Welcome to Programming Scala, Third Edition

Programming Scala, Second Edition was published six years ago, in the fall of 2014. At that time, interest in Scala was surging, driven by two factors.

First, alternative languages for the JVM instead of Java were very appealing. Java’s evolution had slowed, in part because its steward, Sun Microsystems was sold to Oracle Corporation a few years previously. Developers wanted improvements like more concise syntax for some constructs and features they saw in other languages, like support for functional programming.

Second, big data was a hot sector of the software industry and some of the most popular tools in that sector, especially Apache Spark and Apache Kafka, where written in Scala and offered elegant Scala APIs, along with APIs in other languages.

A lot has changed in six years. Oracle deserves a lot of credit for reinvigorating Java after the Sun Microsystems acquisition. The pace of innovation has improved considerably. Java 8 was a ground-breaking release, as it introduced two of the most important improvements needed to address limitations compared to Scala. One was support for anonymous functions, called lambdas, which addressed the biggest missing feature needed for functional programming. The second feature was support for “default” implementations of the methods declared in interfaces, which made Java interfaces more useful as composable “mixins”.

Also, the Kotlin language was created by the tool vendor Jet Brains, as a “better Java” that isn’t as sophisticated as Scala. Kotlin received a big boost when Google endorsed it as the preferred language for Android apps. Around the same time, Apple introduced a language called Swift primarily for iOS development that has a very Scala-like syntax, although it does not target the JVM.

Big data drove the emergence of data science as a profession. Actually, this was just a rebranding and refinement of what data analysts and statisticians had been doing for years. The specialties of data science called deep learning (DL - i.e., using neural networks), reinforcement learning (RL), and artificial intelligence (AI) are currently the hottest topics in the data world. All fit under the umbrella of machine learning (ML). A large percentage of the popular tools for data science, especially ML, are written in Python or expose Python APIs on top of C++ “kernels”. As a result, interest in Python is growing strongly again, while Scala’s growth in the data world has slowed.

But Scala hasn’t been sitting still. This edition introduces you to version 3 of the language, with significant changes to improve the expressiveness and correctness of Scala, and to remove deprecated and less useful features.

Also, Scala is now a viable language for targeting JavaScript applications through Scala.js. Early support for Scala as a native language is now available through Scala Native, although the version of Scala supported tends to lag the JVM version.

I currently split my time between the Python-based ML world and the Scala-based JVM world. When using Python, I miss the concision, power, and correctness of Scala, but when using Scala, I miss the wealth of data-centric libraries available in the Python world. So, I think we’re entering a period of consolidation for Scala, where developers who want that power and elegance will keep the Scala community vibrant and growing, especially in larger enterprises that are JVM-centered. Scala will remain a preferred choice for hosted services, even while the data and mobile communities tend to use other languages. Who knows what the next five or six years will bring, when it’s time for the fourth edition of Programming Scala?

With each addition of this book, I have attempted to provide a comprehensive introduction to Scala features and core libraries, illustrated with plenty of pragmatic examples, tips, and tricks. However, each edition has shifted more towards pragmatism and away from comprehensive surveying of features.

I think this makes the book more useful to you, in an age when so much of our information is gathered in small, ad hoc snippets through Google searches. Libraries come and go. For example, when you need the best way to parse JSON, an Internet search for Scala JSON libraries is your best bet. Second, what doesn’t change so quickly and what’s harder to find on Stack Overflow, is the wisdom of how best to leverage Scala for real-world development. Hence, my goal in this edition is to teach you how to use Scala effectively for a wide class of pragmatic problems, without getting bogged down in corner cases, obscure features, or advanced capabilities that you won’t need to know until you become an advanced Scala developer.

Hence, I also won’t discuss how to use Scala.js or Scala Native, as the best sources of information for targeting those platforms are their respective websites.

Finally, I wrote this book for professional programmers. I’ll err on the side of tackling deeply technical topics, rather than keeping the material “light”. Other books provide less thorough, but more gentle introductions, if that’s what you prefer. This is a book if you are serious about using Scala professionally.

Welcome to Programming Scala, Second Edition

Programming Scala, First Edition was published five years ago, in the fall of 2009. At the time, it was only the third book dedicated to Scala, and it just missed being the second by a few months. Scala version 2.7.5 was the official release, with version 2.8.0 nearing completion.

A lot has changed since then. At the time of this writing, the Scala version is 2.11.2. Martin Odersky, the creator of Scala, and Jonas Bonér, the creator of Akka, an actor-based concurrency framework, cofounded Typesafe (now Lightbend) to promote the language and tools built on it.

