[XeTeX] ifxetex package

[Will Robertson]

Hi Ross,

I think we've started off on the wrong foot.
Here's the package in its entirety:

%%%%%%%%%%%%%%%%%%%%%%%%%%
\ifdefined\ProvidesPackage
   \ProvidesPackage{ifxetex}
     [2006/06/12 v0.1 Provides ifxetex condition]
\fi

\unless\ifdefined\ifxetex
   \newif\ifxetex
\fi

\ifdefined\XeTeXversion
   \xetextrue
\fi
%%%%%%%%%%%%%%%%%%%%%%%%%%

(Your comments about \ifdefined are addressed below.)

On 19/06/2006, at 10:54 , Ross Moore wrote:

>> LaTeX itself ensures than a package is only loaded once, and for
>> Plain I've only added enough smarts that you at least shouldn't get
>> error messages.
>
> Yes, but that assumes the user tried to load using  XeLaTeX .
> Would this not also be useful for XeTeX (without LaTeX) ?
> In particular, it'd be desirable to have it working properly
> with
>    \input miniltx.tex
>    \input ifxetex.sty

Yes, as you can see it is written with this purpose in mind.

> Be careful here.
> \ifdefined is *not* a LaTeX command, nor a TeX one.

No, it's an eTeX command :)
I've bitten the bullet to state that anyone using this package will  
be using an eTeX-based "TeX" -- because all distributions are now  
shipping with pdfeTeX configured for everything, I feel this a  
reasonable assumption.

Anyway, pg 13 of etex-man (`texdoc etex`) has some nice goodies along  
these lines:

"For ε-TEX there are these additional conditionals:
• \ifdefined<token> (test if token is defined)
True if <token> is defined; creates no new hash table entry.
• \ifcsname...\endcsname (test if control sequence is defined)
True if the control sequence \csname...\endcsname would be defined;  
creates
no new hash table entry.
  ...
These are ε-TEX’s additional expandable commands:
• \unless.
The next (unexpanded) token must be a boolean conditional (i.e., not
\ifcase); the truth value of that conditional is reversed."

I don't know if there's much point using
   \unless\ifdefined\XeTeXversion
compare to
   \ifdefined\XeTeXversion\else
but I don't mind the syntax so I've tried it out.

There's more in eTeX that I don't fully understand (e.g., when  
\unexpanded would be useful) but its extras are well worth  
investigating.

> Even better still (why ?) is:
>
>    \begingroup
>     \expandafter\ifx\csname XeTeXversion\endcsname\relax
>      \aftergroup\endinput\fi
>    \endgroup

Erm, something about not retaining the \XeTeXversion macro that's  
equivalent to \relax? Hang on...\endinput?

Anyway, the requirement for such convoluted stuff was the reason  
\ifdefined was created, as I understand it.

So anyway, given how easy it is to write \ifdefined\XeTeXversion, I'm  
not sure there's much point in having an ifxetex package.

All the best,
Will