How to ignore everything in the document environment?

Question

Yes, you did read the title correctly!

I want to ignore everything that is between \begin{document} and \end{document} when I \input a file. I thought that that would be very simple, just use the environ package:

\RenewEnviron{document}{}

But it turn out not be that easy. The MWE below yields the correct results:

but only with the comments intact. If you un-comment the commented lines then end up with:

LaTeX Error: \env@document@save@env undefined

Interestingly, ignoring this error one can continue past this, and still get the correct results. :-)

Notes:

• I realize that in there can be other things in the preamble, so this is by no means a general solution. For the places where I need this functionality, the preamble only has \documentclass and two \input/\usepackage which loads all the .sty files needed so this will suffice for my needs.

Code:

\documentclass{article}
\usepackage{environ}
\usepackage{standalone}

\newtoks{\MyToken}

\usepackage{filecontents}
\begin{filecontents*}{foo.tex}
    \documentclass{standalone}
    \usepackage{MyStandardPackages}

    \MyToken={foo}

%   \begin{document}
%       lots of text here
%
%       \SetSomeVarable{\SomeVar}{Some Value}
%       
%       \begin{SomeEnvironment}
%           lots more stuff here as well
%       \end{SomeEnvironment}
%   \end{document}
\end{filecontents*}

\MyToken={XXX}


\newcommand{\DisablePreamble}{%
    \renewcommand{\documentclass}[2][]{}%  remove def'n of \documentclass
    \renewcommand{\usepackage}[2][]{}%     ignore any \usepackage{}
%   \RenewEnviron{document}{}% Ignore everything within the "document" environment.
}%

\newcommand*{\ExtractMyToken}[1]{%
    \begingroup% What happens in the group stays in the group!
        \DisablePreamble%
        \input{#1}%
        \global\MyToken=\expandafter{\the\MyToken}%
    \endgroup%
}%

\begin{document}
MyToken=\the\MyToken

\ExtractMyToken{foo}

MyToken=\the\MyToken
\end{document}

Answer 1

This is a common problem with environ, and the way I get around it is to define those commands it has a problem with:

\makeatletter
\providecommand{\env@document@save@env}{}% To keep environ happy
\providecommand{\env@document@process}{}%
\RenewEnviron{document}{}% Ignore everything within the "document" environment.
\makeatother

A quick-and-dirty way of achieving what you're after is to use

\renewcommand{\document}{\endgroup\endinput}

which terminates the reading of the \input document as soon as it reaches \begin{document}.

Answer 2

I'd (locally) define

\def\begin#1{\endinput}`

so the file stops at that point, or perhaops (I didn't understand your use case really

 \def\documentclass[#1]#2{\endinput}

and stop even earlier (but if you are doing that just not inputting the file would be easier)