Using \NewDocumentCommand to redefine a table rule

Question

I was trying to redefine an old command \lightrule with the new syntax from xparse.

When I try to use \newlightrule unexpectedly it displays Misplaced \noalign error.

Minimal Working Example and desired result follows.

---

---

\documentclass[margin=2mm]{standalone}
\usepackage[table]{xcolor}
\usepackage{booktabs}
\newcommand{\lightrule}{
    \arrayrulecolor{black!30}%
    \midrule[\lightrulewidth]%
    \arrayrulecolor{black}%
}
\usepackage{xparse}
\NewDocumentCommand{\newlightrule}{}{
    \arrayrulecolor{black!30}%
    \midrule[\lightrulewidth]%
    \arrayrulecolor{black}%
}
\begin{document}
\begin{tabular}{c l}
\toprule
    test & test\
\midrule
    test & test\
\lightrule
    test & test\
\bottomrule
\end{tabular}
\end{document}

Answer 1

You should use \NewExpandableDocumentCommand to define. As David said xparse is not needed unless you have an old latex release.

I also suggest you to use tabularray to typeset your table, it's easy to use.

\documentclass[margin=2mm]{standalone}
\usepackage[table]{xcolor}
\usepackage{booktabs}
% \newcommand{\lightrule}{
%     \arrayrulecolor{black!30}%
%     \midrule[\lightrulewidth]%
%     \arrayrulecolor{black}%
% }
% \usepackage{xparse}
\NewExpandableDocumentCommand{\newlightrule}{}{
    \arrayrulecolor{black!30}%
    \midrule[\lightrulewidth]%
    \arrayrulecolor{black}%
}
\usepackage{tabularray}
\begin{document}
\begin{tabular}{c l}
\toprule
    test & test\
\midrule
    test & test\
\newlightrule
    test & test\
\bottomrule
\end{tabular}
\begin{tblr}
{
colspec={Q[c,m]Q[l,m]},
hline{1,Z}={wd=.08em},
hline{2}={wd=.05em},
hline{3}={wd=.05em,fg=black!30},
}
test & test\
test & test\
test & test\
\end{tblr}
\end{document}