# Conditionals—do something, if a given command is being used in the document

Is it possible to have a conditional that allow us to do something if a given command is being used in the document? For example for the \part command would be:

\@ifcommandisbeingused{\part}{<then>}{<else>}


## Clarification

The following would avoid the \bfseries for the chapter entry in the TOC, because the \part command is being used in the document.

\documentclass{book}

\makeatletter

\@ifcommandisbeingused{\part}
{
\renewcommand*\l@chapter[2]{%
\ifnum \c@tocdepth >\m@ne
\vskip 1.0em \@plus\p@
\setlength\@tempdima{1.5em}%
\begingroup
\parindent \z@ \rightskip \@pnumwidth
\parfillskip -\@pnumwidth
\leavevmode
\hskip -\leftskip
#1\nobreak\hfil \nobreak\hb@xt@\@pnumwidth{\hss #2}\par
\penalty\@highpenalty
\endgroup
\fi}
}{}

\makeatother

\begin{document}

\part{Lorem}
\chapter{Ipsum}

\end{document}


P.S.

I've changed \@ifcommandIsBeingUsed to \@ifcommandisbeingused 'cause I've remembered that in TeX/LaTeX language is common to use uppercase letters only in "very low level" macros.

• Note that usage of such a command would at least require two compiles since you may want to use \@ifcommandIsBeingUsed before it is actually used... – Werner Mar 17 '14 at 20:55
• LaTeX is turring-complete, so yes. I recently used the ifthen package in an answer here: tex.stackexchange.com/questions/165873/… – Argo Mar 17 '14 at 21:23
• To clarify, you wish this command to discern whether a macro has actually been invoked, and not just if it has been defined. Is that correct? – Steven B. Segletes Mar 17 '14 at 22:17
• Also, when you say "is being used", do you mean "will be used at some point" or "has been used to this point" in the document? – Steven B. Segletes Mar 17 '14 at 22:41
• @StevenB.Segletes I hope my update will answer your questions. – Aurelius Mar 17 '14 at 23:34

This solution creates a file \jobname.use which tracks, which commands have been used. You add a command to the list by doing \ifused\<command>{<hook>}. In the example I used \patchcmd from etoolbox to remove \bfseries from \l@chapter. Doing \ifused twice for the same command will produce errors.

\documentclass{book}
\usepackage{xparse,etoolbox}
\ExplSyntaxOn

\file_if_exist:nT { \c_job_name_tl . use }
{ \file_input:n { \c_job_name_tl . use } }

\iow_new:N \l_cloud_used_stream
\iow_open:Nn \l_cloud_used_stream { \c_job_name_tl . use  }
\AtEndDocument { \iow_close:N \l_cloud_used_stream }

\cs_new_protected:Npn \cloud_is_used:N #1
{
\iow_now:Nx \l_cloud_used_stream
{ \exp_not:N \bool_set_true:c { l _ cloud _ if \cs_to_str:N #1 _ bool } }
}

\NewDocumentCommand{\ifused}{mm}
{
\preto #1 { \cloud_is_used:N #1 }
\iow_now:Nx \l_cloud_used_stream
{ \exp_not:N \bool_new:c { l _ cloud _ if \cs_to_str:N #1 _ bool } }
\bool_if:cT { l _ cloud _ if \cs_to_str:N #1 _ bool } { #2 }
}

\ExplSyntaxOff

\makeatletter
\ifused\part{\patchcmd\l@chapter{\bfseries}{}{}{}}
\makeatother

\begin{document}
\tableofcontents
\chapter{Before first part}
\part{First part}
\chapter{After first part}
\part{Second part}
\end{document}


• \preto =? \tl_put_left:Nn? – Manuel Sep 14 '14 at 14:19
• @Manuel Indeed that works, but it shadows the initial intention. Unfortunately there is no \cs_put_left:Nn. – Henri Menke Sep 14 '14 at 14:36
• \cs_put_left:Nn would be more like \pretocmd. I think \tl_put_left:Nn is fine here, but it's true what you say :) – Manuel Sep 14 '14 at 14:38
• Note that \c_job_name_tl has been replaced by \c_sys_jobname_str in the September 2015 release of expl3. – egreg Sep 30 '15 at 9:43