Commenting out lineno and disabling commands related to it via ltxcmds fails to compile unless I manually delete the aux file

Question

From a previous question, take this example

\documentclass{book}
\usepackage{lipsum}
\usepackage[left, pagewise]{lineno} % LINE
\usepackage{ltxcmds}
\makeatletter
\newcommand{\OnlyIfPackageLoaded}[2]{\ltx@ifpackageloaded{#1}{#2}{}}
\makeatother
\makeatletter
\OnlyIfPackageLoaded{lineno}{
\def\makeLineNumberLeft{%
  \linenumberfont\llap{\hb@xt@\linenumberwidth{\LineNumber\hss}\hskip\linenumbersep}% left line number
  \hskip\columnwidth% skip over column of text
  \rlap{\hskip\linenumbersep\hb@xt@\linenumberwidth{\hss\LineNumber}}\hss}% right line number
\leftlinenumbers% Re-issue [left] option
}
\makeatother
\begin{document}
\OnlyIfPackageLoaded{lineno}{\linenumbers}
\lipsum[1]
\end{document}

where \OnlyIfPackageLoaded is used to make lineno-related things disappear if I comment out its \usepackage.

Unfortunately:

if I compile from a clean directory with THAT line commented, the compilation succeeds;
if I decomment THAT line and recompile, the compilation succeeds;

if I re-comment THAT line and recompile, the compilation fails:

...
(./main.aux
! Undefined control sequence.
l.2 \@LN
        {0}{0}
?

if I delete the .aux file and recompile, the compilation succeeds.

What can I do to avoid having to manually delete the AUX file when I go from with-lineno to without-lineno?

it is very common that removing a package makes it necessary to delete the .aux . hyperref, longtable, .... — David Carlisle, Mar 04 '23 at 22:19
why ltxcmds rather than just use the standard latex \@ifpackageloaded ? — David Carlisle, Mar 04 '23 at 22:23

David Carlisle · Accepted Answer · 2023-03-04T23:01:17.747

You just need to not hide the else branch of \@ifpackageloaded so you can define the aux command to do nothing. (ltxcmds does nothing here as this is all already latex)

\documentclass{book}
\usepackage{lipsum}
\usepackage[left, pagewise]{lineno} % LINE
% not useful here \usepackage{ltxcmds}
\makeatletter
\newcommand{\OnlyIfPackageLoadedTF}{@ifpackageloaded}
\newcommand{\OnlyIfPackageLoadedT}[2]{@ifpackageloaded{#1}{#2}{}}
\makeatother
\makeatletter
\OnlyIfPackageLoadedTF{lineno}{
\def\makeLineNumberLeft{%
  \linenumberfont\llap{\hb@xt@\linenumberwidth{\LineNumber\hss}\hskip\linenumbersep}% left line number
  \hskip\columnwidth% skip over column of text
  \rlap{\hskip\linenumbersep\hb@xt@\linenumberwidth{\hss\LineNumber}}\hss}% right line number
\leftlinenumbers% Re-issue [left] option
}
% else
{
\def@LN#1#2{}
}
\makeatother
\begin{document}
\OnlyIfPackageLoadedT{lineno}{\linenumbers}
\lipsum[1]
\end{document}

What is \def\@LN#1#2{} for? And what are T and TF suffixes meant to stand for? — Enlico, Mar 05 '23 at 06:42
@Enlico as you show in your question the aux file has lines \@LN{...}{...} and you need to define them to do nothing. TF is a convention used by several newer latex commands mimicking expl3, the TF version has true and false arguments the T version is your original with just a true branch and no else — David Carlisle, Mar 05 '23 at 08:12

Commenting out lineno and disabling commands related to it via ltxcmds fails to compile unless I manually delete the aux file

1 Answers1