Swift In Depth (Tjeerd in ’t Veen) (z-library.sk, 1lib.sk, z-lib.sk)

Swift in Depth Tjeerd in 't Veen

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Preface I started as an iOS developer in 2011. I loved to make iPhone apps and still do to this day. Besides doing mobile development work, I also was involved in some web development while learning Ruby. I loved the short powerful language, and wished I could use a compile-time language like Objective-C, but with the elegance and expressive nature of Ruby. Then Apple introduced Swift, and it seems like they listened. Swift was a fresh take on programming for me, combining the elegance of a dynamic language with the speed and safety of a static language. I never liked the Objective-C syntax. I used to say things like “Yeah, Objective-C is verbose, but it gets the job done.” But with Swift, however, I find reading and writing code very pleasing again, like I did with Ruby. I could finally use a static language and keep producing work while loving the language I’m working with. It was a good combination for me. However, it wasn’t love at first sight. Before I truly enjoyed Swift, I struggled a lot with it. Swift looks very friendly, but boy, was it tough sometimes. Everything needs to be safe at compile- time, and I could not mix and match types in arrays anymore. Meanwhile, Swift was only an early version and kept changing; it was hard to keep up. “What are generators? Oh, they are called iterators now? And why use guard? Can’t we use an if statement instead? Pfft, optionals are overrated; we can use simple nil checks?”, and so on. I wouldn’t even consider working with generics. However, I persevered and started to embrace these Swift concepts. I realized they were older concepts from other programming languages but wearing a fresh new coat that truly helped me become a better programmer and deliver better work. Over time, I started to love the Swift language and its pretty syntax. Since Swift 2, I had the luxury of working in a big company where we produced Swift code on a large scale, starting with about 20 developers and growing to over 40. After working with Swift with so many developers, and after being involved in hundreds of pull requests, I noticed that other developers had the same struggles as me. Or my fellow developers delivered code just fine, but didn’t realize that a more elegant or robust alternative was hidden, waiting to be discovered. Even though our code was correct, sometimes it could be a bit cleaner, or more succinct, or just a bit safer. I also noticed that we all stayed away from powerful techniques—such as generics or flatMap—because they were hard to grasp. Or we used to love the idea of generics, but weren’t sure why or when to apply it ourselves. After these realizations I started to write. First, these scribbles would be notes for myself on how to cleanly unwrap optionals, how lazy properties work, how to deal with generics, and so on. Then, these notes matured, and before I knew it I had enough content for some chapters. It was time to turn these notes into something more elaborate: a programming book that could help others shorten their Swift journey. With a few rough chapters in hand, I was wondering if I should throw an ebook online. However, with an “impressive” 200 people following me on Twitter and lacking a popular blog

website, I figured I wouldn’t find the audience I wanted. Moreover, I thought that I had to learn a lot of the unknowns about writing a book. I decided to approach a publisher to help me turn these rough chapters into a great book. I approached Manning, and we’ve been working on this book together ever since. I believe these “small notes” have grown into something special. With the help of Manning and Swift friends, I have spent most of my free time for over a year writing and polishing and trying to make Swift’s tough concepts more simple to understand. By reading this book, I hope that it helps you on your path to becoming a Swift master. Also, I hope that you can tell I thoroughly enjoy sharing these concepts with you. I hope this book makes your Swift journey easy and fun. TJEERD IN ’T VEEN

