Reading Input and Arguments

Command-line arguments

An @main method's parameters become command-line arguments — Scala parses and checks them for you:

// illustrative
@main def greet(name: String, times: Int): Unit =
  for _ <- 1 to times do println(s"Hello, $name!")

$ scala run greet.scala -- Ada 2
Hello, Ada!
Hello, Ada!

$ scala run greet.scala -- Ada two
Illegal command line after first argument: java.lang.NumberFormatException: For input string: "two"

The arguments after -- go to your program. For a variable number of arguments, use @main def run(args: String*). The built-in message for invalid input is terse, so real tools often use a library such as mainargs or decline, or parse args themselves.

Reading from the keyboard

// illustrative
import scala.io.StdIn

@main def ask(): Unit =
  val name = StdIn.readLine("What is your name? ")
  val age = StdIn.readLine("How old are you? ").toIntOption
  age match
    case Some(a) => println(s"Hi $name, next year you'll be ${a + 1}.")
    case None    => println(s"Hi $name, that wasn't a number.")

What is your name? Ada
How old are you? 36
Hi Ada, next year you'll be 37.

readLine always returns a String, so convert it with toIntOption (or toDoubleOption) and handle invalid input.

Reading all input lines

// illustrative
import scala.io.Source

@main def sumLines(): Unit =
  val numbers = Source.stdin.getLines().flatMap(_.toIntOption).toList
  println(s"Sum: ${numbers.sum}")

Run it with piped input — printf "1\n2\nx\n3\n" | scala run sum.scala — and it prints Sum: 6, skipping the invalid line.