Preface

Programming with Types is the culmination of multiple years of learning about type systems and software correctness, distilled into a practical book with real-world applications.

I’ve always liked learning how to write better code, but if I were to point out exactly when I started down this path, I’d say it was 2015. I was switching teams at that point and wanted to get up to speed on modern C++. I started watching C++ conference videos, picked up Alexander Stepanov’s books on generic programming, and gained a completely different perspective on how to write code.

In parallel, I was learning Haskell in my spare time and working my way through the advanced features of its type system. Programming in a functional language makes it obvious how some of the features taken for granted in such languages get adopted by more mainstream languages as time goes by.

I read several books on the topic, from Stepanov’s Elements of Programming and From Mathematics to Generic Programming to Bartosz Milewski’s Category Theory for Programmers and Benjamin Pierce’s Types and Programming Languages. As you might be able to tell from the titles, these books are more on the theoretical/mathematical side. While learning more about type systems, I could tell that the code I was writing at work became better. There is a direct link between the more theoretical realm of type system design and the day-to-day production software. This isn’t a revolutionary discovery: fancy type system features exist to address real-world problems.

I realized that not every practicing programmer has the time and patience to read dense books with mathematical proofs. On the other hand, my time wasn’t wasted reading such books: they made me a better software engineer. I figured there is room for a book that covers type systems and the benefits they provide more informally, focusing on practical applications anyone can use in their day job.

Programming with Types aims to provide a walk-through of type system features starting from basic types, covering function types and subtyping, OOP, generic programming, and higher kinded types such as functors and monads. Instead of focusing on the theory behind these features, I describe each one of them in terms of practical applications. The book shows how and when to use each of these features to improve your code.

The code samples were originally supposed to be in C++. The C++ type system is powerful and more feature-rich than languages such as Java and C#. On the other hand, C++ is a complex language, and I didn’t want to limit the audience of the book, so I decided to use TypeScript instead. TypeScript has a powerful type system too, but its syntax is more accessible, so it should be easy to work through most examples even if you’re coming from another language. Appendix B provides a quick cheat sheet for the subset of TypeScript used in this book.

I hope you enjoy reading this book and learn some new techniques that you can apply to your projects right away.