[halst]

Yeah, and picking a language was the hardest decision for the book. I wrote the compiler prototype for it in OCaml, but using it for the book would have narrowed its audience unnecessarily. In the end, I'm very happy with picking TypeScript: it allowed to discuss the matters of static and dynamic typing within the same language. Using Python with mypy would be another option, but then, since I wanted the source language to be a subset of the implementation language, I would have to explain parsing whitespace-sensitive code, which is tricky.