Acknowledgements Thank you, Manning, for helping me publish my first book. I want to give special thanks to Mike Stephens for taking the chance by getting me on board. Thank you, Helen Stergius, for working with me during the whole process. Thank you, Alain Couniot, for the great technical reviews of my chapters; it’s not an easy job to keep pointing out errors and possible improvements; still, I greatly appreciate it. And I also want to thank Alexander Pawlicki for his creative cartoon illustrations used throughout the book. Also thank you to the rest of the Manning team: Aleksandar Dragosavljević, Candace Gillhoolley, Ana Romac, Cheryl Weisman, Deirdre Hiam, Dottie Marsico, Nichole Beard, Mary Piergies, Carol Shields, Darren Meiss, Melody Dolab, and Marija Tudor. I want to give special thanks to friends, coworkers, and others who have been my guinea pigs and reviewed (parts of) the book: Bart den Hollander, Dimitar Gyurov, Dario de Rosa, Rein Spijkerman, Janina Kutyn, Sidney de Koning, Torben Schulz, and Edwin Chun Wing Kwok. Also, I would like to thank the other reviewers: Alessandro Campeis, Ali Naqvi, Axel Roest, David Jacobs, Gustavo Gomes, Helmut Reiterer, Jason Pike, John Montgomery, Kent R. Spillner, Lars Petersen, Marcelo Pires, Marco Giuseppe Salafia, Martin Philp, Monica Guimaraes, Patrick Regan, Tyler Slater, and Weyert de Boer. I knew writing a book was tough, but it was much harder than I imagined it to be. My fiancée Jenika and I just had a baby daughter, and it was quite the struggle to start a new family, have sleepless nights, maintain a full-time job, and write a programming book in a second language. I couldn’t have done it if I didn’t love writing this book while having the support of my fiancée. Thank you, Jenika, for being so patient with me.

About this book Swift is a young language. At the time of writing, Swift has reached the fourth version and yet is not ABI-stable, meaning that there will be breaking changes when Swift 5 comes out. So why is this book in any position to tell you how to write your code? You’d be right to be skeptical, but please bear with me. Even though Swift is relatively new, I think it’s fair to say that some solutions work better than others, which is even more essential to understand if you’re using Swift for real production apps. Swift borrows a lot of important concepts from other programming languages, such as Haskell, Ruby, Rust, Python, C#, and others. Therefore, you’d be wise to keep an eye out for these concepts. By mixing programming paradigms with real-world experience, this book shares some very fun and useful best practices you can instantly apply to your work. Having programmed for over a decade in multiple languages and teams, I would like to share tips, tricks, and guidelines that helped my Swift career tremendously, and I want the same for you. WHY THIS BOOK? Honestly, a lot of software in this world runs on “ugly” code, and that is completely normal. If your product does what it needs to do, that is—like it or not—good enough for businesses. As a developer, you have to make sure your product works and works well. But your users won’t look under the hood and point out ugly if statements. Perfectionism is harmful to software development and the cause to large numbers of unfinished projects. Still, there’s a large gap between “It does what it needs to do” and a project where some excellent decisions were made that pay off in the long run. Having worked on numerous projects, one thing I highly value is writing code that your coworkers and your future self will understand clearly—because elegant code means less chance of bugs, higher maintainability, better understanding for developers who inherit code, increased programmer happiness, and many other benefits. Another aspect I value is the robustness of code, meaning how refactor-proof some pieces are. Will it break if you sneeze on it? Or can you change code without a hassle? In this book, I share my tips, tricks, and guidelines that have worked well for me and companies I’ve worked for. On top of that, it fills in significant knowledge gaps that may arise while working with Swift.

Although this is a Swift book, a lot of the principles shared here are not Swift-centric and carry over to other programming languages as well; this is because Swift borrows a lot of ideas and paradigms from other languages. After you finish this book, you may find it easy to apply concepts in other languages. For instance, you’ll learn a lot about optionals, or how to use the reduce method on arrays. Later, you may decide to learn Kotlin, where you may apply optionals and reduce—called fold—straight away. You may also find Rust—and its similar generics implementation—easier to learn. Because of Swift’s multi-paradigm nature, this book switches without preference between object-oriented programming, functional programming, and protocol-oriented programming paradigms—although admittedly, I do favor other techniques over subclassing. Switching between these paradigms offers you many tools and solutions to a problem, with insights as to why a certain solution works well or not. Whether you’re stuck in a rut or open to many new programming insights, this book challenges you to solve problems in different ways. IS THIS BOOK FOR YOU? This book does assume that you have made one or more applications in Swift. Do you work in a team? Even better—this book shows you how to write good, clear code that gets appreciated in teams, and helps you improve pull requests of others. Your code will be more robust and cause less maintenance for you and your team. This book fills in knowledge gaps for both beginner and seasoned Swift developers. Perhaps you mastered protocols but still struggle with flatMapping on types or asynchronous error handling. Or maybe you create beautiful apps but stay away from generics because they can be hard to interpret. Or perhaps you sort-of know when to use a struct versus a class but aren’t aware that enums are sometimes a better alternative. Either way, this book helps you with these topics. By the end, generics should come as naturally as for loops. You’ll be confident calling flatMap on optionals, know how to work with associated types, and you’ll gladly use reduce in your daily routine when working with iterators. If you’re aiming to get a programming interview for a new job in the future, you’re in for a treat. You’re going to be able to answer a lot of relevant questions in regard to Swift development trade-offs and decisions. This book can even help you write elegant code in your code assignments. If you just want an app in the app store, just keep doing what you’re doing; no need to read this book! But if you want to write code that is more robust, easier to understand, and increases your chances of getting a job, getting better at your job, or giving qualitative comments on pull requests, you’re at the right place. WHAT THIS BOOK IS NOT This book is focused on Swift. It mostly uses framework-free examples because it isn’t about teaching Cocoa, iOS, Kitura, or other platforms and frameworks.

