Thanks - author of the repo here. This really started off by me reading Pierce's Types and Programming Languages textbook and getting interested in implementing typecheckers and whatnot... and kind of spiraled into a full blown compiler. So it's ultimately a learning project for me and not meant to be a production-ready compiler (we already have MLton).

After this experience, I do agree that going with the "vertical" approach you described would be significantly easier - I actually considered doing this for just the raw lambda calculus so that I could get CPS transform and native codegen working on a smaller scale.

Unfortunately, I haven't had time to work on the project in a while (I'm finishing my PhD in chemical biology), but I hope to come back to it soon.