Scala 对象不扩展任何内容

Question

我在我们的代码库中遇到了一段代码，在我看来无效，但编译成功并运行。

object Main extends {
  def main(args: Array[String]): Unit = {
    print("Hello World")
  }
}

Hello World

有人可以解释一下，这里发生了什么？
是否 Main类在这里扩展匿名 class/ trait ?

Answer 1

如果我们使用 scala -Xprint:typer 反编译代码，我们看到 Main扩展 AnyRef :

scalac -Xprint:typer Main.scala                                                                                               
[[syntax trees at end of                     typer]] // Main.scala
package com.yuvalitzchakov {
  object Main extends scala.AnyRef {
    def <init>(): com.yuvalitzchakov.Main.type = {
      Main.super.<init>();
      ()
    };
    def main(args: Array[String]): Unit = scala.Predef.print("Hello World")
  }
}

这也记录在 Scala specification 中。在对象/类定义下:

An object definition defines a single object of a new class. Its most general form is object m
extends t. Here, m is the name of the object to be defined, and t is a template of the form

sc with mt1 with … with mtn { stats } which defines the base classes, behavior and initial state of m. The extends clause extends sc with
mt1 with … with mtn can be omitted, in which case extends scala.AnyRef is assumed.

此语法也适用于 early initializers :

abstract class X {
  val name: String
  val size = name.size
}

class Y extends {
  val name = "class Y"
} with X