The Scala REPL

The Scala REPL (“Read-Evaluate-Print-Loop”) is a command-line interpreter that you use as a “playground” area to test your Scala code. To start a REPL session, just type scala at your operating system command line, and you’ll see this:

$ scala
Welcome to Scala 2.13.0 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_131).
Type in expressions for evaluation. Or try :help.

scala> _

Because the REPL is a command-line interpreter, it just sits there waiting for you to type something. Once you’re in the REPL, you can type Scala expressions to see how they work:

scala> val x = 1
x: Int = 1

scala> val y = x + 1
y: Int = 2

As those examples show, just type your expressions inside the REPL, and it shows the result of each expression on the line following the prompt.

Variables created as needed

Note that if you don’t assign the result of your expression to a variable, the REPL automatically creates variables that start with the name res. The first variable is res0, the second one is res1, etc.:

scala> 2 + 2
res0: Int = 4

scala> 3 / 3
res1: Int = 1

These are actual variable names that are dynamically created, and you can use them in your expressions:

scala> val z = res0 + res1
z: Int = 5

You’re going to use the REPL a lot in this book, so go ahead and start experimenting with it. Here are a few expressions you can try to see how it all works:

val name = "John Doe"
"hello".head
"hello".tail
"hello, world".take(5)
println("hi")
1 + 2 * 3
(1 + 2) * 3
if (2 > 1) println("greater") else println("lesser")

In addition to the REPL there are a couple of other, similar tools you can use:

• Scastie is “an interactive playground for Scala” with several nice features, including being able to control build settings and share code snippets
• IntelliJ IDEA has a Worksheet plugin that lets you do the same things inside your IDE

For more information on the Scala REPL, see the Scala REPL overview