There are also a lot more books about Scala. So, do we really need a second edition of this book? Many excellent beginner’s guides to Scala are now available. A few advanced books have emerged. The encyclopedic reference remains Programming in Scala, Second Edition, by Odersky et al. (Artima Press).

Yet, I believe Programming Scala, Second Edition remains unique because it is a comprehensive guide to the Scala language and ecosystem, a guide for beginners to advanced users, and it retains the focus on the pragmatic concerns of working professionals. These characteristics made the first edition popular.

Scala is now used by many more organizations than in 2009 and most Java developers have now heard of Scala. Several persistent questions have emerged. Isn’t Scala complex? Since Java 8 added significant new features found in Scala, why should I switch to Scala?

I’ll tackle these and other, real-world concerns. I have often said that I was seduced by Scala, warts and all. I hope you’ll feel the same way after reading Programming Scala, Second Edition.

Welcome to Programming Scala, First Edition

Programming languages become popular for many reasons. Sometimes, programmers on a given platform prefer a particular language, or one is institutionalized by a vendor. Most Mac OS programmers use Objective-C. Most Windows programmers use C++ and .NET languages. Most embedded-systems developers use C and C++.

Sometimes, popularity derived from technical merit gives way to fashion and fanaticism. C++, Java, and Ruby have been the objects of fanatical devotion among programmers.

Sometimes, a language becomes popular because it fits the needs of its era. Java was initially seen as a perfect fit for browser-based, rich client applications. Smalltalk captured the essence of object-oriented programming as that model of programming entered the mainstream.

Today, concurrency, heterogeneity, always-on services, and ever-shrinking development schedules are driving interest in functional programming. It appears that the dominance of object-oriented programming may be over. Mixing paradigms is becoming popular, even necessary.

We gravitated to Scala from other languages because Scala embodies many of the optimal qualities we want in a general-purpose programming language for the kinds of applications we build today: reliable, high-performance, highly concurrent Internet and enterprise applications.

Scala is a multiparadigm language, supporting both object-oriented and functional programming approaches. Scala is scalable, suitable for everything from short scripts up to large-scale, component-based applications. Scala is sophisticated, incorporating state-of-the-art ideas from the halls of computer science departments worldwide. Yet Scala is practical. Its creator, Martin Odersky, participated in the development of Java for years and understands the needs of professional developers.

Both of us were seduced by Scala, by its concise, elegant, and expressive syntax and by the breadth of tools it put at our disposal. In this book, we strive to demonstrate why all these qualities make Scala a compelling and indispensable programming language.

If you are an experienced developer who wants a fast, thorough introduction to Scala, this book is for you. You may be evaluating Scala as a replacement for or complement to your current languages. Maybe you have already decided to use Scala, and you need to learn its features and how to use it well. Either way, we hope to illuminate this powerful language for you in an accessible way.

We assume that you are well versed in object-oriented programming, but we don’t assume that you have prior exposure to functional programming. We assume that you are experienced in one or more other programming languages. We draw parallels to features in Java, C#, Ruby, and other languages. If you know any of these languages, we’ll point out similar features in Scala, as well as many features that are new.

Whether you come from an object-oriented or functional programming background, you will see how Scala elegantly combines both paradigms, demonstrating their complementary nature. Based on many examples, you will understand how and when to apply OOP and FP techniques to many different design problems.

In the end, we hope that you too will be seduced by Scala. Even if Scala does not end up becoming your day-to-day language, we hope you will gain insights that you can apply regardless of which language you are using.

Conventions Used in This Book

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Italic

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Constant width

Used for program listings, as well as within paragraphs to refer to program elements such as variable or function names, databases, data types, environment variables, statements, and keywords.

Constant width bold

Shows commands or other text that should be typed literally by the user.

Constant width italic

Shows text that should be replaced with user-supplied values or by values determined by context.

Using Code Examples

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We appreciate, but do not require, attribution. An attribution usually includes the title, author, publisher, and ISBN. For example: “Programming Scala, Third Edition by Dean Wampler. Copyright 2020 Kevin Dean Wampler, 978-1-491-94985-6.”

If you feel your use of code examples falls outside fair use or the permission given above, feel free to contact us at [email protected].

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Acknowledgments for the Third Edition

Working with early builds of Scala 3, I often ran into unimplemented features and incomplete documentation. The members of the Scala community have provided valuable help while I learned what’s new. The EPFL documentation for Dotty, https://dotty.epfl.ch/docs/, provided essential information.

