Problem issuing \egroup ("}") from Lua

Question

I would like to define a command using Lua that will start and end a \savebox depending on if it is starred or not. Let's call this command \switchcolumn.

Here is the code (the first \switchcolumn that starts the box is substituted with the literal text, since it is irrelevant to the question):

\documentclass[11pt, paper = B5]{scrbook}
\usepackage{fontspec}
\usepackage{luacode}
\usepackage{xparse}

\setmainfont{Libertinus Serif}

\begin{luacode}
    dofile(kpse.find_file("mwe.lua"))
\end{luacode}

\NewDocumentCommand{\switchcolumn}{}{\directlua{SwitchColumn()}}

\begin{document}

\newsavebox{\aaa}
\savebox{\aaa}[0.5\textwidth] {
Hello World!
\switchcolumn

\end{document}

mwe.lua:

function SwitchColumn()
    tex.cprint(2, "}")
end

It doesn't work. The closing curly bracket doesn't end up in the TeX's engine input:

Runaway argument?
{ Hello World! \switchcolumn \par \end {document} 
! File ended while scanning use of \@isavebox.

I tried tex.print, tex.sprint, it's all the same.

How to set things right?

Answer 1

The exact Lua form of your function disn't matter as macros and commands are not executed while TeX is scanning for arguments, so the end of file is reached looking for a closing } and \switchcolumn is never expanded.

You could use the environment form lrbox but that doesn't have a width argument so I re-use the box in \makebox to set the width.

Lua (although you could use a tex definition here more easily)

function SwitchColumn()
tex.print('\\end{lrbox}')
end

tex

\documentclass[11pt, paper = B5]{scrbook}
\usepackage{fontspec}
\usepackage{luacode}
\usepackage{xparse}

\setmainfont{Libertinus Serif}

% over complicated:
%\begin{luacode}
%    dofile(kpse.find_file("mwe.lua"))
%\end{luacode}
\directlua{require 'mwe'}

\NewDocumentCommand{\switchcolumn}{}{\directlua{SwitchColumn()}}

\begin{document}

\newsavebox{\aaa}
\begin{lrbox}{\aaa}
Hello World!
\switchcolumn


X\makebox[0.5\textwidth]{\unhbox\aaa}X
\end{document}