Re: unusual behaviour of \if(x)

From: Hans Hagen <pragma@wxs.nl>
To: mailing list for ConTeXt users <ntg-context@ntg.nl>
Cc: Mojca Miklavec <mojca.miklavec.lists@gmail.com>
Subject: Re: unusual behaviour of \if(x)
Date: Wed, 01 Jun 2011 16:08:03 +0200	[thread overview]
Message-ID: <4DE647C3.3010209@wxs.nl> (raw)
In-Reply-To: <BANLkTi==B0TNvWABV+kNShhEu1aca0=NbQ@mail.gmail.com>

On 1-6-2011 3:52, Mojca Miklavec 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.

Actually the code does what it is supposed to do .. tex's scanner does a 
fast scan for \else\fi and nested \if's so when you define one in a to 
be skipped branch you're on your own.

The reason why plain tex behaves a bit different is that there \newif is 
defined as outer. Actually, you might like to stare for a while at the 
definition of \newif, especially if you love catcodes.

All language have their specialities, like

local l = {
     ["in"] = "in",
     out    = "out",
}

in Lua.

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