[tex-eplain] Position of optional argument

[Rodolfo Medina]

On Sun, Apr 26, 2009 at 07:16:55PM +0100, Rodolfo Medina wrote:

>> I've been using the eplain \@getoptionalarg and \@optionalarg commands in a
>> certain macro.  It turns out to me that the optional argument must be put
>> *before* the non optional argument.
>>
>> How can I set things - if possible - so to put it after it?


Oleg Katsitadze <olegkat at gmail.com> writes:

>> > Try this:
>> >
>> >     \input eplain
>> >     \makeatletter
>> >     \def\test#1{%
>> >       \def\test at arg{#1}%
>> >       \@getoptionalarg\@test
>> >     }
>> >     \def\@test{%
>> >       \message{^^JArg: \test at arg}%
>> >       \message{^^JOptional arg: \@optionalarg}%
>> >     }
>> >     \makeatother



Rodolfo:

>>If, in the above code, in place of `\@getoptionalarg\@test' I put:
>>   \ifnum0=0
>>       \@getoptionalarg\@test
>>   \else
>>   \fi
>>then, strangely, it doesn't seem to work any more.  I can't explain why.  Can
>>anybody?
>


Dan Luecking <luecking at uark.edu> writes:

> Not so strange, as \@getoptionalarg looks ahead, sees
> \else rather than [, and concludes there is no optional arg.
>
> You have to get rid of the \else and the \fi before
> \@getoptionalarg is expanded. Try this instead:
>   \ifnum0=0
>     \expandafter\@getoptionalarg\expandafter\@test
>   \else
>   \fi
>
> The first expandafter expands the second which expands the
> \else. The expansion of \else removes everything up to and
> including \fi and the way is clear to see a following [
> (if there is one).
>
> You can omit the \else if there is nothing between it and \fi.


Thanks!  But, if I put:

\ifx\@optionalarg\empty
        \@test
\else
        \expandafter\@getoptionalarg\expandafter\@test
\fi

, it doesn't seem to work either, and neither

\ifx\@optionalarg\empty
        \@test
\else
        \@getoptionalarg\@test
\fi

.  How can I then have such a condition working?

Thanks again
Rodolfo