The code examples used in Programming Scala, 2nd Edition (O'Reilly)
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Programming Scala, 2nd Edition#

README for the Code Examples

Dean Wampler
August 11, 2014

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This archive contains all the code examples found in Programming Scala, Second Edition, with the exception of some trivial code snippets in the text. There are also some examples in this distribution that aren't actually in the book.

The examples use Scala 2.11, which is specified in the sbt build.

In the book's text, when an example corresponds to a file in this distribution, the listing begins with a path in a comment with the following format:

// src/main/scala/progscala2/chapter/../filename

And similarly for Java files (yes, there are Java files!). Following the usual conventions, tests are in src/test/....

Use these comments to find the corresponding source file. This archive also contains ScalaTest and ScalaCheck unit tests to validate some of the code. Most of these tests are not reproduced in the text of the book, except when discussing testing itself.

Naming Conventions

The examples include "scripts" that are run with the scala command (or within SBT using the console), source files that are compiled with scalac, and source files that deliberately fail to compile to demonstrate common errors. To keep them straight and to support building with SBT, the following naming conventions are used for the files:

  • build.sbt - The SBT build script (described below).
  • *.scala - Source files that are compiled with scala. In fact, this is the community-standard extension for all Scala files, code to be compiled or scripts. But to keep the build process simple, I use different conventions for files that aren't compiled, discussed next.
  • *.sc - Script files that are executed directly, e.g., scala This file extension is not a standard, but it is used by the newer IDE worksheet feature I discuss in the book. So, I stole the convention; SBT will ignore these scripts when compiling. These script don't have tests to verify them (TODO).
  • *.javaX, *.scalaX and *.scX - Java and Scala source files and scripts with deliberate errors, so they don't compile and run, or building them would require significant changes to the build that were deemed unnecessary. Most contain comments explaining what's wrong with them or in some cases, the corresponding section of the book provides the details.

Required and Optional Tools

To build and run the examples, all you need to do is install SBT. SBT is the de-facto standard build tool for Scala. When you run SBT, it will bootstrap itself with the correct version of its jar file, Scala, and project dependencies, which are specified in the build.sbt file in the root directory and other build files in the project directory.

Follow these installation instructions.

If you want to install Scala separately and Scaladocs (recommended), go to, but this isn't strictly required.

Editors, Eclipse, and IntelliJ

Most editors and IDEs now include Scala plugins. For Eclipse, see the Scala IDE project. For IntelliJ use the plugin browser to find and install the Scala plugin. For NetBeans, see the Scala page in their wiki. For other editors, Google is your friend. ;)

After installing the required plugin, to load this project in your IDE, IntelliJ can import the SBT project directly. For eclipse, run sbt eclipse task to generate project files, then import them. For Netbeans, see the Scala wiki page.

Building the Code Examples

After installing SBT, open a command/terminal window and run the sbt test command.

There will be a lot of output as it downloads all the dependencies, both internal to the tools and explicitly defined for the code examples.

After resolving dependencies, the code will be compiled and tested. You should see only success messages.

SBT is discussed in more detail in the book, but a few other commands are worth mentioning here.

If you start sbt without any arguments, it puts you into an interactive mode where you can type commands. Use control-D to exit this mode. Once at the SBT prompt (a >), try the following commands, where each # starts a comment; don't type those!

help       # help on tasks and settings
clean      # delete all build outputs
compile    # compile the source, but not test code
test       # compile source and test code, if necessary and run the tests.
+test      # compile and test for all versions of Scala supported.
~test      # continuously compile and test when source changes are saved.
tasks      # show the most common tasks (commands). 
tasks -V   # REALLY show ALL tasks

Note the +test example. It looks at a property named crossScalaVersions in the build file, build.sbt to know which versions of Scala to use. The + can be used for any task, although for some it will make little sense. Similarly, the ~ prefix causes the task to be run continuously each time source code changes are saved. This promotes continuous TDD (test-driven development) and is one of my favorite features!

Outside of SBT, you could, in principle, run the script files manually at the console/terminal prompt.

scala src/main/scala/.../

However, many of the scripts require other project code that has been compiled (which is in target/scala-2.11/classes) and occasionally third-party libraries that are part of the project dependencies.

For example, if build artifacts are required on the class path, use the following command from the root directory of the distribution:

scala -classpath target/scala-2.11/classes src/main/scala/.../

Usually, the best way to run the scripts is to start sbt and run console to start the Scala REPL with all the dependencies added to the classpath. Then, use the REPL :load src/main/scala/.../ to load and run the script.


I welcome feedback on the examples. Please post comments, corrections, etc. at the books site, Programming Scala, Second Edition, or you can post feedback on the O'Reilly forum. There is also a dedicated site for the book where occasional updates, clarifications, corrections, and lame excuses will be posted: