Message-Id: <9309151250.AA23123@sc.ZIB-Berlin.DE>
Reply-To: Mailing list for the LaTeX3 project <LATEX-L@vm.urz.Uni-Heidelberg.de>
Date:         Wed, 15 Sep 93 14:14:38 CET
From: "Erik-Jan Vens" <erikjan@OBELIX.ICCE.RUG.NL>
Sender: Mailing list for the LaTeX3 project <LATEX-L@vm.urz.Uni-Heidelberg.de>
To: Multiple Recipients of <LATEX-L@vm.urz.Uni-Heidelberg.de>
Subject:      Re: when is a bug a bug? or is it? (fwd)
Status: R


Hi,

  some time ago I provided Barbara Beeton with an example where LaTeX seemed
to be getting in an infinite loop. I suspected it had to do with TeX the
program, but yesterday I found the answer, and Barbara asked me to pass it on
to you:

dixit bbeeton:

> well, that's a good one!
> shows the dangers of redefining primitives.
> this should definitely be passed on to the latex3 folks -- would
> you do that, please?  (cc me, though -- i'd like to hear the
> answer.)
> i'm going to hand this over to my reliable bug-checker for his
> opinion.  it may be desirable to report this to knuth even though
> it's a bad thing in latex and a possible "misuse" of the facilities.

This was what I did:

\documentstyle{article}

\begin{document}

\begin{tabular}{rl}
\ &$\vbox to 2\ht\strutbox{\ \vfill}$\\
\end{tabular}

\end{document}

Don't ask me why I did just this, it was part of a much larger structure, but
this was the minimal example I could make that seemed to hang TeX. Now,
Barbara told me that DEK only accepts plain input, so I rewrote it in TeX:

\vbox{\halign{\hfil#&#\hfil\cr
\ &$\vbox to 2\ht\strutbox{\ \vfill}$\cr}}
\bye

But TeX happily created a DVI file. It was much later that I tried a
\tracingall (yes, I always throw in the whole weight). This reveiled that
LaTeX redefined \par. So I inserted this redefinition in the TeX example:

\vbox{\halign{\hfil#&#\hfil\cr
\ &\let\par\relax$\vbox to 2\ht\strutbox{\ \vfill}$\cr}}
\bye

And of course, TeX now also gets in an infinite loop: for the \vfill TeX has
to be in vertical mode, so it inserts a \par. The \par is redefined to
\relax, so it doesn't do anything. Now it is time to examine the \vfill again,
which needs vertical mode, so TeX inserts a \par. This is an endless loop.

EJee.

--

Erik-Jan Vens.  E.J.Vens@icce.rug.nl