Scala:什么是::in { case x::y::_ => y}？方法还是案例类？

Question

object Test1 extends App {
    val list: List[Int] => Int = {
        case x :: y :: _ => y    //what is ::? method or case class?
    }
    println(list(List(1, 2, 3)))    //result is 2.
}

我在 scala IDE 中设置了“语法着色”，方法 的前景色我设置为红色。代码截图: 而且我无法打开black::的声明，所以我不知道它是什么。

如果black::是方法，应该这样调用:

... {case _.::(y).::(x) => y}     //compile failed!

那么，什么是black::？方法还是案例类？

非常感谢!

Answer 1

我认为这是描述的方法 here .为了历史的缘故，以防该页面消失，这里是简介:

About pattern matching on Lists

If you review the possible forms of patterns explained in Chapter 15, you might find that neither List(...) nor :: looks like it fits one of the kinds of patterns defined there. In fact, List(...) is an instance of a library-defined extractor pattern. Such patterns will be treated in Chapter 24. The "cons" pattern x :: xs is a special case of an infix operation pattern. You know already that, when seen as an expression, an infix operation is equivalent to a method call. For patterns, the rules are different: When seen as a pattern, an infix operation such as p op q is equivalent to op(p, q). That is, the infix operator op is treated as a constructor pattern. In particular, a cons pattern such as x :: xs is treated as ::(x, xs). This hints that there should be a class named :: that corresponds to the pattern constructor. Indeed there is such as class. It is named scala.:: and is exactly the class that builds non-empty lists. So :: exists twice in Scala, once as a name of a class in package scala, and again as a method in class List. The effect of the method :: is to produce an instance of the class scala.::. You'll find out more details about how the List class is implemented in Chapter 22.

所以它是 scala.::(a,b)