scala2-3-MonoidTypeClassDef-revisited.scala

import scala.annotation.targetName

trait Semigroup[T]:
  extension (t: T)
    infix def combine(other: T): T
    @targetName("plus") def <+>(other: T): T = t.combine(other)

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

given NumericMonoid[T](using num: Numeric[T]): Monoid[T] with
  def unit: T = num.zero
  extension (t: T) infix def combine(other: T): T = num.plus(t, other)