Scala 3 Implicit

July 4, 2024

This is a Scala 3 port of the Scala 2 Type Classes example by Rock the JVM. For reference of the updated Implicits in Scala 3, read the reference on Given Instances.

Scala 3

In Scala 3, given and using replace the original implicit object and implicit, respectively. Define the implicit object with given and specify where it can be used with using.

trait Summable[T] {
  def sumElements(list: List[T]): T
}

given intSum: Summable[Int] with
  def sumElements(list: List[Int]) = list.sum

// With braces (aka curly brackets)
given stringSum: Summable[String] with {
  def sumElements(list: List[String]) = list.mkString("")
}

def processMyList[T](list: List[T])(using summable: Summable[T]): T =
  summable.sumElements(list)

Scala 2

Define the implicit object and override the trait methods.

trait Summable[T] {
  def sumElements(list: List[T]): T
}

implicit object IntSummable extends Summable[Int] {
  override def sumElements(list: List[Int]) = list.sum
}

implicit object StringSummable extends Summable[String] {
  override def sumElements(list: List[String]) = list.mkString("")
}

def processMyList[T](list: List[T])(implicit summable: Summable[T]): T = {
  summable.sumElements(list)
}

Full Scala 3 Example

@main def hello(): Unit = {
  val intSum = processMyList(List(1, 2, 3))
  val stringSum = processMyList(List("A", "B", "C"))
  
  println(intSum)
  println(stringSum)
}

trait Summable[T] {
  def sumElements(list: List[T]): T
}

given intSum: Summable[Int] with
  def sumElements(list: List[Int]) = list.sum

given stringSum: Summable[String] with
  def sumElements(list: List[String]) = list.mkString("")

def processMyList[T](list: List[T])(using summable: Summable[T]): T =
  summable.sumElements(list)

Output

6
ABC

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