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

On Wed, Jun 1, 2011 at 3:52 PM, 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.

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

On 1-6-2011 3:46, luigi scarso 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)

On Wed, Jun 1, 2011 at 3:26 PM, Mojca Miklavec wrote: 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 -----------------------------------------------------------------

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