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// Variation of
import scala.annotation.infix
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 as Monoid[String]:
def unit: String = ""
extension (s: String) @infix def combine(other: String): String = s + other
given IntMonoid as Monoid[Int]:
def unit: Int = 0
extension (i: Int) @infix def combine(other: Int): Int = i + other
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