On 2013–05–07 Roland Thiers wrote:

> Hi,
> I did this code to get a regular polygon with metafun, it works but
> I would like to have an user command  like  \define or \def
> to be able to choose the number of sides, n. I don't know how to do
> that ...

Here's one way to do it:

\startuniqueMPgraphic{polygoneregulier}{sides}
  drawoptions (withpen pencircle scaled 2pt);
  numeric u ; u:=1*cm ;
  numeric n ; n:=\MPvar{sides};
  pair a[];
  a[0]:=(0,3u);
  path polygone[] ;
  for i=1 upto n :
  a[i]:=a[0] rotatedaround(origin,i*360/n) ;
  polygone[i]:=a[i-1]--a[i];
  draw polygone[i]; endfor;
  draw fullcircle scaled 6u withcolor blue;
\stopuniqueMPgraphic

%% \polyreg{<number>}
\define[1]\polyreg
  {\setupMPvariables[polygoneregulier][sides=#1]%%
   \uniqueMPgraphic{polygoneregulier}}

\starttext
  \dontleavehmode
  \dorecurse{9}{\polyreg\recurselevel\space}
\stoptext

You can extent this to be able to use key-value syntax or default
values if you like.

Marco