How is it possible to put hidden data in LaTeX?

Question

In my LaTeX document, I would like to put hidden data, like a marker or tag. The idea behind it is to be able to return an explicit error message. For example in this latex file, I would like to put a tag or marker where the comment line is with stars:

\documentclass[12pt]{article}

\usepackage[parfill]{parskip}
\usepackage[none]{hyphenat}
\sloppy
\usepackage[usenames,dvipsnames]{xcolor}
\usepackage{array,ragged2e, calc}
\usepackage{enumitem}
\usepackage{lipsum}
\usepackage{needspace}

\definecolor{shadecolor}{RGB}{217,217,217}  % Color used for highlighting
\definecolor{light-blue}{RGB}{0,175,236}    % Color for the footer

% ------------------------------------------------------------------------------------
% Header and footer management.
% ------------------------------------------------------------------------------------
\usepackage{fancyhdr}
\pagestyle{fancy}

% clear any old style settings
\fancyhf{}
\fancyheadoffset{0in}
\headheight = 53pt
\renewcommand{\headrulewidth}{0pt}

% ------------------------------------------------------------------------------------
% Defining the section style
\newcommand{\mysectionstyle}[1]{\colorbox{shadecolor}{\begin{tabular}{>{}p{\rectanglelength}}{\fontsize{13}{6}\selectfont\textbf{#1}}\end{tabular}}\vspace{6pt}}
% ------------------------------------------------------------------------------------
\addtolength{\footskip}{0.6cm}
\renewcommand{\footrulewidth}{1pt}
\renewcommand{\footrule}{{\color{light-blue}%
\vskip-\footruleskip\vskip-\footrulewidth
\hrule width\headwidth height\footrulewidth\vskip\footruleskip}}
% ------------------------------------------------------------------------------------

\usepackage[left=0.75in,top=1.5in,right=0.75in,bottom=1in]{geometry} % Document margins
\usepackage{titlesec}

\newlength{\rectanglelength}
\setlength{\rectanglelength}{\textwidth}
\addtolength{\rectanglelength}{-6pt}

\newlength{\foo}

\begin{document}
\lipsum[1]

\settototalheight{\foo}{\parbox[t]{\linewidth}{\begin{minipage}[t]{\linewidth} \begin{tabular}{@{}>{\RaggedRight}p{11cm}>{\RaggedLeft}p{6.35cm}@{}}\fontsize{13}{6}\selectfont{LaTeX}&\textbf{2011 to 2012}\end{tabular}\\\begin{tabular}{@{}>{\RaggedRight}p{11cm}>{\RaggedLeft}p{6.35cm}@{}}\textbf{Lead Programmer}&\textbf{}\end{tabular}%
\vspace{3pt}\begin{itemize}[parsep=0pt, topsep=0pt, itemsep=0pt, leftmargin=2ex]\item \lipsum[1]\item \lipsum[1]\item \lipsum[1]\item \lipsum[1]\item \lipsum[1]\item \lipsum[1]\item \lipsum[1]\item \lipsum[1]\item \lipsum[1]\end{itemize}%
\vspace{7pt}\textbf{End Title}\\\lipsum[1]

\vspace{10pt}\end{minipage}}
}
\needspace{\foo}{\setlength{\parskip}{4pt}

% ******* I WANT TO INSERT A TAG OR MARKER HERE.*******
\mysectionstyle{Experiences}

\begin{tabular}{@{}>{\RaggedRight}p{11cm}>{\RaggedLeft}p{6.35cm}@{}}\fontsize{13}{6}\selectfont{LaTeX}&\textbf{2011 to 2012}\end{tabular}\\\begin{tabular}{@{}>{\RaggedRight}p{11cm}>{\RaggedLeft}p{6.35cm}@{}}\textbf{Lead Programmer}&\textbf{}\end{tabular}%
\vspace{3pt}\begin{itemize}[parsep=0pt, topsep=0pt, itemsep=0pt, leftmargin=2ex]\item \lipsum[1]\item \lipsum[1]\item \lipsum[1]\item \lipsum[1]\item \lipsum[1]\item \lipsum[1]\item \lipsum[1]\item \lipsum[1]\item \lipsum[1]\end{itemize}%
\vspace{7pt}\textbf{End Title}\\\lipsum[1]

\vspace{10pt}}
\end{document}

I want to insert a tag or a marker into the LaTeX file so when an error occurred, I can retrieve the marker and then return a user-friendly message. Something like, there's an error in section X.

Is it possible in LaTeX? If yes, how can I do that?

Answer 1

You can just make a definition such as

\def\thissection{section X}

then in error messages that you control you can use that command

\PackageError{mypackage}{You made a mistake in \thissection: try harder}{}

and it will pick up whatever is the current definition.

However you can not customise the error handler of TeX itself. If for example the user mistypes a command \fooobar TeX will issue its undefined command error and you can not customise the error message it uses.

Answer 2

I'm going off the use case described in your last paragraph:

I want to insert a tag or a marker into the LaTeX file so when an error occurred, I can retrieve the marker and then return a user-friendly message. Something like, there's an error in section X.

The \show command will halt typesetting and print to the terminal a description of the next token. Similarly, \showthe will halt and print the value of a counter.

When typesetting halts, the i command will allow you to insert tokens for processing.

Putting these together, you can write a macro which shows the section number and call for it on errors.

\documentclass{article}
\usepackage{lipsum}

\makeatletter
\newcommand{\showsection}{%
   \showthe\c@section 
}
\makeatother

\begin{document}

\section{First}

\lipsum

\section{Second}

\lipsum

\foo% undefined cs

\section{Third}

\lipsum

\end{document}

Here is a sample console session:

! Undefined control sequence.
l.20 \foo
         % undefined cs
? i \showsection
> 2.
\showsection ->\showthe \c@section 

l.20 \foo
         % undefined cs
? s
OK, entering \scrollmode...

I typed i \showsection on the terminal input line and it printed 2. You can prettify the output as you like.

As far as how to "tag" the document I would use what's already in place and hook into those macros. For instance, noticed I used the existing counter for section numbers rather than created something new. If I wanted to print the section name I could add commands to the \section macro that would save the section title to a macro and \show that instead. (I didn't do this because if you read the source you will see that \section is not a simple macro.)