Is there a symbol that is \longto minus the tip

Question

I wonder if there is a horizontal line math symbol that

• has the same length as \longto, \longmapsto, etc but does not contain any arrow tips, and
• is a \mathrel?

I am currently using \mathrel{-\!-} and have difficulty figuring out the length of the line.

Answer 1

If one ignore the broken \longmapsto length under \scriptscriptstyle, this should do:

\documentclass{article}
\DeclareDocumentCommand{\longdash}{}{%
  \mathrel{\relbar\mkern-6.5mu\relbar\mkern-6.5mu\relbar}}
\begin{document}
$a \longmapsto b_{a \longmapsto b_{a \longmapsto b}}$
$a \longdash b_{a \longdash b_{a \longdash b}}$
\end{document}

Answer 2

Here's a way that also avoids the hole in \scriptscriptstyle:

\documentclass{article}
\makeatletter
\DeclareRobustCommand{\longdash}{%
  \mathrel{\mathpalette\longvdash@\relax}%
}
\DeclareRobustCommand{\longvdash}{\mapstochar\longdash}
\newcommand{\longvdash@}[2]{%
  \begingroup
  \sbox\z@{$\m@th#1\longrightarrow$}%
  \hb@xt@\wd\z@{\dashfill{#1}}%
  \endgroup
}
\newcommand{\dashfill}[1]{% modelled on \rightarrowfill
  $\m@th#1%
  \smash-\mkern-7mu
  \cleaders\hbox{$#1\mkern-2mu\smash-\mkern-2mu$}\hfill
  \mkern-7mu\smash-$%
}
\makeatother
\begin{document}
$a \longrightarrow b_{a \longrightarrow b_{a \longrightarrow b}}$
$a \longmapsto b_{a \longmapsto b_{a \longmapsto b}}$
$a \longdash b_{a \longdash b_{a \longdash b}}$
$a \longvdash b_{a \longvdash b_{a \longvdash b}}$
\end{document}