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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?

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3 Answers 3

Reset to default
5 
\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}.)

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  • Ah, very short. Must have missed it when looking up \savebox etc... But it doesn't work before \begin{document} though (but I could defer it using AtBeginDocument) Commented Jan 15, 2011 at 3:17
  • @pascal: That can be fixed. See my edits above. I think it would be safe to forgo the usage of \newsavebox and just use \setbox0 as in TH's answer, even without the extra group that he uses – since box registers with numbers below 10 are reserved for temporary use. Commented Jan 15, 2011 at 10:10
  • Perfect. I used \long\def\ignoreoutput#1{\setbox0\vbox{\everypar{}#1}} and it works. (since I use local macros in the ignored command, I prefer manual \gdef) Commented Jan 15, 2011 at 18:03
  • Good call on the \everypar{}. Commented Jan 15, 2011 at 21:00
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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.

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  • thanks, didn't know about \globaldefs or temporary boxes. Commented Jan 15, 2011 at 18:03
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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.

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