如何具体设置类型绑定的抽象类型？

Question

我试图将case对象的类型用作抽象类型.我很惊讶地看到下面的(类似)代码编译:

sealed abstract class Bar

case object BarOne extends Bar

case object BarTwo extends Bar

sealed abstract class Foo {
  type A <: Bar

  def f: A
}

object Foo {
  object FooOne extends Foo {
    type A = BarOne.type
    val f = BarTwo
  }

  object FooTwo extends Foo {
    type A = BarTwo.type
    val f = BarOne
  }
}

在我的实例中Foo是参数化并用作案例类.所以我不能只做A一个类型参数.

怎么f = BarTwo编译,什么时候A设置BarOne.type？

如果Ain f: A被解释为A <: Bar,为什么会这样？

有没有办法A为每个对象实例具体设置Foo？

---

我正在使用Scala 2.11.8.

---

更新:当我val attributeType = ...用def attributeType = ...in FooOne&FooTwocompilation 替换失败时(如预期的那样).

Answer 1

有人建议您升级到 Scala 的现代版本吗？（玩笑。）

有关覆盖的错误为该类型提供了一个很好的路径。

$ scala
Welcome to Scala 2.12.0-M5 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_92).
Type in expressions for evaluation. Or try :help.

scala> :pa
// Entering paste mode (ctrl-D to finish)

sealed abstract class Bar

case object BarOne extends Bar

case object BarTwo extends Bar

sealed abstract class Foo {
  type A <: Bar

  def f: A
}

object Foo {
  object FooOne extends Foo {
    type A = BarOne.type
    val f = BarTwo
  }

  object FooTwo extends Foo {
    type A = BarTwo.type
    val f = BarOne
  }
}

// Exiting paste mode, now interpreting.

<console>:26: error: overriding method f in class Foo of type => Foo.FooOne.A;
 value f has incompatible type
           val f = BarTwo
               ^
<console>:31: error: overriding method f in class Foo of type => Foo.FooTwo.A;
 value f has incompatible type
           val f = BarOne
               ^

错误就是这个，重复的问题来自去年 11 月。

这个错误的本质也很聪明：它是由 引入的-Yoverride-objects，这不是一个非常有用的选项，但出现在我的几个 SO 答案中，现在又出现在一个问题中。

编辑：

$ scala
Welcome to Scala 2.11.8 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_92).
Type in expressions for evaluation. Or try :help.

scala> object X ; object Y
defined object X
defined object Y

scala> class C { def f: X.type = X }
defined class C

scala> class D extends C { override def f: Y.type = Y }
defined class D

scala> :quit
$ scalam
Welcome to Scala 2.12.0-M5 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_92).
Type in expressions for evaluation. Or try :help.

scala> scala> object X ; object Y

// Detected repl transcript. Paste more, or ctrl-D to finish.

defined object X
defined object Y

scala> class C { def f: X.type = X }
defined class C

scala> class D extends C { override def f: Y.type = Y }
defined class D
// Replaying 3 commands from transcript.

scala> object X ; object Y
defined object X
defined object Y

scala> class C { def f: X.type = X }
defined class C

scala> class D extends C { override def f: Y.type = Y }
<console>:13: error: overriding method f in class C of type => X.type;
 method f has incompatible type
       class D extends C { override def f: Y.type = Y }
                                        ^