Lenses and monad transformers are libraries, not part of the language. It doesn't look like the book uses them, so you don't need to understand them in order to

Lenses and monad transformers are libraries, not part of the language. It doesn't look like the book uses them, so you don't need to understand them in order to follow it: https://nostarch.com/learn-physics-functional-programming https://nostarch.com/learn-physics-functional-programming