How to use in Metafun, a function defined in Lua

[Otared Kavian <otared@gmail.com>]

Hi all,

I would like to use a math function defined in lua in Metafun in order to draw its graph, together with the graphs of some other functions.
As a simple example consider the following:

%%% begin lua-metafun-example.tex
\startluacode 
	function document.myfunction(n,x)
		local S
		S = math.sin(n*x)
		return S
	end
\stopluacode

\starttext

Here \type{document.myfunction(n,x)} is $\sin(nx)$. For instance when $n = 2$ and $x = 355/113$ we have:

\type{document.myfunction(2,355/113)} $=\cldcontext{document.myfunction(2,355/113)} \sim 0$.

\startplacefigure[title={Nothing works\dots}]
\startMPcode{GridAxis}
	numeric gu ; gu = 5mm ; % gu = graph unit
	draw image (
	draw function (1,"x","document.myfunction(2,x)", % <-- this does not work
%	draw function (1,"x","sin(2*x)", % <-- this works
	-355/113,355/113,.01)
	scaled gu 
	withpen pencircle scaled .075pt
	withcolor darkred ;
	) xsized .7TextWidth ;
\stopMPcode
\stopplacefigure

\stoptext
%%% end lua-metafun-example.tex

Here one sees that the function « document.myfunction » is well defined in Lua and is known in ConTeXt, but I cannot use it in a Metapost code: how can one do that?

Thanks in advance for any help.
Best regards: Otared
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