Error when subtitle contains absolute value ("pipe")

[ChristopherChudzicki]

I'm getting an error when I compile documents where an exercise subtitle contains the pipe | character. Here's a MWE:

% test.tex
\documentclass{article}

\usepackage{xsim}

\begin{document}

\begin{exercise}[subtitle={$|w|$}]
    Hello, world!
\end{exercise}

\end{document}

When I run pdflatex test.tex twice, the second run throws an error:

Runaway argument?
$\q_stop \tl_set_rescan:Nnx \l__xsim_tmpc_tl {}{\l__xsim_tmpc_str }\__xsim_set_
property:VVnV \ETC.
! File ended while scanning use of \__xsim_extract_property_list_entry:NNNwww.
<inserted text> 
                \par 
<*> test.tex

(Although, looking at the generated PDF, the document appears OK. But it makes me nervous.)

[cgnieder]

Hi, thanks. This is not really a bug but should be resolved nonetheless. The problem stems from this. If you have several exercises

\documentclass{article}

\usepackage{xsim}

\begin{document}

\begin{exercise}[subtitle=foo]
  Hello, world!
\end{exercise}
\begin{exercise}[subtitle=bar]
  Hello, world!
\end{exercise}

\end{document}

then you'll find in the aux file lots of lines like this one

\XSIM {subtitle}{exercise-1=={foo}|exercise-2=={bar}}

where the | character is used to separate entries. I'll implement an option so that you'd be able to say

\documentclass{article}

\usepackage{xsim}
\xsimsetup{
  split-aux-lists = ||
}
\begin{document}

\begin{exercise}[subtitle={$|w|$}]
  Hello, world!
\end{exercise}
\begin{exercise}[subtitle=bar]
  Hello, world!
\end{exercise}

\end{document}

[cgnieder]

I will also make || the default so hopefully most people will not have to worry about this…
I wonder why you are using | in the first place. From what it looks like I would probably use something like

\usepackage{mathtools}
\DeclarePairedDelimiter\abs{\lvert}{\rvert}

and

\begin{exercise}[subtitle={$\abs{w}$}]

[cgnieder]

This is included in v0.11:

\documentclass{article}

\usepackage{xsim}
\xsimsetup{
  split-aux-lists = ||
}
\begin{document}

\begin{exercise}[subtitle={$|w|$}]
  Hello, world!
\end{exercise}
\begin{exercise}[subtitle=bar]
  Hello, world!
\end{exercise}

\end{document}

[ttnick]

I considered opening a new issue but I think this one is closely enough related. A colleague of mine encountered the following error (xsim v0.20c, TeX Live 2022):
File ended while scanning use of \__xsim_extract_property_list_entry:NNNwww.
on every second run of pdflatex, when he uses || within an exercise or solution environment. I believe this is essentially the same issue as the original one here that a || causes xsim to confuse it for a separator in the *.xsim file.
MWE:

\documentclass[a4paper,12pt]{article}
\usepackage{xsim}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}

\begin{document}
\begin{exercise}
	Consider the following set
	\[
		B(c, r) = \{x \in \mathbb{R}^n \mid ||x - c||_2 \leq r\}.
	\]
\end{exercise}
\end{document}

I told him that that use of || is not good anyway but the error seems strange enough that I thought I report it to you.
(If that matters, there is also notation from scheduling theory that might create the double pipe, e.g. P||C_\max is short for the scheduling problem with multiple machines that wants to minimize the makespan.)

[muzimuzhi]

@ttnick I would suggest to use either \| or \Vert to typeset double vertical bar. For the problem with ||, see #109.