Re: What does "\c!" means?

[Wolfgang Schuster <schuster.wolfgang@googlemail.com>]

Am 15.03.2009 um 16:15 schrieb Wei-Wei Guo:

> I just finished reading the 'System_Macros'. I still don't get the  
> meaning
> of '\??pb @lang@' in the following codes:
>
>  \def\lang#1%
>   {\def\biblanguage{#1}%
>    \ifcsname \??pb @lang@#1\endcsname
>        \expanded{\mainlanguage[\getvalue{\??pb @lang@#1}]}%
>        \expanded{\language[\getvalue{\??pb @lang@#1}]}%
>     \fi \ignorespaces}
>
> In \lang[zh], for example, '\??pb @lang@' will be expanded to  
> \@@pblangen,
> will it? Is \??pb a command? Is '\??pb @lang@' a command? Why can  
> there be
> a whitespace in '\??pb @lang@'?

\lang{zh} forms in the \ifcsname ... \endcsname the command  
\@@pp@lang@zh
and looks if the commands is defined, if this is true the language is
switched to chinese.

> Sorry for some many questions. I'm so confused and don't know what to
> search in wiki, manuals, etc..

The \??bp at the begin of the macro is ConTeXt's system to create a  
namespace.

To understand I have to go a little bit away from the above macro and  
will
explain it on ConText's command to create key-val list. With the  
internal
command \getparameters is used to create key-val-lists, this is done  
with

   \getparameters[mycommand][width=1cm,height=2cm,align=right]

This creates the commands \mycommandwidth with the value 1cm,  
\mycommandheight
with the value 2cm and \mycommandalign with the value right. The text  
in the
first brace is the namespace which is used as prefix for each created  
command.
To optimize this operation and save a little bit of TeX's register we  
replace
'mycommand' with '\mycommand' which expand itself \mycommand, to make  
this
system more consystem ConTeXt use the convetion to use namespaces with  
two
question marks and two (or more) letter like the above \??bp, you can  
more
of this tricks in a article from Hans [1].

The space after '\??bp' and the \lang macro above is necessary because  
TeX
thinks otherwise '@lang@' is a part of the '\??bp' command and to  
prevent
this Taco puts the space there.

As a normal user you don't want to write macros or as beginner in  
ConTeXT
programming you don't have to care about this but it's good to know  
why are
things done in this way to understand system macros.

[1] http://www.ntg.nl/maps/22/27.pdf

Wolfgang

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