Retain side-effects but discard command output

Question

The following code works, although I have a feeling my \settowidth method of discarding the output is not how it should be done:

\documentclass{article}
\newlength{\trashlength}
\long\def\ignoreoutput#1{%
 \settowidth{\trashlength}{#1}%
}
\long\def\command#1{
 This \gdef\stuff{Stored: #1} shouldn't be here.
}
%% Should not raise Missing \begin{document} error here:
%\ignoreoutput{\command}
\begin{document}
 % Should produce "nothing":
 no\ignoreoutput{\command{Side %\par %%%%% this breaks everything.  
 effect!}}thing.

 % Should output the text previously stored:
 \ifx\stuff\undefined No side effect.
 \else \stuff \fi
\end{document}

When uncommenting the \par though, it breaks: with Paragraph ended before \@settodim was complete.

So how do I ignore commands that allow long input?

Answer 1

\newsavebox{\trashbox}
\long\def\ignoreoutput#1{\setbox\trashbox\vbox{\everypar{}\globaldefs1 #1}}

seems to work.

(Edit: make assignments global as per TH's answer, and locally disable LaTeX's \everypar-based mechanism for complaining about a missing \begin{document}.)

Answer 2

I would use something similar to Harald's example (which appeared just before I posted this).

\long\def\executeglobally#1{
        \begingroup
        \setbox0\vbox{
                \globaldefs1
                #1%
        }%
        \endgroup
}

\def\foo{foo}
\executeglobally{asdf\def\foo{bar}asdf}
\foo
\bye

The \globaldefs1 makes assignments global.

Answer 3

Just tried this, but it seems even fishier:

\documentclass{article}
\newlength{\trashlength}
\newsavebox{\trashbox}
\long\def\ignoreoutput#1{%
    \sbox{\trashbox}{#1}%
    \settowidth{\trashlength}{\usebox{\trashbox}}%
}
% etc like above

But it can break with Dimension too large.