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From: Joseph Wright <joseph.wright@MORNINGSTAR2.CO.UK>
Subject: Re: xparse processors
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Will Robertson wrote:
> On 23/10/2009, at 6:55 AM, Joseph Wright wrote:
> 
>> This would allow things like:
>>
>> > \ReturnVariable { \MakeHarmless\ReturnVariable }
>>  % Will absorb one more argument, as required
>>
>> (using \MakeHarmless from xdoc). That doesn't work at present as the 
>> return has to be in a toks.
>>
>> As Lars points out, that is quite a bit more flexible at the cost of a 
>> little complexity in the syntax.
> 
> 
> I'm not sure that I see how this is any more flexible?
> Oh, simply that your argument processor doesn't have to be hard-coded to 
> use \l_xparse_arg_toks.
> But hang on, can't you do the same thing at present by just writing
> 
>     > { \MakeHarmless\l_xparse_arg_toks }
> 
> ?

No. \MakeHarmless has the syntax:

\MakeHarmless <macro> <input>

where <macro> is used for the output. So if you try to use a toks here, 
it will fail to work correctly. You'd need a wrapper function, which is 
what Lars is suggesting we should avoid.
--
Joseph Wright