What does happen in this book is I often make use of Apple’s Foundation, which is hard to avoid if you want real-world examples. If you’re on Linux, you can use swift.org’s Foundation alternative to get similar results. A BIG EMPHASIS ON PRACTICAL SCENARIOS This book is very practical, showcasing tips and tricks you can apply straight away in your daily programming. Don’t worry: it’s not a theory-dense book. You’ll learn a lot of theory, but only via the use of real- world problems that any Swift developer runs into sooner or later. It doesn’t, however, reach an academic level where it discusses Swift’s LLVM representation or machine code. Also, I made sure to avoid a personal pet peeve of mine: I do not subclass “Animal” with “Dog” or add a “Flyable” protocol to “Bird.” I also don’t add “Foo” to “Bar.” You’ll deal with real-world scenarios, such as talking to APIs, loading local data, and refactoring and creating functions, and you’ll see useful bits and pieces of code you can implement in your projects. ROADMAP The following sections provide an overview of the book, divided into chapters. The book is quite modular, and you can start with any chapter that interests you. Some chapters I consider crucial chapters. Chapter 4, “Making optionals second nature,” is key, because optionals are so prevalent in Swift and return over and over again in chapters. To understand the abstract side of Swift, I highly recommend reading chapter 7, “Generics,” chapter 8, “Putting the pro in protocol-oriented programming,” and chapter 12, “Protocol extensions.” Together, these chapters lay a solid foundation for key Swift skills. Be sure not to skip these! As a bonus, if you’re interested in learning functional programming techniques, direct your attention to chapter 2, “Modeling data with enums,” chapter 10, “Understanding map, flatMap, and compactMap,” and chapter 11, “Asynchronous error handling with Result.” Chapter 1: Introducing Swift in depth This warmup chapter shows the current state of Swift, what it’s good at, what it’s not so good at, and what you’ll be doing in this book. It’s not very technical, but it sets expectations and prepares you for what you’ll learn. Chapter 2: Modeling data with enums This chapter is excellent if you want to flex your brain and think differently about modeling data and see how far enums can go to help you.

You’ll see how to model data with structs and enums, and how to reason about it so that you can turn structs into enums and back again. You’ll be challenged to step away from the usual class, subclass, and struct approach and see how to model data with enums instead, and why you would want to. You’ll also see other interesting uses for enums and how to use enums to write safer code. By the end of this chapter, you may catch yourself writing enums a lot more. Chapter 3: Writing cleaner properties Swift has a rich property system with many options to pick from. You’ll learn to pick the right type of properties for the right types of situations. You’ll also create clean computed properties and stored properties with behavior. Then you’ll discover when to use lazy properties, which can cause subtle bugs if they’re not carefully handled. Chapter 4: Making optionals second nature This chapter leaves no stone unturned regarding optionals. Optionals are so pervasive that this chapter takes a very thorough look at them. Both for beginners and Swift masters, this chapter is riddled with best practices and tips and tricks that will boost your day-to-day Swift code. It covers optionals in many scenarios, such as when handling optional Booleans, optional strings, optional enums, implicitly unwrapped optionals, and force unwrapping. Chapter 5: Demystifying initializers Life in the programming world starts with initializers. Avoiding them in Swift is impossible, and of course, you work with them already. Still, Swift has a lot of weird rules and gotchas regarding structs and classes and how their properties are initialized. This chapter uncovers these strange rules to help you avoid boxing matches with the compiler. It isn’t just theory either; you’ll see how you can write less initialization code to keep your codebase clean, and you’re going to gain an understanding of subclassing and how the initializer rules apply there. Chapter 6: Effortless error handling This book has two error handling chapters covering two different idioms: one for synchronous error handling, and one for asynchronous error handling.

