"Programming Scala, 3rd Edition" Code Examples
This repo contains all the code examples in O'Reilly's Programming Scala, Third Edition. (The second edition is available here.) There are also many code files in this distribution that aren't included in the book.
master branch and the
3.X.Y tag releases are for the third edition. The code examples for the second edition are still available. Download the release tagged 2.1.0 or check out the
release-2.1.0 branch. While the second edition was published for 2.11. The latest
2.1.0 release and
release-2.1.0 are updated for 2.12 and 2.13. (No more
release-2.X.Y releases are planned.)
TIP: Several sections offer troubleshooting tips if you encounter problems.
How the Code Is Used in the Book
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:
Following the usual conventions, tests are in
Use these comments to find the corresponding source file. This archive also contains MUnit and ScalaCheck unit tests to validate some of the code.
The examples include "scripts" that are intended for interactive use in the
scala command-line "REPL" (read, eval, print loop), for example using
sbt console (where
sbt is the de facto build tool for Scala that I use). Other files are compiled.
To keep these different kinds of files straight and to support building with
sbt, the following conventions are used for the files:
src/main/scala/.../*.scala- All Scala 3 source files built with
src/test/.../*.scala- All Scala 3 test source files built and executed with
src/script/.../*.scala- "Script" files that won't compile with
scalac, but can be interpreted with the
scalaREPL or used as a worksheet (see below).
src/*/scala-2/.../*.scala- Some Scala 2 source files that won't compile with Scala 3. They are not built with
Other Notes about the Code
You won't find many comments in the code, except of the form
// <1>, which get converted into markers corresponding to notes in the book. All the descriptions of the code are in the book, so they aren't repeated as code comments.
Some files have sections marked like this:
// tag::section1 // end::section1
These are used to mark sections that are selectively included in the book. Sometimes the whole file is included in sections, while other times the file has extra bonus content that doesn't appear in the book.
Required and Optional Tools
To build and run the examples, all you need is a recent version of the the JDK and
sbt. 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
sbt installation instructions.
If you want to install Scala separately and Scala's Scaladocs, go to the scala-lang.org Getting Started guide for details. However, this isn't required.
If you want to play with the Spark example,
src/script/scala-2/progscala3/bigdata/SparkWordCount.scala, you'll need to download a Spark distribution from https://spark.apache.org. Assuming that
$SPARK_HOME refers to the root directory of your Spark installation, run the following command in the root directory of this project:
$ $SPARK_HOME/bin/spark-shell ... scala>
Then copy and paste the content of
src/script/scala-2/progscala3/bigdata/SparkWordCount.scala at the prompt. After it runs, there will be a new directory,
README.md.wordcount with the partition files of the output.
Tip: For more on Spark, see my free tutorial on GitHub, spark-scala-tutorial.
Editors, IntelliJ, Visual Studio Code, and Other IDEs
NOTE: Support for Scala 3 may be limited for a while in the following tools.
Most editors and IDEs now have some sort of Scala support:
- IntelliJ: Either the Community or Ultimate additions will work. Install the Scala plugin, which has built-in support for
- Visual Studio Code: Use the new Scala Metals plugin instead of older plugins.
- Eclipse Scala IDE: Old, no longer recommended.
For other IDEs and text editors, try Scala Metals first (I've used it with Sublime Text, for example) or the older ENSIME project. You may also need additional, third-party tools for syntax highlighting, etc.
After installing the required plugins, load this project in your IDE, which should detect and use the
sbt project automatically. For eclipse, run the
sbt eclipse task to generate project files, then import them.
Troubleshooting with IntelliJ
One reader reported a problem when trying to run examples in IntelliJ:
scalac: Flag -encoding set repeatedly. I could confirm this problem and I fixed it as follows:
- Open the preferences ("cmd-," on MacOS)
- Search for "scala"
- Select "Build, Execution, Deployment > Compiler > Scala Compiler"
- Select the "sbt" configuration in the list of Scala build configurations.
- Select "Additional compiler options:".
-encoding utf-8from the text, since it is already in the
After that, you should be able to select a type with a
main and run it.
The same reader also reported errors where multiple occurrences of the same name in a
@targetName annotation collided. I believe this happens if you use
sbt in a terminal to compile and then allow IntelliJ to do its own build. There are probably two copies of the class files on the resulting runtime classpath. For example, I saw this error when attempting to run
sbt console in IntelliJ's
sbt shell, but not when I used
sbt in a terminal window.
So, what worked for me was to only use the terminal to run
sbt clean, then let IntelliJ build the software itself, but when I need to use
sbt console, I use a terminal window.
Using Scala Worksheets
If you like working with Scala worksheets in your IDE or editor, you may be able to load any of the REPL "script" files under
src/script as a worksheet. If your environment is more restrictive, for example about the file extension used, then run the included
./make-worksheets.sh to copy all the REPL "script" examples to worksheet files. This command copies the tree
src/worksheet and changes the
.scala extension for all the files to
.worksheet.sc, the VSCode convention. These behaviors are configurable. Use the
--help option to see the details. If you are using Windows and you don't have
bash available, e.g., through the Linux subsystem, then modify individual files as you see fit.
See this Dotty documentation page for more information about worksheets.
Building the Code Examples
sbt, open a command/terminal window and run the
sbt test command.
You'll see lots of output as it downloads all the dependencies, compiles the code and runs the tests. You should see
[success] messages at the end.
sbt is discussed in more detail in the book and the
sbt website, but a few useful 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 (
sbt:programming-scala-3rd-ed-code-examples>), 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 # continuously compile and test when source changes are saved. console # run the Scala REPL; dependencies and code are on the CLASSPATH tasks # show the most common tasks (commands). tasks -V # REALLY show ALL tasks
~ 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!
sbt, you could, in principle, run the REPL and load the script files manually at the prompt:
$ scala scala> :load src/script/scala/.../Foo.scala
However, it's easier to run most of the scripts using
sbt console, because
sbt will configure the
CLASSPATH with the third-party libraries and compiled code examples that a script file might use.
Also, new for the Scala 3 REPL, for those
src/main/... files that define one (and only one) entry point, meaning a
main method (Scala 2 compatible) or annotated with
@main (new Scala 3 technique), you can compile and run them in one step:
$ scala src/main/scala/progscala3/introscala/UpperMain2.scala Hello World! HELLO WORLD! $
I welcome feedback on the Book and these examples. Please post comments, corrections, etc. to one of the following places:
- This GitHub repo's Gitter channel, Discussion forum, or Issues.
- The book's Twitter account, @ProgScala.
- The O'Reilly book and errata sites (coming soon).
There is also my dedicated site for the book where occasional updates, clarifications, corrections, and lame excuses will be posted: programming-scala.org.
A Little History
|August 11, 2014||2nd edition examples|
|May 27, 2019||Updated for Scala 2.12 and 2.13|
|June 18, 2019||New support for Maven builds, courtesy of oldbig|
|October 12, 2019||Updated for Scala 2.13.1, sbt 1.3.2, and other dependencies. Also now compiles with JDK 11|
|October 13, 2019||Renamed the repo from
|December 31, 2019||Renamed the
|March 1, 2020||Completed conversion to Scala 3|
|March 20, 2020||Started incorporating new Scala 3 syntax, idioms|
|May 15, 2021||Scala
|May 22, 2021||Final updates for Programming Scala, Third Edition!|
|July 24, 2021||Scala 3.0.1. Notes on using IntelliJ.|
|November 6, 2021||Scala 3.1.0 and a fix for locale settings (PR 42).|