\definetypeface [mainface][mm][math] [euler] [default] [text=rm] \setupbodyfont[mainface, 12pt]
\starttext \showmathcharacters \stoptext
\definetypeface [mainface] [mm][math] [euler] [default] [text=rm] mainface, 12pt]
\setupbodyfont[
\starttext
\showmathcharacters
\stoptext