Typescript actually does have some forms of type dependency (eg. conditional types), but these things have been added in a sort of ad-hoc way, and it's unclear

Typescript actually does have some forms of type dependency (eg. conditional types), but these things have been added in a sort of ad-hoc way, and it's unclear how sound they actually are, and they are quite limited. The book here uses a language, pie, that is designed in a much more principled way, with greater expressiveness. The book probably won't be directly applicable to TS work, but I'm sure you would benefit from getting a more rounded understanding of what is possible with type systems.