[texhax] FW: Re: Implementing control sequences with key values

[Uwe Lueck]

"Paul Isambert" <zappathustra at free.fr>, 15.11.2010 12:34:36:
>Le 14/11/2010 22:47, Uwe Lueck a écrit :
>> Correcting my 14.11.2010 19:02:33:
>>>> Wolfgang Schuster wrote:
>>>>> \def\direction#1=#2#3
>>>>>     {\def\currentdirection{#2#3}}
>> I commented:
>>> Wolfgang's code is redundant both in the definition and testing.
>>> The core of his idea is:
>>>     \def\direction=#1 {\def\currentdirection{#1}}
>> But
>>
>>      \def\direction=#1#2 {\def\currentdirection{#1#2}}
>>
>> is superior in allowing ("optional"!) spaces after "="
>> in using \direction.
>
PI> And Wolfgang's original
PI>
PI> \def\direction#1=#2#3 {...}
PI>
PI> also allows optional space before "=", which might happen under some 
PI> circumstances. 

My previous posting was the result of carefully checking this, 
it turns out that the parameter at the left of "=" is not needed 
for allowing 

    \direction        = RTL

it already works with my suggestion above.
I did post code that demonstrated this -- 
finally this is anyway clear from TeX's parsing rules 
(under usual catcodes): 
"=" is the next token after \direction that TeX realizes 
(I am unable to learn the "mouth/stomach" terms 
 for a better explanation right now, and cannot 
 look up the TeXbook chapter, I think this happens 
 already at tokenizing, i.e., the tokenizing function 
 is TeX's only part that knows about spaces after 
 "\direction".).

So the "#1=#2#3" really is redundant, I only admitted 
that it is not as redundant as I had claimed before.

PI> It actually allows any "filler".

You may be right with some additional requirements, 
e.g. when somebody insists that 

    \Direction{} = RTL

should work with 

    \def\Direction#1{\direction}

("! Use of \direction doesn't match its definition.")

Cheers, 

    Uwe.