Acknowledgments for the Second Edition

As I, Dean Wampler, worked on this edition of the book, I continued to enjoy the mentoring and feedback from many of my Typesafe colleagues, plus the valuable feedback from people who reviewed the early-access releases. I’m especially grateful to Ramnivas Laddad, Kevin Kilroy, Lutz Huehnken, and Thomas Lockney, who reviewed drafts of the manuscript. Thanks to my long-time colleague and friend, Jonas Bonér, for writing an updated Foreword for the book.

And special thanks to Ann who allowed me to consume so much of our personal time with this project. I love you!

Acknowledgments for the First Edition

As we developed this book, many people read early drafts and suggested numerous improvements to the text, for which we are eternally grateful. We are especially grateful to Steve Jensen, Ramnivas Laddad, Marcel Molina, Bill Venners, and Jonas Bonér for their extensive feedback.

Much of the feedback we received came through the Safari Rough Cuts releases and the online edition available at http://programmingscala.com. We are grateful for the feedback provided by (in no particular order) Iulian Dragos, Nikolaj Lindberg, Matt Hellige, David Vydra, Ricky Clarkson, Alex Cruise, Josh Cronemeyer, Tyler Jennings, Alan Supynuk, Tony Hillerson, Roger Vaughn, Arbi Sookazian, Bruce Leidl, Daniel Sobral, Eder Andres Avila, Marek Kubica, Henrik Huttunen, Bhaskar Maddala, Ged Byrne, Derek Mahar, Geoffrey Wiseman, Peter Rawsthorne, Geoffrey Wiseman, Joe Bowbeer, Alexander Battisti, Rob Dickens, Tim MacEachern, Jason Harris, Steven Grady, Bob Follek, Ariel Ortiz, Parth Malwankar, Reid Hochstedler, Jason Zaugg, Jon Hanson, Mario Gleichmann, David Gates, Zef Hemel, Michael Yee, Marius Kreis, Martin Süsskraut, Javier Vegas, Tobias Hauth, Francesco Bochicchio, Stephen Duncan Jr., Patrik Dudits, Jan Niehusmann, Bill Burdick, David Holbrook, Shalom Deitch, Jesper Nordenberg, Esa Laine, Gleb Frank, Simon Andersson, Patrik Dudits, Chris Lewis, Julian Howarth, Dirk Kuzemczak, Henri Gerrits, John Heintz, Stuart Roebuck, and Jungho Kim. Many other readers for whom we only have usernames also provided feedback. We wish to thank Zack, JoshG, ewilligers, abcoates, brad, teto, pjcj, mkleint, dandoyon, Arek, rue, acangiano, vkelman, bryanl, Jeff, mbaxter, pjb3, kxen, hipertracker, ctran, Ram R., cody, Nolan, Joshua, Ajay, Joe, and anonymous contributors. We apologize if we have overlooked anyone!

Our editor, Mike Loukides, knows how to push and prod gently. He’s been a great help throughout this crazy process. Many other people at O’Reilly were always there to answer our questions and help us move forward.

We thank Jonas Bonér for writing the Foreword for the book. Jonas is a longtime friend and collaborator from the aspect-oriented programming (AOP) community. For years, he has done pioneering work in the Java community. Now he is applying his energies to promoting Scala and growing that community.

Bill Venners graciously provided the quote on the back cover. The first published book on Scala, Programming in Scala (Artima), that he cowrote with Martin Odersky and Lex Spoon, is indispensable for the Scala developer. Bill has also created the wonderful ScalaTest library.

We have learned a lot from fellow developers around the world. Besides Jonas and Bill, Debasish Ghosh, James Iry, Daniel Spiewak, David Pollack, Paul Snively, Ola Bini, Daniel Sobral, Josh Suereth, Robey Pointer, Nathan Hamblen, Jorge Ortiz, and others have illuminated dark corners with their blog entries, forum discussions, and personal conversations.

Dean thanks his colleagues at Object Mentor and several developers at client sites for many stimulating discussions on languages, software design, and the pragmatic issues facing developers in industry. The members of the Chicago Area Scala Enthusiasts (CASE) group have also been a source of valuable feedback and inspiration.

Alex thanks his colleagues at Twitter for their encouragement and superb work in demonstrating Scala’s effectiveness as a language. He also thanks the Bay Area Scala Enthusiasts (BASE) for their motivation and community.

Most of all, we thank Martin Odersky and his team for creating Scala.