BaSiX – A Basic interpreter written in TeX (1990) [pdf]

Also in the tradition of ambitious TeX is this 2008 winner of the Judge's Prize for the ICFP Programming Contest: http://sdh33b.blogspot.com/2008/07/icfp-contest-2008.html (contest link: http://web.cecs.pdx.edu/~sheard/2008IcfpContest/).

I suspect this is just the code equivalent of performance art (with the emphasis on art). People do a lot of funny things just because they can, and, since source is embedded in the paper, I’m guessing this wasn’t just an April Fools prank.

It looks like there's a copy in CTAN: https://ctan.org/pkg/basix