Hello,
Hans suggested a macro to draw the vectors . It works well but I find that in some situations, the arrow is too big.
thank you,
Fabrice


https://mailman.ntg.nl/pipermail/ntg-context/2016/085828.html

\useMPlibrary[mat]

\startMPextensions
     vardef math_stacker_arrow_shape =
         image (
             drawarrow
                 (OverlayWidth,OverlayOffset) -- (0,OverlayOffset)
             withcolor
                 OverlayLineColor ;
             setbounds currentpicture to boundingbox currentpicture
bottomenlarged (OverlayOffset/2) topenlarged (OverlayOffset/2) ;
         )
     enddef ;
     vardef math_stacker_leftarrow_shape =
         math_stacker_arrow_shape
     enddef ;
     vardef math_stacker_rightarrow_shape =
         math_stacker_arrow_shape rotated 180
     enddef ;
\stopMPextensions

\startuniqueMPgraphic{math:stacker:\number"2190}
     math_stacker_draw(math_stacker_leftarrow_shape) ;
\stopuniqueMPgraphic

\startuniqueMPgraphic{math:stacker:\number"2192}
     math_stacker_draw(math_stacker_rightarrow_shape) ;
\stopuniqueMPgraphic

% so far

\setupmathstackers
   [both] % vfenced]
   [color=darkred,
    alternative=mp]

\setupmathstackers
   [top]
   [color=darkred,
    alternative=mp]

\setupmathstackers
   [bottom]
   [color=darkred,
    alternative=mp]

\starttext

$\overrightarrow  {i}$

\stoptext