val vs var

val vs var

val declares an immutable binding -- once assigned, it can never be reassigned -- while var declares a mutable variable that can be reassigned later. Scala idiom strongly favors val by default, only reaching for var when mutation is genuinely needed. This preference for immutability is a core part of Scala's functional programming style.

Warning: Attempting to reassign a val, like x = 5 after val x = 5, is a compile error, not a runtime one.

Example: val vs var

object Main extends App {
  val x = 10
  var y = 20

  y = 30 // allowed, y is a var
  println(s"x = $x, y = $y")
}