How to output math environment code alongside typeset math?

Question

I have a preamble which defines lots of symbols and a LaTeX document for testing them. In particular, I have source like this:

\[\wlength{abc} = 3\]
\[\lang{A} = \lang{\varphi}\]
\[\kplus{L} \defeq \kstar{L} \setminus \emptyword\]

Which faithfully creates this:

Now I would like to use the same PDF also as a reference which requires the source code to show up. That is, I want something like that:

Ideally, what I did by hand here (copy source into a Verbatim environment from fancvrb) would be done automatically; why write the same thing down twice, blowing up everything?

Can this be done and if, how? I guess you would have to rewrite how math environments are translated, but that is beyond me.

Solutions using pdflatex are preferred.

Answer 1

For short math you can use the following. It might not give you good results for longer equations.

\documentclass{article}

\let\mopen\[
\let\mclose\]

\def\[#1\]{%
    \par\noindent
    \framebox[\linewidth]{\texttt{\detokenize{#1}}}%
    \mopen #1 \mclose
}


\begin{document}
\[a = b^C_D \]
\[A = \varphi\]
\end{document}

---

For left aligned output you could use:

\documentclass{article}

\def\[#1\]{%
    \par\medskip\par\noindent
    \framebox[\linewidth][l]{\texttt{\detokenize{#1}}}%
    \par\smallskip
    \(\displaystyle #1 \)
}


\begin{document}
\[a = b^C_D \]
\[A = \varphi\]
\end{document}

---

Update:

Here a real verbatim implementation which doesn't add spaces after macros. It goes the other way around. Instead of reading the code normally and then turning it into verbatim, it reads it verbatim and then turns it back to code when required.

I also added line breaking support.

\documentclass{article}
\usepackage{amsmath}

\makeatletter
\def\[{%
    \begingroup
    \let\do\@makeother
    \dospecials
    \obeyspaces
    \readmath
}
\long\expandafter\def\expandafter\readmath\expandafter#\expandafter1\string\]{%
    \endgroup
    \par\medskip\par\noindent
    \fbox{\minipage{\dimexpr\linewidth-2\fboxsep-2\fboxrule\relax}\raggedright\verbatim@font #1\endminipage}%
    \par\smallskip
    \(\displaystyle \scantokens{#1} \)%
    \par
}
\makeatother

\begin{document}

\[a = b^C_D \]

\[A = \varphi\]


\[a = b^C_\text{i} \]


\[ a = 
\alpha_1 + \beta_1 + \gamma_1 +
\alpha_2 + \beta_2 + \gamma_2 +
\alpha_3 + \beta_3 + \gamma_3 +
\alpha_4 + \beta_4 + \gamma_4
\]

\end{document}

---

This all requires e-TeX, which is part of any modern LaTeX compiler. I could code an alternative if wanted.