Insert negative of \widthof into \hspace without defining a newlength

Question

Following MWE does not work:

\documentclass{article}
\usepackage{calc}
\newlength{\charwidth}
\setlength{\charwidth}{\widthof{$-$}}
\begin{document}
\centering $\infty$

\centering \hspace{-\charwidth}$-\infty$

\centering \hspace{-\widthof{$-$}}$-\infty$
\end{document}

\hspace{-\widthof{$-$}} throws following error

! Missing number, treated as zero. <to be read again> \widthof l.10 \centering \hspace{-\widthof{$-$}} $-\infty$ ?

Is there a possibility to insert negative \widthof into \hspace without defining a newlength?

Answer 1

This answer is valid in TexLive 2019 and 2013, but not in 2016. Use it at your own risk.

I just tweaked @DavidCarlisle answer. Instead of just defining the -widthof{$-$}, I substracted it from zero 0pt-widthof{$-$} (it should yield the same ;))

So here is the way to get negative lengths from the \width_ commands \setlength{\dimen0}{0pt-\widthof{$-$}}\hspace{\dimen0}

So your code would look like that

\documentclass{article}
\usepackage{calc}
\newlength{\charwidth}
\setlength{\charwidth}{\widthof{$-$}}
\begin{document}
   \centering $\infty$
\centering \hspace{-\charwidth}$-\infty$
\centering \setlength{\dimen0}{0pt-\widthof{$-$}}\hspace{\dimen0}$-\infty$
\end{document}

Technically no new length is defined as David pointed out, although the scratch register is used.

God bless you, guys.

Edition: result image added