insert-image-file "hello.png"))))
>>       (line-beginning-position))
>>
>> This yields 592, whereas I expect it to yield 1.
>
> Your expectations are unjustified, because line-beginning-position is
> not supposed to work on visual lines, it works on logical/physical
> lines, i.e. it scans the buffer for newline characters.
>
> For visual-line operation, you can use either beginning-of-visual-line
> or vertical-motion (the former actually calls the latter internally).
>
>> Is there a way to make line-beginning-position yield the line beginning
>> position in cases like this? Would you please consider adding this feature?
>
> We already have features to query about the visual lines, so I don't
> see why we would need line-beginning-position work in terms of visual
> lines.  It will fail Lisp programs that want to disregard the visual
> appearance of the buffer.

As Eli has correctly explained, this is the expected behaviour.  So I'm
therefore closing this bug report.

If this conclusion is incorrect, please reply to this email (use "Reply
to all" in your email client) and we can reopen the bug report.

Best regards,
Stefan Kangas