[texhax] Implementing control sequences with key values

[Uwe Lueck]

"Paul Isambert" <zappathustra at free.fr>, 14.11.2010 17:28:25
>Selon "Philip Taylor (Webmaster, Ret'd)" :
>> Wolfgang Schuster wrote:
>> > \def\direction#1=#2#3
>> >    {\def\currentdirection{#2#3}}
>> >
>> > \tracingmacros1
>> >
>> > \direction=LTR Direction: \currentdirection\par
>> > \direction=RTL Direction: \currentdirection\par
>> >
>> > \vskip\baselineskip
>> >
>> > \direction =LTR Direction: \currentdirection\par
>> > \direction =RTL Direction: \currentdirection\par
>> >
>> > \vskip\baselineskip
>> >
>> > \direction = LTR Direction: \currentdirection\par
>> > \direction = RTL Direction: \currentdirection\par
>> >
>> > \bye
>>
>> 11/10 for effort, Wolfgang, but there are some common cases
>> for which it does not work :
>>
>> \direction = LTR\relax Direction: \currentdirection\par
>> \direction = RTL\relax Direction: \currentdirection\par
>>
>> \vskip\baselineskip
>>
>> \direction = LTR%
>> Direction: \currentdirection\par
>> \direction = RTL%
>> Direction: \currentdirection\par
>
>
> If the direction is always made of three letters, as I think it is, then
>
> \def\direction=#1#2#3{...}
>
> might work well. Of course it forbids
>
> \direction={RTL}
>
> Anyway, Vafa, are you trying to mimick TeX assignments or key-value's? In the
> former case (which is investigated here), nothing will be perfect. In the
> latter, though, the "key = value" pair is normally well delimited (e.g. by a
> comma or the end of the list), so the value can be easily retrieved.

Wolfgang's code is redundant both in the definition and testing. 
The core of his idea is:

    \def\direction=#1 {\def\currentdirection{#1}}

The problem (noted by Paul and Phil) is that it is somewhat difficult with TeX 
to catch a "parameter word" with varying "word" delimiters. Wolfgang chose 
delimiters "=" and " ", which of course fails when " " is replaced by anything else. 
And even the "=": TeX allows assignments with or without "=", 
while Wolfgang's suggestion fails with 

    \direction RTL

I.e., it is "difficult" to emulate TeX's assignment parsing by macros. 
(Well, \@ifnextchar= etc. ...)
A similar problem has been noted with the FiNK package: 
It is "difficult" to replace \input by a self-defined macro.
-- Ah, cf. Phil on "\magnification hacks" ...

"Wolfgang Schuster" , 14.11.2010 14:52:54:
[...]
>Yes but i would recommend i method which fits to the macro package, e.g. 
> \setdirection{LTR} or \setdirection[RTL]

\set... is so obvious, and with this construction I wonder why anyone 
should want to have an "=" assignment syntax. I have defined such commands 
so often and now think that I could use "\MakeSet":

    \def\MakeSet#1{%
      \expandafter\edef\csname set#1\endcsname{%
        \def\expandafter\noexpand\csname#1\endcsname}}
    \MakeSet{direction}
    \setdirection{RTL}Direction: \direction\par
    \setdirection{LTR}Direction: \direction\par

When this has been understood, \@bsphack and \@esphack might be added ...

Cheers, 

    Uwe.