New Chapter: First Class Families
Hands down, the hardest part about writing The Book of Types is that type-level programming is an area of active research. Last week, Lyxia posted "Haskell with only one type family" which provides a framework for writing reusable higher-order type families---something I previously thought impossible.

Today's update includes this new research into the book (with Lyxia's blessing! thanks!!), although I haven't yet gone back and thought too much about how this breakthrough will affect some of my earlier written material.

As always, you can find the updated copy on leanpub.