Type classes in Scala 3

type_classes.worksheet.sc

trait Semigroup[T]:
  def mappend(x: T, y: T): T

trait Monoid[T] extends Semigroup[T]:
  def empty: T

object Semigroup:
  given sumIntSemigroup: Semigroup[Int] with
    override def mappend(x: Int, y: Int): Int = x + y

  given prodIntSemigroup: Semigroup[Int] with
    override def mappend(x: Int, y: Int): Int = x * y

  given stringSemigroup: Semigroup[String] with
    override def mappend(x: String, y: String): String =
      x + y
import Semigroup.given

object Monoid:
  given prodIntMonoid: Monoid[Int] with
    override def mappend(x: Int, y: Int): Int =
      prodIntSemigroup.mappend(x, y)
    def empty = 1

  given sumIntMonoid: Monoid[Int] with
    override def mappend(x: Int, y: Int): Int =
      sumIntSemigroup.mappend(x, y)
    def empty = 0

  given stringMonoid: Monoid[String] with
    override def mappend(x: String, y: String) =
      stringSemigroup.mappend(x, y)
    override def empty = ""
end Monoid
import Monoid.given

// it doesn't compile
// (1:Int) mappend (2:Int)

sumIntMonoid.mappend(1, 2)
stringMonoid.mappend("1", "2")

case class SomeRecord(x: Int, y: String)

object SomeRecord:
  val emptyRecord = SomeRecord(x = sumIntMonoid.empty, y = stringMonoid.empty)

  given someRecordMonoid: Monoid[SomeRecord] with
    override def mappend(self: SomeRecord, other: SomeRecord): SomeRecord =
      SomeRecord(x = sumIntMonoid.mappend(self.x, other.x),
                 y = stringMonoid.mappend(self.y, other.y))
    override def empty =
      SomeRecord(x = sumIntMonoid.empty, y = stringMonoid.empty)
import SomeRecord.given

someRecordMonoid.mappend(SomeRecord(x = 1, y = "1"), SomeRecord(2, "2")) // mappend SomeRecord.empty
// SomeRecord(x = 1, y = "1") mappend SomeRecord(2, "2") mappend SomeRecord.emptyRecord