Scala 3: new but optional syntax

This is the first article in my series that provides an overview of the changes in Scala 3.

Let's start with the most controversial innovations: optional curly braces and

new syntax for control constructs.

The optional curly braces make Scala code more like Python or Haskell, which uses indentation to group expressions. Let's take a look at examples taken from the 3rd edition of my book Programming Scala , which is now in preparation for publication.

Optional curly braces

First, let's look at declaring a type using the old and new syntax. This also works for packages if we declare multiple packages in the same file.

//  
trait Monoid2[A] {
  def add(a1: A, a2: A): A
  def zero: A
}

//  
trait Monoid3[A]:
  def add(a1: A, a2: A): A
  def zero: A
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

The new syntax closely resembles Python, and it can be confusing if you constantly switch between the two languages.

You can mix old and new style, the Dotty compiler (soon to be renamed to Scala 3) will compile such code without errors.

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def m2(s: String): String = {
  val result = s.toUpperCase
  println(s"output: $result")
  result
}

def m3(s: String): String =
  val result = s.toUpperCase
  println(s"output: $result")
  result
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

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partial functions, match expressions try-catch-finally ( ):

val o2:Option[Int] => Int = {
  case Some(i) => i
  case None => 0
}

val o3:Option[Int] => Int =
  case Some(i) => i
  case None => 0

      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

0 match {
  case 0 => "zero"
  case _ => "other value"
}

0 match
  case 0 => "zero"
  case _ => "other value"

      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

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import scala.annotation.tailrec

@tailrec def loop(whileTrue: => Boolean)(f: => Unit): Unit =
  f
  if (whileTrue) loop(whileTrue)(f)

var i=5
loop(i > 0) {
  println(i)
  i -= 1
}

var j=5
loop(j > 0):       // ERROR
  println(j)
  j -= 1
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

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for (i <- 0 until 5) println(i)   //  
for i <- 0 until 5 do println(i)  //  
for i <- 0 until 5 yield 2*i
for i <- 0 until 10
  if i%2 == 0
  ii = 2*i
yield ii

val i = 10
if (i < 10) println("yes")        //  
else println("no")
if i < 10 then println("yes")     //  
else println("no")
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

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