We'll start with 0 lines of code and end up with a fully working interpreter for the Monkey programming language. Monkey has been specifically designed for this book: it's a language that looks a lot like C, has first class functions, closures, strings, hashes and arrays and its only implementation is the one we build in the book. In contrast to text books on interpreters & compilers, the focus of this book is working code. Code is not just found in the appendix -- no, nearly every page contains a snippet! And not only that, but also tests. The code presented in the book is fully tested and the test suite is included.