scala2-3-AliasGivens.scala

scala> import scala.annotation.targetName
     
scala> trait Semigroup[T]:             // also used in other examples for Type Classes
     |   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
     
scala> given NumericMonoid2[T : Numeric]: Monoid[T] = new Monoid[T]:
     |   println("Initializing NumericMonoid2")
     |   def unit: T = summon[Numeric[T]].zero
     |   extension (t: T) infix def combine(other: T): T =
     |     summon[Numeric[T]].plus(t, other)
def NumericMonoid2[T](using evidence$1: Numeric[T]): Monoid[T]
     
scala> given StringMonoid2: Monoid[String] = new Monoid[String]:
     |   println("Initializing StringMonoid2")
     |   def unit: String = ""
     |   extension (s: String)
     |     infix def combine(other: String): String = s + other    
lazy val StringMonoid2: Monoid[String]

// Use them. Note the output:
scala> 2.2 <+> (3.3 <+> 4.4)             // 9.9
     | (2.2 <+> 3.3) <+> 4.4             // 9.9
Initializing NumericMonoid2
Initializing NumericMonoid2
Initializing NumericMonoid2
Initializing NumericMonoid2
val res0: Double = 9.9
val res1: Double = 9.9

scala> "2" <+> ("3" <+> "4")             // "234"
     | ("2" <+> "3") <+> "4"             // "234"
Initializing StringMonoid2
val res2: String = 234
val res3: String = 234