模式匹配`@`符号

Question

鉴于此Person案例类:

scala> case class Person(name: String, age: Int) {}
defined class Person

......而这个例子

scala> val b = Person("Kevin", 100)
b: Person = Person(Kevin,100)

是否有理由喜欢这个代码(有@)

scala> b match {
     |    case p @ Person(_, age) => println("age")
     |    case _ => println("none")
     | }
age

...以下？

scala> b match {
     |    case Person(_, age) => println("age")
     |    case _ => println("none")
     | }
age

也许我错过了意义/力量@？

Answer 1

您只需要包含@您想要处理对象本身的时间.因此:

that match{
  case p @ Person(_, age) if p != bill => age
  case Person(_, age) => age - 15
  case _ => println("Not a person")
}

否则,包含它并没有真正的意义.

Answer 2

关于上述答案的评论.

考虑这个案例类.

case class Employee(name: String, id: Int, technology: String)

在进行模式匹配时.

case e @ Employee(_, _, "scala") => e.name // matching for employees with only scala technology ... it works

case x: Employee => x.name // It also works

case e: Employee(_, _, "scala") => e.name // matching for employees with only scala technology ... **wont't work**