This chapter deals with synchronous error handling. You’ll discover best practices related to throwing errors, handling errors, and maintaining a good state in your programs. But it also touches on propagating, adding technical information, adding user-facing information, and bridging to NSError. You’ll also find out how to make your APIs a bit more pleasant by making them throw fewer errors while respecting the integrity of an application. Chapter 7: Generics Generics are a rite of passage for Swift developers. They can be hard to understand or work with at first. However, once you’re comfortable with them, you’ll be tempted to use them often. This chapter makes sure you know when and how to apply them by creating generics functions and types. You’ll see how you can make code polymorphic with generics so that you’ll be able to write highly reusable components and shrink down your codebase at the same time. Generics become even more interesting when you constrain them with protocols for specialized functionality. You’ll discover core protocols, such as Equatable, Comparable, and Hashable, and see how to mix and match generics with them. Generics won’t be intimidating after you have read this chapter, I promise. Chapter 8: Putting the pro in protocol-oriented programming Protocols—similar to typeclasses in Haskell or traits in Rust—are the holy grail of Swift. Because Swift can be considered a protocol-oriented language, this chapter provides a look at applying protocols in useful ways. It covers generics and shows how they fare against using protocols as types. You’ll be able to clearly choose (or switch) between either. Protocols with associated types can be considered advanced protocols. This chapter makes sure that you understand why and how they work so that you don’t have to refrain from using them. It models a piece of a program with protocols, and keeps running into shortcomings, which it ultimately solves with associated types. Then you’ll see how to pass protocols with associated types around in functions and types, so that you can create extremely flexible, yet abstract code. This chapter puts a lot of focus on how to use protocols at compile time (static dispatch) and how to use them at runtime (dynamic dispatch) and their trade-offs. This chapter aims to provide a strong foundation for protocols so that you can tackle more difficult patterns in later chapters.

Chapter 9: Iterators, sequences, and collections It’s not uncommon to create a data structure in Swift that isn’t only using the core types, such as sets arrays and dictionaries. Perhaps you’ll need to create a special caching storage, or maybe a pagination system when downloading a Twitter feed. Data structures are often powered up by the Collection protocol and the Sequence protocol. You’ll see how Sequence in turn is using the IteratorProtocol. With these combined, you’ll be able to extend and implement core functionalities in your data types. First, you’ll take a look at how iteration works with the IteratorProtocol and Sequenceprotocols. You’ll discover some useful iterator patterns, such as reduce(), reduce(into:), and zip, as well as how lazy sequences work. You’ll create a data structure called a bag, also known as a multiset, using the Sequenceprotocol. Then you’ll discover the Collection protocol and the landscape of all the collection protocols Swift offers. At the end, you’ll create another data structure and see how to make it conform to the Collection protocol. This part is highly practical, and you can apply the same techniques to your code straight away. Chapter 10: Understanding map, flatMap, and compactMap This chapter highlights key concepts commonly found not only in Swift but also other frameworks and programming languages. Sooner or later you’ll run into map, flatMap, and compactMap on arrays, optionals, error types, and perhaps even functional reactive programming such as RxSwift. You’ll get a proper look at how to clean up code by applying map and flatMap on optionals. But you’ll also see how to map over dictionaries, arrays, and other collection types. You’ll also learn the benefits of flatMapping over strings. Lastly, you’ll get to review compactMap and how it elegantly handles optionals in collections. Understanding map, flatMap, and compactMap on a deeper level is a good base for understanding how to read and write more concise yet elegant code, and a good base for working with Resultin chapter 11. Chapter 11: Asynchronous error handling with Result Swift’s error handling falls a bit short on asynchronous error handling. You’re going to take a closer look and see how to get compile-time safety for asynchronous programming by making

