From: luigi scarso <luigi.scarso@gmail.com>
To: mailing list for ConTeXt users <ntg-context@ntg.nl>
Subject: Re: unusual behaviour of \if(x)
Date: Wed, 1 Jun 2011 16:10:32 +0200 [thread overview]
Message-ID: <BANLkTinCXZQ+QZy8V1U1Lo_+aqe0Z4OAGw@mail.gmail.com> (raw)
In-Reply-To: <BANLkTi==B0TNvWABV+kNShhEu1aca0=NbQ@mail.gmail.com>
On Wed, Jun 1, 2011 at 3:52 PM, Mojca Miklavec
<mojca.miklavec.lists@gmail.com> wrote:
> 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.
see TeX by Topic e.g.
13.7 Evaluation of conditionals
--
luigi
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next prev parent reply other threads:[~2011-06-01 14:10 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-06-01 13:26 Mojca Miklavec
2011-06-01 13:36 ` Wolfgang Schuster
2011-06-01 13:52 ` Mojca Miklavec
2011-06-01 14:08 ` Hans Hagen
2011-06-01 14:10 ` luigi scarso [this message]
2011-06-01 13:46 ` luigi scarso
2011-06-01 13:56 ` Hans Hagen
2011-06-01 14:01 ` luigi scarso
2011-06-01 14:09 ` Wolfgang Schuster
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