Passing variables into context

[Matthijs Kooijman <matthijs@stdin.nl>]

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Hi all,

I've been trying to make a metapost graphic, based on the \textwidth. However,
it seems I can't just pass in \textwidth as an option. For example, the
following breaks:

  \startuniqueMPgraphic{HLine}
  draw origin -- (\MPvar{width}, 0mm); 
  \stopuniqueMPgraphic

  \starttext

  \setupMPvariables[HLine][width=\textwidth]
  \uniqueMPgraphic{HLine}

  \stoptext

with the following error:

  mplib | mp terminal: >> textwidth
    ! Undefined x coordinate has been replaced by 0.
    <to be read again>
                   ;
    <*> ...d:=3441; draw origin -- (\textwidth , 0mm);

It seems that the \textwidth option is not expanded before being passed to MP.
Is there any way to force this expansion? I've tried \expandafter (without
really understanding what that does), but that didn't help.

I've tried putting this graphic as an overlay under an \framed, which allows
me to use the \overlaywidth macro in metapost (which I assume is expanded
specially by context?). This worked, but is not very elegant. Is there
something more straightforward possible?

Gr.

Matthijs

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