use of a so-called Result type, which is unofficially offered by Apple via the Swift Package Manager. Perhaps you’re using some version of Result already, found in multiple frameworks. But even if you’re acquainted with Result, I’d wager that you’ll see new and useful techniques in this chapter. You’ll start by learning the downsides of traditional Cocoa-style error handling and why Result can help with that. Then you’ll see how to transform a traditional call to one that uses Result. Also, you’re going to take a look at transforming throwing functions to Result and back again. You’ll be applying the special AnyError type to create more flexibility, avoiding NSError, and making sure that you get a lot of compile-time safety. As a cool trick, you’ll learn about the Never type, which is a unique way to tell the Swift compiler that a Result can never succeed or fail. Lastly, you’ll use what you learned from map and flatMap on optionals to understand how to map over values and errors, and even how to flatMap with Result. You’ll end up with a so- called monadic style of error handling, which gives you the power to very cleanly and elegantly propagate an error up in the call stack with very little code while keeping a lot of safety. Chapter 12: Protocol extensions This chapter is all about modeling data in a decoupled way, offering default implementations via protocols, making use of clever overrides, and seeing how to extend types in interesting ways. As a start, you’ll learn about modeling data with protocols versus subclasses. Then, you’re going to model data two ways: one approach entails protocol inheritance, and the other uses protocol composition. Both have their pros and cons, which you’ll discover when you go over the trade-offs. Also, you’ll see how protocol extensions work when overridden by protocol inheritance and concrete types. It’s a little theoretical, but it’s useful to understand protocols on a deeper level. You’ll also see how you can extend in two directions. One direction is extending a class to adhere to a protocol, and the other is extending a protocol and constraining it to a class. It’s a subtle but important difference. At the end of the chapter you’re going to extend Collection, and then you’ll dive deeper and extend Sequence to create highly reusable extensions. You’ll get acquainted with ContiguousArray and functions that have the rethrows keyword, while you create useful methods you can directly apply in your projects.

Chapter 13: Swift patterns This may be the hardest chapter in the book, but it’s a great mountain to climb. This chapter’s goal is to handle common obstacles that you may run into. The patterns described here are not a rehash of SOLID principles—plenty of books cover that! Instead, it focuses on modern approaches for a modern language. You’ll discover how to mock an API with protocols and associated types—something that comes in handy frequently—so that you can create an offline version of an API and a testing version of an API. Then, you’ll see how conditional conformance works in accordance with generic types and protocols with associated types. Next, you’ll create a generic type, and power it up by using the powerful technique of conditional conformance, which is another way to deliver highly flexible code. After that, you’ll deal with an issue you may run into when trying to use a protocol as a concrete type. You’ll use two techniques to combat it: one involves enums, and the other involves an advanced technique called type erasure. Lastly, you’re also going to examine whether protocols are a good choice. Contrary to popular belief, protocols are not always the answer. You’ll look at an alternative way to create a flexible type, involving a struct and higher-order functions. Chapter 14: Delivering quality Swift code This is the least code-centric chapter in the book, but it may be one of the most important ones. It’s about writing clean, easy-to-understand code that creates fewer headaches for everybody on your team (if you’re on one). It challenges you about establishing naming conventions, adding documentation and comments, and cutting up large classes into small generic components. You’ll also set up SwiftLint, a tool that adds style consistency and helps avoid bugs in your projects. Also you’ll get a peek at architecture, and how to transform large classes with too many responsibilities into smaller generic types. This chapter is a good check to see if your code is up to standards and styles, which will help when creating pull requests or finishing code assignments for a new job. Chapter 15: Where to Swift from here At this point, your Swift skills will be seriously powered-up. I share some quick pointers on where to look next so you can continue your Swift journey.

