[texhax] Eplain TeX: problem with \@optionalarg

[Rodolfo Medina]

Arnaud Schmittbuhl <aschmittbuhl at libertysurf.fr> writes:

> Le Sun, 13 Feb 2011 13:50:01 +0000,
> Rodolfo Medina <rodolfo.medina at gmail.com> a écrit :
>
Rodolfo Medina <rodolfo.medina at gmail.com> writes:

> I posted the following problem to eplain mailing list but couldn't solve it.
> Anybody there accustomed with eplain commands?
>
> At:
>
>  http://tug.org/eplain/doc/eplain/Macro-arguments.html
>
> it is suggested the following code:
>
>      \makeatletter
>      % \mo{m}[o]
>      \def\mo#1{\def\mo at arg{#1}\@getoptionalarg\fin at mo}
>
>      \def\fin at mo{\vskip1pc
>       \message{^^JArg: \mo at arg}%
>       \message{^^JOptional arg: \@optionalarg}%
>      }
>
>> Now, I wish to put a condition there:
>> 
>>    \ifx\@optionalarg\empty
>>         \fin at mo
>>    \else
>>         \@getoptionalarg\fin at mo
>>    \fi
>
> Hi,
> try that in \else \fi
>   \expandafter\@getoptionalarg\expandafter\fin at mo


Strange: it works only with

\mo{test}[optionaltext]
\mo{test}

; instead, with

\mo{test}
\mo{test}[optionaltext]

it doesn't.
Rodolfo