Dotty 提供什么来替换类型投影？

Question

我一直在阅读有关 Dotty 的文章，因为它看起来即将成为 scala 3，并注意到类型投影被认为是“不合理的”并从语言中删除...

这看起来很糟糕，因为我已经看到了几个真正有用的用例。例如：

trait Contents
class Foo extends Contents
class Bar extends Contents

trait Container[T <: Contents] { type ContentType = T }
class FooContainer extends Container[Foo]
class BarContainer extends Container[Bar]

trait Manager[T <: Container[_]] { 
  type ContainerType = T 
  type ContentType = T#ContentType
  def getContents: ContentType 
  def createContainer(contents: ContentType): ContainerType
}

如何在 Dotty 中做这样的事情？向Manager添加第二个类型参数？但是，除了使创建和操作Manager 的实例变得非常乏味之外，它也不太有效，因为没有办法强制这两种类型之间的关系（Manager[FooContainer, Bar] 不应该合法）。

然后，还有其他用途，如类型 lambdas 和部分应用类型，它们对于创建偏向函子等很有用......或者这些（部分应用类型）是否成为 Dotty 中的“一等公民”？

编辑

为了回答 cmets 中的问题，这里有一个他可以使用的有点做作的例子。假设，我的Managers实际上是阿卡Actors：

abstract class BaseManager[T <: Container[_]](
  val storage: ContentStorage[T#ContentType]
) extends Actor with Manager[T] {
    def withContents(container: T, content: ContentType): ContainerType
    def withoutContents: T

    var container: T = withoutContents

    def receive: Receive {
       case ContentsChanged => 
          container = withContents(container, storage.get)
       case ContainerRequester => 
           sender ! container
       // ... other common actions 
    }
}

class FooManager(storage: FooStorage) extends BaseManager[FooContainer](storage) {
   def withContents(container: FooContainer, content: Foo) = 
       container.copy(Some(content))
   def withoutContent = FooContainer(None)

   override def receive: Receive = super.receive orElse { 
    // some additional actions, specific to Foo
   }
}

case class FooContainer(content: Option[Foo]) extends Container[Foo]{
  // some extremely expensive calculations that happen when 
  // content is assigned, so that we can cache the result in container
}

Answer 1

在 Scala 2.12 中，类型投影有时可以替换为类型类 + 路径依赖类型

trait ContentType[T <: Container[_]] {
  type Out
}
object ContentType {
  type Aux[T <: Container[_], Out0] = ContentType[T] { type Out = Out0 }
  def instance[T <: Container[_], Out0]: Aux[T, Out0] = new ContentType[T] { type Out = Out0 }

  implicit def mk[T <: Contents]: Aux[Container[T], T] = instance
}

abstract class Manager[T <: Container[_]](implicit val contentType: ContentType[T]) {
  type ContainerType = T
  def getContents: contentType.Out
  def createContainer(contents: contentType.Out): ContainerType
}

---

签入 Dotty 0.16.0-bin-20190529-3361d44-NIGHTLY（在 0.16.0-RC3 中 delegate 应该代替 implied）

trait Contents
class Foo extends Contents
class Bar extends Contents

trait Container[T <: Contents] { type ContentType = T }
class FooContainer extends Container[Foo]
class BarContainer extends Container[Bar]

trait ContentType[T <: Container[_]] {
  type Out
}
object ContentType {
  implied [T <: Contents] for ContentType[Container[T]] {
    type Out = T
  }
}

trait Manager[T <: Container[_]] given (val contentType: ContentType[T]) {
  type ContainerType = T
  type ContentType = contentType.Out
  def getContents: ContentType
  def createContainer(contents: ContentType): ContainerType
}

---

另一种选择是使用match types

trait Contents
class Foo extends Contents
class Bar extends Contents

trait Container[T <: Contents] { type ContentType = T }
class FooContainer extends Container[Foo]
class BarContainer extends Container[Bar]

type ContentType[T <: Container[_]] = T match {
  case Container[t] => t
}

trait Manager[T <: Container[_]] {
  type ContainerType = T
  def getContents: ContentType[T]
  def createContainer(contents: ContentType[T]): ContainerType
}