[XeTeX] newcommand with optional arguments

[Ross Moore]

Hi Manuel,

On 04/03/2008, at 11:36 AM, Manuel Pégourié-Gonnard wrote:

> Hi,
>
> Ross Moore a écrit :
>>


>> A more bullet-proof approach is:
>>
>> \makeatletter
>> \def\myempty@{}
>> \makeatother
>> \let\testtheargument\relax
>>
>
> \makataletter here ? To use \myempty at ...

Yes; so that the exceptional case becomes un-typeable,
at least in a normal LaTeX document.

>> \newcommand{\nuu}[2][]{%
>>    \def\testtheargument{#1}%
>>    \textit{#2}%
>>    \expandafter\ifx\expandafter\myempty@\testtheargument\myempty@
>
> Using \def and the expandafters is equivalent to using just #1,  
> with to
> problems added :
> - #1 should not contain any # now;

Yes, there can be an extra complication here,
depending upon how that # is intended to be used.
If, for example, it is as \# , then it has already
been tokenized, so it'll be OK; but others not so.


> - the test becomes non fully expandable;
> (which should not be a problem in this case, however).

Yes, again; another complication in some circumstances.

>
> If you choose to use a \def, the simpler and best solution is
> \def\arg at one{#1}
> \ifx\arg at one\@empty

Indeed; which is why I quickly followed my first post
with a correction.  :-)

>   \ETC.
>
>> But even this has the side-effect of leaving \testtheargument
>> with a value different from what it was before \nuu is encountered.
>>
> This should not be a problem, since \testtheargument should be used  
> only for this.

Yes, but when it comes to generalising the code for
other similar purposes, with more complicated testing
and processing, ...

>> This technicality is overcome using the following:
>>
> Useless, IMHO.

   ...  it can pay to be very careful.

>
>> And this may not be the end of the story; but enough for now.
>>
> Well, if you want to test wether an argument is empty or not, the  
> best is to
> use the ifmtarg package : it's reliable (no problems with \ifs or  
> string
> beginning with \empty) and fully expandable (this can be important).

Yes; that can well be.
Donald Arseneau writes good stuff; often needing to be studied
very carefully to understand exactly how/why it works.

> The only
> particularity is that one string consisting only of space tokens is  
> considered
> empty.
>
> By the way, all this discussion has little (if at all) to do with  
> XeTeX, and
> would surely be more natural in comp.text.tx or so...

True. But the question arose here, and was answered (incorrectly) here,
so a correction and further discussion should be made on this list.
It would be wrong to have the posting with the original answer as
the final message in this thread.

>
> Manuel.

Cheers,

	Ross

------------------------------------------------------------------------
Ross Moore                                         ross at maths.mq.edu.au
Mathematics Department                             office: E7A-419
Macquarie University                               tel: +61 +2 9850 8955
Sydney, Australia  2109                            fax: +61 +2 9850 8114
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