sharing metapost variables between mplibcode environments: a proof of concept

mpvarshare.tex

\documentclass{article}
\usepackage{luamplib}
\everymplib{
  if (readfrom "mymplibtmp.mp" <> EOF):
    input mymplibtmp;
  fi;
  closefrom "mymplibtmp.mp";
  if unknown u: u := 10; fi;
  beginfig(0);
}
\everyendmplib{
  endfig;
  write "u := " & decimal u &";" to "mymplibtmp.mp";
  write EOF to "mymplibtmp.mp";
}
\begin{document}
\raggedright\leavevmode
\newcount\n \n=0
\loop \ifnum\n<50 \advance\n by 1
\begin{mplibcode}
  draw fullcircle scaled u;
  %randomseed := u; u := uniformdeviate 50;
  u := 1.25u;
  if u > 50: u := 2 fi;
\end{mplibcode}
\repeat
\end{document}

out of date due to recent update of luamplib.