Your First Program

Create a file named hello.scala:

@main def hello(): Unit =
  println("Hello, world!")
  println("Welcome to Scala.")

Run it:

scala run hello.scala

Expected output

Hello, world!
Welcome to Scala.

Line by line:

• @main marks the method where the program starts.
• def hello() defines a method called hello with no parameters.
• : Unit is the return type — Unit means "returns nothing useful", like void in Java.
• = introduces the method's body.
• The indented lines below are the body. Scala 3 uses indentation to group code, like Python.
• println prints a line of text.

Braces are optional

Scala 3 also accepts curly braces, the only style in Scala 2 — you'll see both in real code:

@main def hello(): Unit = {
  println("Hello, world!")
}

Hello, world!

This course uses the indentation style of the official Scala 3 documentation.

Common mistakes

• Inconsistent indentation: lines in the same block must line up.
• Forgetting the = after the method signature. In Scala 3 that's a syntax error.

Practice

Write a program that prints your name on one line and your favorite food on the next.

@main def aboutMe(): Unit =
  println("Ada")
  println("Pizza")

Ada
Pizza