How to create an active tilde in LaTeX 3

Question

I have some LaTeX3 code that generates matrices and uses the hhline package internally. In LaTeX3 the ~ character is a special character, used for inserting spaces in strings, and using \c_tilde_str results in hhline ignoring the character and giving the warning:

Package hhline Warning: macro:->~ ignored in \hhline argument

I have tried various tricks such as defining

\cs_new_protected:Npn \__active_tilde { \~ }
\group_begin:
\char_set_active_eq:nN  {`~} \__active_tilde
\group_end:

and then using \__active_tilde but this does not work either.

Here is a minimal working example that exhibits my problem:

\documentclass{article}
\usepackage{hhline}

\usepackage{expl3}
\ExplSyntaxOn
\newcommand\mat{
\typeout{----------------------------------------}
  \begin{array}{cc} \hhline{~-} 1&2 \end{array}
\typeout{----------------------------------------}
  \begin{array}{cc} \hhline{\c_tilde_str-} 1&2 \end{array}
\typeout{----------------------------------------}
}
\ExplSyntaxOff

\begin{document}

$\mat$

\end{document}

This produces the output

with the overline over the 1's instead of the 2's. The log file contains the lines:

Package hhline Warning: macro:->~ ignored in \hhline argument
(hhline)                 on input line 24.

This shows that the first ~ is being ignored, as it should be since ~ is a space in expl code, and that the \c_tilde_str is not being recognised as a ~ by hhline.

[I know that the tabularray package gives other ways of doing this but it does not work in my real use-case because it does not support expansion in its key-value syntax -- and for the number of tables that I need to produce there may be other performance issues...]

Answer 1

Something like

\documentclass{article}
\usepackage{hhline}
\let\foo~
\usepackage{expl3}
\ExplSyntaxOn
\newcommand\mat{
\typeout{----------------------------------------}
  \begin{array}{cc} \hhline{\foo-} 1&2 \end{array}
\typeout{----------------------------------------}
  \begin{array}{cc} \hhline{\foo-} 1&2 \end{array}
\typeout{----------------------------------------}
}
\ExplSyntaxOff

\begin{document}

$\mat$

\end{document}