some_examples.scala

trait Semigroup[T]:
  type C = T
  extension (a: C) def |+|(b: C): C

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

def mkMonoid[T: Semigroup as s](e: T) = 
  new:
    def empty = e
    export s.*


given additive: Semigroup[Int] with
   extension (a: C) def |+|(b: C) = a + b

given multiplicative: Semigroup[Int] with
 extension (a: C) def |+|(b: C) = a * b

extension [T: Semigroup](t: T) def double = t |+| t

@main
def tst =
  given Semigroup[Int] = additive
  val a = 10.double
  val b = 10.double(using multiplicative)
  println(s"a = $a, b = $b")