> FWIW, I am working to reimplement parts of METAFONT in my current project (need the curves, and want an implementation which will also allow me to write out a .mp file) and I'm finding _METAFONT: The Program_ very helpful.
I have a reimplementation of parts of METAFONT as a hobby project. In fact, it is no longer METAFONT; it is a language based on METAFONT. However, I used the same Hobby's algorithm to generate curves. It understands pens, paths and renders it to an OpenGL texture. The results are not compatible with Knuth's METAFONT, as I use floating-point numbers instead of the fixed-point arithmetic. It is still under development and needs some cleaning, but if what you are developing is for personal use or is free software compatible with GNU Affero GPL, perhaps parts of the code, or even the whole, could be of use.
When I write Literate Programs, it's mostly for future me so that I can remember why a particular approach was taken, or what the significance of the two slightly differently named variable is and why they are not interchangeable. If it's for others to use, then user documentation is a specific section of the program code, and possibly a totally separate document (only written after the fact when the UI and so forth is stable enough that things won't change).
FWIW, I am working to reimplement parts of METAFONT in my current project (need the curves, and want an implementation which will also allow me to write out a .mp file) and I'm finding _METAFONT: The Program_ very helpful.