You now have the foundation to go into more depth. In chapter 5, I/O could only be covered superficially because the Julia type system had not yet been covered. Chapter 17 expands on chapter 5 by focusing on the I/O type hierarchy.
In chapter 10, you saw how parametric types are used to represent various forms of nothing, and in chapter 6 how they help define strongly typed collections. In chapter 18, I will explain how parametric types aid in type correctness, performance, and memory usage.