为什么 Semigroup 有一个 Option 类型并且在 Monoid 中表现得像一个中性元素？

Question

Why does Semigroup has an Option type and None behave like a neutral element in Monoid?

val two: Option[Int] = Option(2)
val n: Option[Int] = None

n |+| two should be(Some(2))//why do we have the opportunity to do this?
two |+| n should be(Some(2))//n look like neutral here

Semigroup[Option[Int]].combine(Option(1), None) should be(Some(1))//why does semigroup has it?

Answer 1

Every Monoid is also a Semigroup . Semigroup doesn't have to have an identity element, but every Semigroup that is also a Monoid will always have one (without "knowing" about the concept).

Another example: "addition of integers" semigroup doesn't formally posses an identity element (zero). It doesn't define one. But you can still add 3 + 0 = 3.