ABOUT THE CODE This book contains many examples of source code, both in numbered listings and in line with normal text. In both cases, source code is formatted in a fixed-width font like this to separate it from ordinary text. Sometimes code is also in bold to highlight code that has changed from previous steps in the chapter, such as when a new feature adds to an existing line of code. In many cases, the original source code has been reformatted; we’ve added line breaks and reworked indentation to accommodate the available page space in the book. In rare cases, even this was not enough, and listings include line-continuation markers (➥). Additionally, comments in the source code have often been removed from the listings when the code is described in the text. Code annotations accompany many of the listings, highlighting important concepts. The source code for all listings in this book is available for download from the Manning website at https://www.manning.com/books/swift-in-depth and from GitHub at https://github.com/tjeerdintveen/manning-swift-in-depth. With the exception of chapters 14 and 15, every chapter has source code included. BOOK FORUM Purchase of Swift in Depth includes free access to a private web forum run by Manning Publications where you can make comments about the book, ask technical questions, and receive help from the authors and from other users. To access the forum, go to https://forums.manning.com/forums/swift-in-depth. You can also learn more about Manning’s forums and the rules of conduct at https://forums.manning.com/forums/about. Manning’s commitment to our readers is to provide a venue where a meaningful dialogue between individual readers and between readers and the authors can take place. It is not a commitment to any specific amount of participation on the part of the authors, whose contribution to the forum remains voluntary (and unpaid). We suggest you try asking the authors some challenging questions lest their interest stray! The forum and the archives of previous discussions will be accessible from the publisher’s website as long as the book is in print.

ABOUT THE AUTHOR Tjeerd in ’t Veen is an avid Swift fan and a freelance iOS developer, having experience working for agencies, co-founding a small startup, and at time of writing, helping ING scale up their mobile development. Starting out as a Flash developer in 2001, his career progressed to iOS development with Objective-C, web development with Ruby, and some tinkering in other programming languages. When he’s not developing in Swift, he’s busy spending time with his two daughters, making cringy dad jokes, and dabbling on an acoustic guitar. You can find him on Twitter via @tjeerdintveen. ABOUT THE COVER ILLUSTRATION The figure on the cover of Swift in Depth is captioned “Man from Omišalj, island Krk, Croatia.” The illustration is taken from the reproduction, published in 2006, of a nineteenth-century collection of costumes and ethnographic descriptions entitled Dalmatia by Professor Frane Carrara (1812–1854), an archaeologist and historian, and the first director of the Museum of Antiquity in Split, Croatia. The illustrations were obtained from a helpful librarian at the Ethnographic Museum (formerly the Museum of Antiquity), itself situated in the Roman core of the medieval center of Split: the ruins of Emperor Diocletian’s retirement palace from around AD 304. The book includes finely colored illustrations of figures from different regions of Dalmatia, accompanied by descriptions of the costumes and of everyday life. Dress codes have changed since the nineteenth century, and the diversity by region, so rich at the time, has faded away. It is now hard to tell apart the inhabitants of different continents, let alone different towns or regions. Perhaps we have traded cultural diversity for a more varied personal life—certainly for a more varied and fast-paced technological life. At a time when it’s hard to tell one computer book from another, Manning celebrates the inventiveness and initiative of the computer business with book covers based on the rich diversity of regional life of two centuries ago, brought back to life by illustrations from collections such as this one.

