On 13/10/2022 09:44, Ihor Radchenko wrote:
Max Nikulin writes:
I am considering \noalign{} instead of \relax. I was never aware of its
effect, but accordingly to The TeXbook it should keep TeX in vertical
mode without any action due to empty argument. (Actually I surprised
that \relax causes any change of internal state besides parser.)
If \noalign has less side effects compared to \relax, I'd prefer
\noalign. Can you confirm this?
My understanding of TeX is not solid enough to confirm this. From The
TeXbook I have an impression that \noalign is added specially for
table-like blocks to be used immediately after \cr (low level command
similar to \\). So I would not use it outside of tables however it may
be safe. I am not aware of problems with \\\noalign{} in tables. The
exact effect of \relax is not clear to me yet. I do not have an example
of negative effect of \relax similar to \hline but outside of tables.
I can try to ask for a better suggestion at stackexchange, but I am
unsure if the question will be noticed by some person from a TeX engine
or core LaTeX developers or at least with better understanding of TeX
internals.
Unfortunately \noalign{} just as \relax will not allow @@latex:[1cm]@@
on the next line, perhaps @@latex:\noalign{\vskip 1cm}@@ is a workaround
to introduce additional vertical space.
What you are talking about appears to be abusing our exporter. We give
no guarantees about how \\ is going to be exported internally into
LaTeX. We should have no obligation to keep use-cases like this.
On the other hand LaTeX backend was transparent to such hacks, so the
change might be breaking to some users. That is why conditionally adding
\noalign{} or \relax if \\ is not followed by an export snippet may be a
better solution.
I never used \\* or \\*[10pt] variants of the command. Current stable
release should has problems when the line next to \\ starts from * that
is not bold marker, besides square brackets.
I feel like I am missing what you are talking about here.
The rabbit hole is deeper than I thought. The \\ command has its star
counterpart \\* to prevent page break at this point, and both of them
allow optional [<LENGTH>] argument. In vanilla LaTeX2e space is allowed
between \\ and *. However \usepackage{amsmath} redefines \\ command. It
still can recognize following *, but only on the same line. Unsure if it
is a bug or feature. So before your commit the following is not a
problem in *default* Org configuration:
Star\\
@@ignore:@@* on the next line.
Star and brackets\\
*[1em] on the next line.
| Star |
| * in the next row. |
|-|
| Star and brackets |
| *[1em] in the next row. |
However it might be if someone changes list of default packages. Adding
\noalign{} and \relax should fix the issue just for square brackets.
So at least as a temporary fix \noalign{} should be used for tables
instead of \relax to mitigate negative effect of the recent patch.
P.S. Packages like longtable might bring more surprises.
\documentclass{article}
%\usepackage{amsmath}
\begin{document}
\section{Regular usage}
Just\\
line break.
Brackets\\[1em]
on the same line.
Star\\*
on the same line.
Star and brackets\\*[1em]
on the same line.
\begin{tabular}{l}
tabular\\
\hline
Two\\
rows\\
\hline
Brackets \\[1em]
in the same row\\[1pt]
\hline
Star \\*
in the same row\\*
\hline
Start and brackets\\*[1em]
in the same row\\*[1pt]
\hline
\end{tabular}
\section{Problems}
Brackets and starts disappear in the beginning of
lines and table rows.
Brackets\\
[1em] on the next line.
\begin{tabular}{l}
tabular\\
\hline
Brackets \\
[1em] in the next row\\
[1pt]
\hline
\end{tabular}
\section{Problems when usepackage\{amsmath\} absent}
Star\\
* on the next line.
Star and brackets\\
*[1em] on the next line.
\begin{tabular}{l}
tabular\\
\hline
Star\\
* in the next row. \\
\hline
Star and brackets\\
*[1em] in the next row. \\
\hline
\end{tabular}
\end{document}
