"Programming Scala, 3rd Edition" Code Examples

• Dean Wampler
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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.

The 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.)

[!WARNING] Scala 3 is evolving, as are the tools that support it. I try to keep the main branch up to date with the latest versions, including changing the examples as required to handle new and changed features (see, e.g., issue #131). Hence, sometimes an example (or how to run it) will be different from what you see in the book. So, if you are reading the book and want the examples exactly as they appear there, with the same tool versions used at that time, then grab the 3.0.0-final release.

In particular, running a scala program on the command line has changed as of 3.5.0. So, for example, at the top of page 12 of the book, change this command for running a program at the shell prompt:

$ cp="target/scala-3.5.0/classes/"  # Note the book has "3.0.0"
$ scala -classpath $cp progscala3.introscala.Hello2 Hello Scala World!

to this:

$ cp="target/scala-3.5.0/classes/"  # Note the book has "3.0.0"
$ scala -classpath $cp -M progscala3.introscala.Hello2 -- Hello Scala World!

Note the required -M (or --main-class) flag before the “main” class and the -- to separate scala arguments from your programs arguments. Use these changes for all subsequent examples in the book that use the scala command to run code.

It appears that sbt syntax has not changed when using runMain at the SBT prompt, for example:

runMain progscala3.introscala.Hello2 Hello Scala World!

(Use of sbt is discussed further below.)

[!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:

// src/main/scala/progscala3.introscala.UpperMain1

Following the usual conventions, tests are in src/test/....

Use these comments to find the corresponding source file. This archive also contains MUnit and ScalaCheck unit tests to validate some of the code.

Naming Conventions

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 sbt.
• src/test/.../*.scala - All Scala 3 test source files built and executed with sbt.
• src/script/.../*.scala - "Script" files that won't compile with scalac, but can be interpreted with the scala REPL 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 sbt.

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 project directory.

Follow these 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 sbt.
• 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:

1. Open the preferences ("cmd-," on MacOS)
2. Search for "scala"
3. Select "Build, Execution, Deployment > Compiler > Scala Compiler"
4. Select the "sbt" configuration in the list of Scala build configurations.
5. Select "Additional compiler options:".
6. Remove -encoding utf-8 from the text, since it is already in the build.sbt file.

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 bash script ./make-worksheets.sh to copy all the REPL "script" examples to worksheet files. This command copies the tree src/script to 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

After installing 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

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 REPL and load the script files manually at the prompt, for example:

$ scala
scala> :load src/script/scala/progscala3/introscala/Upper1.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, for example:

$ scala src/main/scala/progscala3/introscala/UpperMain2.scala -- Hello World!
HELLO WORLD!
$

[!NOTE] The -- argument separator is required for Scala 3.5.0 and later. It is not used for Scala 3.4.X and earlier.

Feedback

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