1

是否有一种单行方式来提供对一个值的命名using引用，该值在没有即将被弃用的implicit关键字的情况下隐式可用（即通过语法可用）？根据文档，我希望以下工作（在 SBT 下，scalaVersion := "3.0.0-M2"：

trait Greeter {
  def sayHello(username: String): Unit
  def shutdown(): Unit
}

def greet(username: String)(using g: Greeter): Unit = {
  g.sayHello(username)
}

object Foo extends App {
  given greeter: Greeter = new Greeter {
    override def sayHello(username: String): Unit = println(s"Hello, ${username}!")

    override def shutdown(): Unit = println("Shutting down")
  }
  
  try {
    greet("world")
  } finally {
    greeter.shutdown()
  }
}

但这失败了

[error] -- Error: /path/to/project/src/main/scala/Foo.scala:12:15 
[error] 12 |  given greeter: Greeter = new Greeter {
[error]    |               ^
[error]    |               end of statement expected but ':' found
[error] -- [E040] Syntax Error: /path/to/project/src/main/scala/Foo.scala:12:17 
[error] 12 |  given greeter: Greeter = new Greeter {
[error]    |                 ^^^^^^^
[error]    |                 ';' expected, but identifier found

现在：我可以通过多种方式解决这个问题，但要么文档令人困惑（或错误），要么我误解了一些相当基本的东西。

解决方法 1（如此处建议）：

lazy val greeter: Greeter = new Greeter {
  override def sayHello(username: String): Unit = println(s"Hello, ${username}!")
  override def shutdown(): Unit = println("Shutting down")
}
given Greeter = greeter
...

但我希望能够用一个表达式而不是两个表达式来做到这一点。相同的注释适用于先定义给定，然后将其绑定到名称：

given Greeter with {
  override def sayHello(username: String): Unit = println(s"Hello, ${username}!")

  override def shutdown(): Unit = println("Shutting down")
}
val greeter: Greeter = implicitly

特别是因为我认为implicitly将在 3.1 中弃用并在 3.2 中消失。

我们还可以将给定对象的调用包装在函数中来解决这个问题：

def greet(username: String)(using g: Greeter): Unit = {
  g.sayHello(username)
}

def shutdown(using greeter: Greeter): Unit = {
  greeter.shutdown()
}  

given Greeter with {
  override def sayHello(username: String): Unit = println(s"Hello, ${username}!")

  override def shutdown(): Unit = println("Shutting down")
}

try {
  greet("world")
} finally {
  shutdown
}

但这对我来说似乎是样板。

4

1 回答 1

3

原来，

given x: T = ...

在修订版 M2 和修订版 RC1 之间添加了语法。以下适用于scalaVersion := "3.0.0-RC1"：

trait Greeter {
  def sayHello(username: String): Unit
  def shutdown(): Unit
}

def greet(username: String)(using g: Greeter): Unit = {
  g.sayHello(username)
}

object Foo extends App {
  given greeter: Greeter with {
    override def sayHello(username: String): Unit = println(s"Hello, ${username}!")
  
    override def shutdown(): Unit = println("Shutting down")
  }
  
  try {
    greet("world")
  } finally {
    greeter.shutdown()
  }
}
于 2021-03-06T20:15:50.873 回答