Chapter 1. Introducing Swift in depth In this chapter • A brief overview of Swift’s popularity and supported platforms • The benefits of Swift • A closer look at Swift’s subtle downsides • What we will learn in this book It is no secret that Swift is supported on many platforms, such as Apple’s iOS, macOS, watchOS, and tvOS. Swift is open source and also runs on Linux, and it’s gaining popularity on the server side with web frameworks such as Vapor, Perfect, Zewo, and IBM’s Kitura. On top of that, Swift is slowly not only encompassing application programming (software for users) but also starting to enter systems programming (software for systems), such as SwiftNIO or command-line tools. Swift is maturing into a multi-platform language. By learning Swift, many doors open to you. Swift was the most popular language on Stack overflow in 2015 and remains in the top five most-loved languages in 2017. In 2018, Swift bumped to number 6 (https://insights.stackoverflow.com/survey/2018). Swift is clearly here to stay, and whether you love to create apps, web services, or other programs, I aim for you to get as much value as possible out of this book for both yourself and any team you’re on. What I love about Swift is that it’s easy to learn, yet hard to master. There’s always more to learn! One of the reasons is that Swift embraces many programming paradigms, allowing you to pick a suitable solution to a programming problem, which you’re going to explore in this book. 1.1. THE SWEET SPOT OF SWIFT One thing I love about a dynamic language—such as Ruby—is its expressive nature. Your code tells you what you want to achieve without getting too much in the way of memory management and low-level technology. Writing in Ruby one day, and Objective-C the other, made me believe that I either had to pick between a compiled language that performed well or an expressive, dynamic language at the cost of lower performance. Then Swift came out and broke this fallacy. Swift finds the right balance where it shares many benefits of dynamic languages—such as Ruby or Python—by offering a friendly syntax and strong polymorphism. Still, Swift avoids some of the downsides of dynamic languages, most notably the performance, because Swift compiles to native machine code via the LLVM compiler. Because Swift is compiled via LLVM, you get not only high performance, but also tons of safety checks, optimizations, and the guarantee that your code is okay before even running it. At the same time, Swift reads like an expressive dynamic language, making it pleasant to work with and effortless to express your intent.

Swift can’t stop you from writing bugs or poor code, but it does help reduce program errors at compile time using various techniques, including, but not limited to, static typing and strong support for algebraic data types (enums, structs, and tuples). Swift also prevents null errors thanks to optionals. A downside of some static languages is that you always need to define the types. Swift makes this easier via type inference and can deduce concrete types when it makes sense. This way, you don’t need to explicitly spell out every single variable, constant, and generic. Swift is a mixtape of different programming paradigms, because whether you take an object- oriented approach, functional programming approach, or are used to working with abstract code, Swift offers it all. As the major selling point, Swift has a robust system when it comes down to polymorphism, in the shape of generics and protocol-oriented programming, which gets pushed hard as a marketing tool, both by Apple and developers (see figure 1.1). Figure 1.1. The sweet spot of Swift 1.2. BELOW THE SURFACE Even though Swift reels you in with its friendly syntax and promises to build amazing apps, it’s merely the tip of the iceberg. An enticing entry to learning Swift is to start with iOS development, because not only will you learn about Swift, you will also learn how to create beautiful apps that are composed of crucial components from Apple frameworks.

But as soon as you need to deliver components yourself and start building more elaborate systems and frameworks, you will learn that Swift works hard to hide many complexities—and does so successfully. When you need to learn these complexities, and you will, Swift’s difficulty curve goes up exponentially. Even the most experienced developers are still learning new Swift tricks and tidbits every day! Once Swift has you hooked, you’ll likely hit speed bumps in the shape of generics and associated types, and something as “simple” as handling strings may cause more trouble than you might expect (see figure 1.2). Figure 1.2. The tip of Swift’s iceberg This book shines in helping you handle the most common complexities. You will cover and tackle any issues and shortcomings with Swift, and you’ll be able to wield the powers that these complexities bring while having fun doing so. 1.3. SWIFT’S DOWNSIDES Swift is my favorite programming language, but let’s not look at it through rose-tinted glasses. Once you acknowledge both Swift’s strong and weak points, you can adequately decide when and how you’d like to use it. 1.3.1. ABI stability Swift is still moving fast and is not ABI-stable, which means that code written in Swift 4 will not be compatible with Swift 5, and vice versa. Imagine writing a framework for your application. As soon as Swift 5 comes out, an application written in Swift 5 can’t use your framework until you’ve updated your framework to Swift 5. Luckily, Xcode offers plenty of help to migrate, so I expect that this migration won’t be as painful. 1.3.2. Strictness Swift is a strict and rigid language, which is a common criticism of static languages but even more so when working with Swift. Before getting comfortable with Swift, you may feel like