unusual behaviour of \if(x)

unusual behaviour of \if(x)

[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

or

abc false breaks
! Extra \fi.

system          > tex > error on line 7 in file iftest.tex: Extra  ...

 1     \starttext
 2
 3     \ifx\hbox\undefined
 4       \message{invisible to tex}
 5       \newif\ifabc \abcfalse
 6       \ifabc \message{abc true breaks} \else \message{abc false breaks} \fi
 7 >>  \fi
 8
 9     \stoptext
10

l.7 \fi

Mojca
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Re: unusual behaviour of \if(x)

[Wolfgang Schuster]

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 from \ifabc which isn’t defined and
ignored by TeX, the last \fi is left which generates the error message

Wolfgang

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Re: unusual behaviour of \if(x)

[luigi scarso]

On Wed, Jun 1, 2011 at 3:26 PM, Mojca Miklavec
<mojca.miklavec.lists@gmail.com> wrote:
> 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
(see wolfgang)
this works

\newif\ifabc
\starttext

\ifx\dhbox\undefined
 \message{dhbox invisible to tex}
   \abcfalse
   \ifabc \message{abc true breaks} \else \message{abc false breaks} \fi
\fi


\ifx\hbox\undefined
 \message{hbox invisible to tex}
   \abcfalse
   \ifabc \message{abc true breaks} \else \message{abc false breaks} \fi
\fi


\stoptext



-- 
luigi
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___________________________________________________________________________________

Re: unusual behaviour of \if(x)

[Mojca Miklavec]

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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___________________________________________________________________________________

Re: unusual behaviour of \if(x)

[Hans Hagen]

On 1-6-2011 3:46, luigi scarso wrote:
> On Wed, Jun 1, 2011 at 3:26 PM, Mojca Miklavec
> <mojca.miklavec.lists@gmail.com>  wrote:
>> 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
> (see wolfgang)
> this works
>
> \newif\ifabc
> \starttext
>
> \ifx\dhbox\undefined
>   \message{dhbox invisible to tex}
>     \abcfalse
>     \ifabc \message{abc true breaks} \else \message{abc false breaks} \fi
> \fi
>
>
> \ifx\hbox\undefined
>   \message{hbox invisible to tex}
>     \abcfalse
>     \ifabc \message{abc true breaks} \else \message{abc false breaks} \fi
> \fi

hm, you want to start an \ifx pissing contest ... ?

\ifx\hbox\undefined
     \message{invisible to tex}
     \newif\ifabc \abcfalse
     \ifcsname ifabc\endcsname \message{abc true breaks} \else 
\message{abc false breaks} \fi
\fi



-----------------------------------------------------------------
                                           Hans Hagen | PRAGMA ADE
               Ridderstraat 27 | 8061 GH Hasselt | The Netherlands
     tel: 038 477 53 69 | voip: 087 875 68 74 | www.pragma-ade.com
                                              | www.pragma-pod.nl
-----------------------------------------------------------------
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If your question is of interest to others as well, please add an entry to the Wiki!

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___________________________________________________________________________________

Re: unusual behaviour of \if(x)

[luigi scarso]

On Wed, Jun 1, 2011 at 3:56 PM, Hans Hagen <pragma@wxs.nl> wrote:
> On 1-6-2011 3:46, luigi scarso wrote:
>>
>> On Wed, Jun 1, 2011 at 3:26 PM, Mojca Miklavec
>> <mojca.miklavec.lists@gmail.com>  wrote:
>>>
>>> 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
>>
>> (see wolfgang)
>> this works
>>
>> \newif\ifabc
>> \starttext
>>
>> \ifx\dhbox\undefined
>>  \message{dhbox invisible to tex}
>>    \abcfalse
>>    \ifabc \message{abc true breaks} \else \message{abc false breaks} \fi
>> \fi
>>
>>
>> \ifx\hbox\undefined
>>  \message{hbox invisible to tex}
>>    \abcfalse
>>    \ifabc \message{abc true breaks} \else \message{abc false breaks} \fi
>> \fi
>
> hm, you want to start an \ifx pissing contest ... ?
>
> \ifx\hbox\undefined
>    \message{invisible to tex}
>    \newif\ifabc \abcfalse
>    \ifcsname ifabc\endcsname \message{abc true breaks} \else \message{abc
> false breaks} \fi
> \fi


no, --- infact I've said "this works", not "it's the only solution" or
" it's the perfect solution".
\ifcsname...
looks a bit strange if I'm working with \newif .



-- 
luigi
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___________________________________________________________________________________

Re: unusual behaviour of \if(x)

[Hans Hagen]

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.


-----------------------------------------------------------------
                                           Hans Hagen | PRAGMA ADE
               Ridderstraat 27 | 8061 GH Hasselt | The Netherlands
     tel: 038 477 53 69 | voip: 087 875 68 74 | www.pragma-ade.com
                                              | www.pragma-pod.nl
-----------------------------------------------------------------
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maillist : ntg-context@ntg.nl / http://www.ntg.nl/mailman/listinfo/ntg-context
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___________________________________________________________________________________

Re: unusual behaviour of \if(x)

[Wolfgang Schuster]

Am 01.06.2011 um 16:01 schrieb luigi scarso:

> no, --- infact I've said "this works", not "it's the only solution" or
> " it's the perfect solution".
> \ifcsname...
> looks a bit strange if I'm working with \newif .

And he’s cheating because he tests only the existence of the \ifabc command.

It’s easier to understand with the \ifdefined:

\ifx\hbox\undefined
   \newif\ifabc \abcfalse
   \ifdefined\ifabc … \else … \fi
\fi

Wolfgang

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___________________________________________________________________________________

Re: unusual behaviour of \if(x)

[luigi scarso]

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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