型クラス（Catsスタイル）

// Define
trait Semigroup[A] {
  def combine(x: A, y: A): A
}

// Instances
implicit val intSum: Semigroup[Int] = (x, y) => x + y
implicit val strConcat: Semigroup[String] = (x, y) => x + y

// Generic function
def combineAll[A](xs: List[A])(implicit s: Semigroup[A]): A =
  xs.reduce(s.combine)

combineAll(List(1, 2, 3))           // 6
combineAll(List("a", "b", "c"))     // "abc"

// Cats provides Monoid, Functor, Monad, etc. the same way