\f@family in math mode

Question

I am using this answer on TeX.SX to get multi-letter variables in math mode, so that $abc$ typesets as $\mathit{abc}$, instead of being typeset as if it was the product of a, b and c.

Unfortunately, that code forces the \mathit even if another math font command was used, i.e. $\mathtt{abc}$ typesets as $\mathtt{\mathit{abc}}$. My goal is therefore to detect what is the current font, and apply \mathit to multi-letter variables only if the default math font is used.

I tried using \f@family as shown in this answer, but it does not seem to work in math mode:

\documentclass{article}
\begin{document}
\makeatletter
\newcommand{\showfont}{
  \message{encoding: \f@encoding,
    family: \f@family,
    series: \f@series,
    shape: \f@shape,
    size: \f@size
  }
}
\makeatother

$foo \showfont bar$
$\mathtt{foo \showfont bar}$

\end{document}

The file above shows in the log:

encoding: OT1, family: cmr, series: m, shape: n, size: 10
encoding: OT1, family: cmr, series: m, shape: n, size: 10

It seems to me that \f@family and the other \f@xxx commands show the text mode font, instead of showing the math mode font.

Is there an equivalent to \f@family which gives the information concerning the math mode font?

Answer 1

Not a command, a number: \the\fam as I set to 4 actually in my answer below to make things default to \mathit. However the number allocation is dynamic: it depends on the order that fonts are used in the document, as there are only 16 slots (in classic TeX) so if for example you declare \mathbb but never use it that font is never allocated a family number at all, they are just set up the first time they are used. In the code below I use \mathit first to get the number and then set that for the rest of the expression.

---

You are looking at the text font settings which are not used here, you just want to set the default family to that of \mathit

\documentclass{article}
\begin{document}




$ffoo  bar$
$ffoo \mathtt{ffoo  bar} bar$

\everymath{\mathit{\xdef\tmp{\fam\the\fam}\aftergroup\tmp}}\sbox0{$$}


$ffoo  bar$
$ffoo \mathtt{ffoo  bar} bar$


\end{document}

Answer 2

Thanks David Carlisle's answer to and comment, I built a solution for my original problem using \the\fam to detect the current math font, based on Heiko Oberdiek's answer:

\documentclass[fleqn]{article}

\makeatletter
\usepackage{etexcmds}
\newtoks\m@toks@math
\newtoks\m@toks@text

\edef\m@tmp@restore{%
  \lccode\number`\X=\the\lccode`\X\relax
  \lccode\number`\~=\the\lccode`\~\relax
}

\newcommand*{\m@activate}{}
\newcommand*{\m@check@letter}{}
\newcommand*{\m@check@fi}{}

\newif\ifm@single

\let\m@start\relax
\def\m@loop{%
  \lccode`\X=\count@
  \lccode`\~=\count@
  \lowercase{%
    \expandafter\mathchardef\csname m@code@X\endcsname=\mathcode\count@
    \edef\m@activate{%
      \etex@unexpanded\expandafter{\m@activate}%
      \mathcode\the\count@="8000\relax
      \def\noexpand~{\m@start\csname m@code@X\endcsname X}%
    }%
    \g@addto@macro\m@check@letter{%
      \ifx\@let@token X\else
    }%
  }%
  \g@addto@macro\m@check@fi{\fi}%
  \advance\count@\@ne
}
% A-Z
\count@=`\A\relax
\@whilenum\count@<\numexpr`\Z+1\relax\do{\m@loop}
% a-z
\count@=`\a\relax
\@whilenum\count@<\numexpr`\z+1\relax\do{\m@loop}

\newcommand*{\m@start}[2]{%
  \bgroup
  \m@toks@math{#1}%
  \m@toks@text{#2}%
  \m@singletrue
  \futurelet\@let@token\m@check
}
\edef\m@check{%
  \etex@unexpanded{%
    \let\m@next\m@add
    \ifx\@let@token\space
      \let\m@next\m@finish
    \else
      \ifx\@let@token\egroup
        \let\m@next\m@finish
      \else
  }%
  \etex@unexpanded\expandafter{%
        \m@check@letter
  }%
  \etex@unexpanded{%
          \let\m@next\m@finish
  }%
  \etex@unexpanded\expandafter{%
        \m@check@fi
  }%
  \etex@unexpanded{%
      \fi
    \fi
    \m@next
  }%
}

\newcommand*{\m@add}[1]{%
  \m@singlefalse
  \m@toks@math\expandafter{%
    \the\expandafter\m@toks@math
    \csname m@code@#1\endcsname
  }%
  \m@toks@text\expandafter{%
    \the\m@toks@text
    #1%
  }%
  \futurelet\@let@token\m@check
}

\newcommand*{\m@finish}{%
  \ifm@single
    \expandafter\mprintsingle\expandafter{%
      \the\expandafter\m@toks@math\expandafter
    }\expandafter{%
      \the\expandafter\m@toks@text\expandafter
    }%
  \else
    \expandafter\mprintmulti\expandafter{%
      \the\expandafter\m@toks@math\expandafter
    }\expandafter{%
      \the\expandafter\m@toks@text\expandafter
    }%
  \fi
  \egroup
}

% \let\mprintsingle\@firstoftwo
% \let\mprintmulti\@firstoftwo
\def\mprintsingle#1#2{{\color{blue}#1}}%
\def\mprintmulti#1#2{%
  % Test whether we are using the default math font or not:
  \ifnum\the\fam=-1\relax%
  \mathit{\color{red}#1}%
  \else%
  {\color{green!50!black}#1}%
  \fi%
}

\everymath{\m@activate}
\everydisplay{\m@activate}

\m@tmp@restore
\makeatother

\usepackage{xcolor}
\usepackage{amstext}

\begin{document}
\textcolor{blue}{Single letters are set in blue},
\textcolor{red}{multiple letters in red},
\textcolor{green!50!black}{multiple letters inside \texttt{\textbackslash mathtt}
 or \texttt{\textbackslash mathit} etc. in green}.

\[ F_{force (implicit\ mathit)} = m_\mathit{mass (explicit\ mathit)} * a_\mathtt{acceleration (explicit\ mathtt)} \]

Multiplication of single-letter variables
\[a f f b\]

Multiplication of multi-letter variables
\[aaa fff fff bbb\]

\end{document}