Hi,

I am looking for a way to assign the value of a TeX macro to an MP variable.
However, this only works the first time, then the value is not updated any
longer. Example:

\startMPdefinitions
  size = \somesize ;
\stopMPdefinitions

\startuseMPgraphic{alpha}
  fill unitcircle scaled size;
\stopuseMPgraphic

\starttext
  \def\somesize{1cm}
  \useMPgraphic{alpha} %% diameter = 1cm

  \def\somesize{2cm}
  \useMPgraphic{alpha} %% diameter = 1cm
\stoptext

I looked into \setupMPvariables, but I couldn't manage to assign them to an
MP variable. The setups key from \defineMPinstance seems to to the job, but
it's TeX, not MetaPost. What comes to mind is a \processMPbuffer before
every MPgraphic:

\startbuffer[foo]
  size := \somesize ;
\stopbuffer

\startuseMPgraphic{alpha}
  fill unitcircle scaled size;
\stopuseMPgraphic

\starttext
  \def\somesize{1cm}
  \processMPbuffer[foo]
  \useMPgraphic{alpha} %% diameter = 1cm

  \def\somesize{2cm}
  \processMPbuffer[foo]
  \useMPgraphic{alpha} %% diameter = 2cm
\stoptext

Is there a better way?

Marco