Scala 3 Syntax Rewriting

Scala 3 adds new syntax for control structures, and makes braces optional through significant indentation. Both are optional: the Scala 2 style is still valid in Scala 3, and converting is not part of the migration. The Optional Braces and New Control Syntax pages of the reference describe them.

The compiler can rewrite code to either style, using one of these options together with -rewrite:

Only one of them can be used per compilation.

Example

Given this code in Scala 2 style:

case class State(n: Int, minValue: Int, maxValue: Int) {

  def inc: State =
    if (n == maxValue)
      this
    else
      this.copy(n = n + 1)

  def printAll: Unit = {
    println("Printing all")
    for {
      i <- minValue to maxValue
      j <- 0 to n
    } println(i + j)
  }
}

Compiling with -new-syntax -rewrite, then with -indent -rewrite, gives:

case class State(n: Int, minValue: Int, maxValue: Int):

  def inc: State =
    if n == maxValue then
      this
    else
      this.copy(n = n + 1)

  def printAll: Unit =
    println("Printing all")
    for
      i <- minValue to maxValue
      j <- 0 to n
    do println(i + j)

Run -new-syntax first: -indent keeps the braces around the generators of a for that uses the old syntax.

Compiling with -no-indent -rewrite, then with -old-syntax -rewrite, goes back to the classic style.

Enforcing a Specific Syntax

Without -rewrite, -new-syntax, -old-syntax and -no-indent make the compiler report an error on code that uses the other syntax. For example, under -new-syntax:

-- Error: State.scala:4:7 ------------------------------------------------------
4 |    if (n == maxValue)
  |       ^^^^^^^^^^^^^^^
  |This construct is not allowed under -new-syntax or -language:future.
  |This construct can be rewritten automatically under -rewrite -source 3.0-migration -new-syntax or -language:future.

Significant indentation cannot be enforced.