scala2-3-MonoidTypeClassDef-Infix.scala

// Variation of https://gist.github.com/deanwampler/e806a3301380d5d70c3247264afb076a

trait Semigroup[T]:
  extension (t: T)
    infix def combine(other: T): T    // infix allows "foo combine bar".
    def <+>(other: T): T = t.combine(other)

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

// Redefine the monoid instances. Note that infix is required for the two combine definitions, even though the trait
// had infix.
given StringMonoid: Monoid[String] with
  def unit: String = ""
  extension (s: String) infix def combine(other: String): String = s + other

given IntMonoid: Monoid[Int] with
  def unit: Int = 0
  extension (i: Int) infix def combine(other: Int): Int = i + other