How could I use \input within soul arguments?

Question

So, I basically want to do something like this:

\documentclass{article}
\usepackage{soul}
\begin{document}

\so{Hello}

\so{\input{key.tex}}

\end{document}

Now the first "so" works but the second does not. It does not find the file no matter how I specify the file location. Why does this not work and how could I fix it? Perhaps modifying the soul source code but what should I modify?

I would rather use the soul to do this because this is continuation from How to make overlaid text?.

Answer 1

The soul package scans its argument and doesn't typeset it directly, therefore the content isn't inserted by \input. You need to read the file content manually and feed it to \so (or any other soul macro). This can be done easily using the catchfile package:

\documentclass{article}
\usepackage{catchfile}
\usepackage{soul}
\begin{document}

\CatchFileDef\filecontent{key.tex}{}
\expandafter\so\expandafter{\filecontent}

\end{document}

If you need this more often define a macro for this:

\newcommand{\soinput}[1]{%
    \begingroup
    \CatchFileDef\filecontent{#1}{}%
    \expandafter
    \endgroup
    \expandafter\so\expandafter{\filecontent}%
}
% ...
\soinput{key}

Note: The group is there to keep the assignment of \filecontent local and avoid any name clash if this macro is used somewhere else.