On Wed, Jun 1, 2011 at 15:36, Wolfgang Schuster wrote:
>
> Am 01.06.2011 um 15:26 schrieb Mojca Miklavec:
>
>> Dear magitians,
>>
>> I discovered this in LaTeX, but ConTeXt behaves the same (plain TeX
>> behaves differently). Can somebody please explain me why this code
>> fails to work? (A workaround is to move \newif on top which I'm
>> willing to do, but I'm still curious.)
>>
>> \starttext
>>
>> \ifx\hbox\undefined
>> \message{invisible to tex}
>> \newif\ifabc \abcfalse
>> \ifabc \message{abc true breaks} \else \message{abc false breaks} \fi
>> \fi
>>
>> \stoptext
>>
>>
>> systems : begin file iftest at line 1
>> abc false breaks
>> ! Extra \fi.
>> l.7 \fi
>
> The \ifx ends with the first \fi
I figured that out, but I still find it very weird.
> from \ifabc which isn’t defined and
> ignored by TeX
This part somehow makes sense (TeX should not keep defining stuff and
writing out stuff) ...
> the last \fi is left which generates the error message
The code works when using \ifx\hboxx\undefined ... But despite the
explanation, this is very very very weird. Or to quote Taco: "The
trick of TeX being bug-free is that all the bugs and limitations are
documented." I hope that this behaviour is documented in TeXbook.
To some extent this resembles
/* I want to comment out this code
c = a + b;
/* a more complex equation */
c = a * b;
*/
but it is not quite the same.
Mojca
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