Re: description fails on \if

["Wolfgang Schuster" <schuster.wolfgang@googlemail.com>]

On Tue, Mar 25, 2008 at 8:38 PM, Hans van der Meer <hansm@science.uva.nl> wrote:
> There is the following problem on code that did work around july 2007,
> but no longer under the current context version (2008-03-11).
>
> Define a description, I did:
> \definedescription[answerblock][%
>        \c!margin=\v!no,\c!location=\v!left,%
>        \c!headstyle=\@@exmintrostyle,\c!style=\@@exmstyle,%
>        \c!before=,\c!inbetween=,\c!after=,%
>        \c!hang=\@@exmhang,\c!width=\questionblockwidth,%
>        \c!text=\placeanswerstart]

Could you try to set "title=no".

> Typeset with this description inside the following macro:
> \def\shortanswer#1\par{%
>        \par % be sure to end paragraph here
>        \answerblock{}% % {} needed here in description call
>        \ifanswers #1\else\dotfill\strut\fi % <<<<<< crashing on this if
>        \par            %  final \par of description ... \par
>        }
> The above code crashes with: ! Incomplete \ifx; all text was ignored
> after ...
>
> After some detective work in which Wolfgang Schuster had a large share
> and has found the most elegant workaround, the addition of a \relax
> just after the description call prevents the error:
> \def\shortanswer#1\par{%
>        \par % be sure to end paragraph here
>        \answerblock{}% % {} needed here in description call
>        \relax          % <<<<<<<<<<<<<<<<<   \relax prevents crash on if
>        \ifanswers #1\else\dotfill\strut\fi
>        \par            %  final \par of description ... \par
>        }
>
> The way \relax stops the error reminds me of the \relax that one
> places after a number in order to separate that from the code following.
> I did compare core-des.tex, both the last version and the archived
> from 2007-04-17. But I must admit, I did not succeed finding a clue
> among the differences about what could have happened. The code is a
> bit too intricate to just jump in and pinpoint the culprit -- at least
> for me.
>
> Allthough Wolfgang's workaround saves the day, I feel this problem
> deserves solving inside ConTeXt, not outside it. Someday somewhere
> another person will stumble on the same problem -- a situation that
> can and should be avoided, in my view.
> Therefore I explain the problem here as fully as possible hoping
> someone knowledgeable with descriptions <<we might need to fall back
> to Taco or Hans>> will find the best remedy.

I would say the message comes from \dodoublegroupempty in \@@description.

Wolfgang
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