On Wed, 12 Apr 2023, Gavin via ntg-context wrote:

> Hi Aditya,
> 
> > Have you tried using feynmp (the metapost macros) directly instead of the LaTeX wrapper? Can you create an example that works with metapost but fails in ConTeXt?
> 
> No, I’m not sure how to do that. I looked at the manual, and it has some examples where new styles are written in MetaPost, but every diagram is drawn with the LaTeX wrapper. I looked in the source files, and maybe feynmf.dtx has everything to produce documentation for the MetaPost macros, but if so, I’m not sure how to generate that documentation.
> 
> Can you point me to an example that works with metapost macros directly?

I have never used feynMP, so I am just copy-pasting an example from 

https://osksn2.hep.sci.osaka-u.ac.jp/~taku/osx/feynmp/fmfsamples.pdf

    \documentclass{article}
    \usepackage{feynmp}
    \begin{document}
    \unitlength = 1mm
    \begin{fmffile}{simple}
    \begin{fmfgraph}(40,25)
    \fmfleft{i1,i2}
    \fmfright{o1,o2}
    \fmf{fermion}{i1,v1,o1}
    \fmf{fermion}{i2,v2,o2}
    \fmf{photon}{v1,v2}
    \end{fmfgraph}
    \end{fmffile}
    \end{document}

This creates a file simple.mp in the same directory (kind of like the old mkii way of including MP in context):

    % simple.mp -- do not edit, generated automatically by test1.tex
    input feynmp
    require_RCS_revision "1.30";
    beginchar(1, 40*2.84526pt#, 25*2.84526pt#, 0);
    "feynmf: 1";
    LaTeX_unitlength:=2.84526pt;
    subgraph (0, 0, w, h);
    vinit;
    pickup pencircle scaled thin;
    vleft (__i1, __i2);
    vright (__o1, __o2);
    vconnect ("fermion", __i1, __v1, __o1);
    vconnect ("fermion", __i2, __v2, __o2);
    vconnect ("photon", __v1, __v2);
    vfreeze;
    vdraw;
    endsubgraph;
    endchar;
    % the end.
    end.
    endinput;

So, I created a context file:

    \startMPinclusions
    input feynmp;
    require_RCS_revision "1.30";
    \stopMPinclusions

    \starttext
    \startMPpage
    beginchar(1, 40*2.84526pt#, 25*2.84526pt#, 0);
    "feynmf: 1";
    LaTeX_unitlength:=2.84526pt;
    subgraph (0, 0, w, h);
    vinit;
    pickup pencircle scaled thin;
    vleft (__i1, __i2);
    vright (__o1, __o2);
    vconnect ("fermion", __i1, __v1, __o1);
    vconnect ("fermion", __i2, __v2, __o2);
    vconnect ("photon", __v1, __v2);
    vfreeze;
    vdraw;
    endsubgraph;
    endchar;
    \stopMPpage
    \stoptext

but compiling that fails with 

metapost        > trace >
metapost        > trace > loading metafun for lmtx, including the plain 1.004 base definitions
metapost        > trace >
metafun         > log >
metafun         > log > error: Isolated expression
metafun         > log >
metapost        > trace > <error> 1
metapost        > trace > <to be read again> #
metafun         > log >
metafun         > log > I couldn't find an '=' or ':=' after the expression that is shown above this
error message, so I guess I'll just ignore it and carry on.
metafun         > log >
metapost        > trace > <line 135> bp# := bp;
metapost        > trace >

My guess is that it could be something to do with bp# being used as a variable, but I am not 100% sure on that. 

In principle, it should be easier to adapt the feynmp.mp (attached) code to make it work with LMTX. The feynmp.sty package, simply provide a high level macro wrapper around this mp code, and it is relatively simply to do something similar in context; but the MP file isn't that different from the latex code anyways.

Aditya