The code is here: https://github.com/mech-lang/mech/tree/main/src/core/src/pro...
This article is yet another reminder I need to learn Haskell (I've been meaning to for a decade), although the code from this article is approachable considering the topic. However, I've just started using Rust for professional projects, so the code you've posted is a bit easier to read, if more verbose, though the concepts are still unfamiliar to me.
I'm assuming this isn't your first go at writing a compiler?
> I'm assuming this isn't your first go at writing a compiler?
Not quite, the first real language I worked on was called Eve: https://witheve.com