无法证明与路径相关类型的等价性

Question

为什么最后一个summon 编译失败？我该怎么做才能让它编译？

import java.time.{LocalDateTime, LocalTime}

trait Circular[T]:
  type Parent

given localTimeCircular: Circular[LocalTime] with
  type Parent = LocalDateTime

final class CircularMap[K, +V]()(using val circular: Circular[K])

val summoned = summon[Circular[LocalTime]]
val daily = new CircularMap[LocalTime, Int]()
println(summoned == daily.circular) // Prints true

summon[localTimeCircular.Parent =:= LocalDateTime]
summon[summoned.Parent =:= LocalDateTime]

summon[daily.circular.Parent =:= LocalDateTime]

Scastie link

在 Scala 3.0.2 和 3.1.0-RC1 上失败

Answer 1

这件事不在 Scala 3 中。在 Scala 2.13.6 中，类似代码无法编译

import shapeless.the

import java.time.{LocalDateTime, LocalTime}

trait Circular[T] {
  type Parent
}

implicit object localTimeCircular extends Circular[LocalTime] {
  type Parent = LocalDateTime
}
//  implicit val localTimeCircular: Circular[LocalTime] {type Parent = LocalDateTime} = new Circular[LocalTime] {
//    type Parent = LocalDateTime
//  }

final class CircularMap[K, +V]()(implicit val circular: Circular[K])

val summoned = the[Circular[LocalTime]]
val daily = new CircularMap[LocalTime, Int]()
println(summoned == daily.circular) // Prints true

implicitly[localTimeCircular.Parent =:= LocalDateTime]
implicitly[summoned.Parent =:= LocalDateTime]

//implicitly[daily.circular.Parent =:= LocalDateTime] // doesn't compile

https://scastie.scala-lang.org/DmytroMitin/lsX3fS3ET0ajzChwl2Mctw

（我将implicitly 替换为the 在一处，因为implicitly 会破坏类型细化，summon 更像the）。

让我们简化您的代码，以便弄清楚发生了什么。让我们移除隐含（让我们明确地解决它们）

import java.time.{LocalDateTime, LocalTime}

trait Circular[T] {
  type Parent
}

val localTimeCircular = new Circular[LocalTime] {
  type Parent = LocalDateTime
}

final class CircularMap[K, +V](val circular: Circular[K])

val summoned = localTimeCircular
val daily = new CircularMap[LocalTime, Int](localTimeCircular)
println(summoned == daily.circular) // Prints true

implicitly[localTimeCircular.Parent =:= LocalDateTime]
implicitly[summoned.Parent =:= LocalDateTime]

//implicitly[daily.circular.Parent =:= LocalDateTime] // doesn't compile

问题在于路径相关类型的设计使得即使x == x1 也不需要x.T =:= x1.T

In the latest release of scala (2.12.x), is the implementation of path-dependent type incomplete?

Force dependent types resolution for implicit calls

How to create an instances for typeclass with dependent type using shapeless

summoned == daily.circular 是真的，但 summoned 的类型为 Circular[LocalTime] { type Parent = LocalDateTime }，而 daily.circular 的类型为 Circular[LocalTime]。您这样指定：val circular: Circular[K]。因此，您将精炼类型Circular[LocalTime] { type Parent = LocalDateTime }（我们将其表示为Circular.Aux[LocalTime, LocalDateTime]）向上转换为其超类型，即没有精炼的类型Circular[LocalTime]（又名存在类型Circular.Aux[LocalTime, _]）。所以类型 localTimeCircular.Parent 和 summoned.Parent 是 LocalDateTime 但类型 daily.circular.Parent 是抽象的。例如，如果你想恢复类型细化，你可以定义

final class CircularMap[K, +V, P](val circular: Circular[K] {type Parent = P})

另一种方法是使用单例类型

final class CircularMap[K, +V](val circular: localTimeCircular.type)

---

有没有在不引入类型参数P的情况下不丢失细化Circular[LocalTime] { type Parent = LocalDateTime }的方法 班级CircularMap？就像是 final class CircularMap[K, +V](val circular: Circular[K] { type Parent = X }) 其中X 将在CircularMap 主体的范围内引入一个新的类型变量。

您想要的实际上是类上的多个类型参数列表（以便在class CircularMap[K, +V][P] 中您可以指定K、V 并推断P）

https://github.com/scala/bug/issues/4719

https://contributors.scala-lang.org/t/multiple-type-parameter-lists-in-dotty-si-4719/2399

你可以模仿他们

import java.time.{LocalDateTime, LocalTime}

trait Circular[T] {
  type Parent
}

val localTimeCircular = new Circular[LocalTime] {
  type Parent = LocalDateTime
}

trait CircularMap[K, +V] {
  type P
  val circular: Circular[K] {type Parent = P}
}
object CircularMap {
  def apply[K, V]: PartiallyApplied[K, V] = new PartiallyApplied[K, V]

  class PartiallyApplied[K, V] {
    def apply[_P](_circular: Circular[K] {type Parent = _P}): CircularMap[K, V] {type P = _P} = new CircularMap[K, V] {
      override type P = _P
      override val circular: Circular[K] {type Parent = _P} = _circular
    }
  }
}

val summoned = localTimeCircular
val daily = CircularMap[LocalTime, Int](localTimeCircular)
println(summoned == daily.circular) // Prints true

implicitly[localTimeCircular.Parent =:= LocalDateTime]
implicitly[summoned.Parent =:= LocalDateTime]
implicitly[daily.circular.Parent =:= LocalDateTime] // compiles