在生成类型类实例时,无法证明单例类型是单例类型

Question

假设我有一个类型类,证明Shapeless副产品中的所有类型都是单例类型:

import shapeless._

trait AllSingletons[A, C <: Coproduct] {
  def values: List[A]
}

object AllSingletons {
  implicit def cnilSingletons[A]: AllSingletons[A, CNil] =
    new AllSingletons[A, CNil] {
      def values = Nil
    }

  implicit def coproductSingletons[A, H <: A, T <: Coproduct](implicit
    tsc: AllSingletons[A, T],
    witness: Witness.Aux[H]
  ): AllSingletons[A, H :+: T] =
    new AllSingletons[A, H :+: T] {
      def values = witness.value :: tsc.values
    }
}

我们可以证明它适用于简单的ADT:

sealed trait Foo
case object Bar extends Foo
case object Baz extends Foo

然后:

scala> implicitly[AllSingletons[Foo, Bar.type :+: Baz.type :+: CNil]].values
res0: List[Foo] = List(Bar, Baz)

现在我们想要将它与Shapeless的Generic机制结合起来,它将为我们提供ADT的联合产品表示:

trait EnumerableAdt[A] {
  def values: Set[A]
}

object EnumerableAdt {
  implicit def fromAllSingletons[A, C <: Coproduct](implicit
    gen: Generic.Aux[A, C],
    singletons: AllSingletons[A, C]
  ): EnumerableAdt[A] =
    new EnumerableAdt[A] {
      def values = singletons.values.toSet
    }
}

我希望implicitly[EnumerableAdt[Foo]]能够工作,但事实并非如此.我们可以-Xlog-implicits用来获取有关原因的一些信息:

<console>:17: shapeless.this.Witness.apply is not a valid implicit value for
  shapeless.Witness.Aux[Baz.type] because:
Type argument Baz.type is not a singleton type
              implicitly[EnumerableAdt[Foo]]
                        ^
<console>:17: this.AllSingletons.coproductSingletons is not a valid implicit
  value for AllSingletons[Foo,shapeless.:+:[Baz.type,shapeless.CNil]] because:
hasMatchingSymbol reported error: could not find implicit value for parameter
  witness: shapeless.Witness.Aux[Baz.type]
              implicitly[EnumerableAdt[Foo]]
                        ^
<console>:17: this.AllSingletons.coproductSingletons is not a valid implicit
  value for AllSingletons[Foo,this.Repr] because:
hasMatchingSymbol reported error: could not find implicit value for parameter
  tsc: AllSingletons[Foo,shapeless.:+:[Baz.type,shapeless.CNil]]
              implicitly[EnumerableAdt[Foo]]
                        ^
<console>:17: this.EnumerableAdt.fromAllSingletons is not a valid implicit
  value for EnumerableAdt[Foo] because:
hasMatchingSymbol reported error: could not find implicit value for parameter
  singletons: AllSingletons[Foo,C]
              implicitly[EnumerableAdt[Foo]]
                        ^
<console>:17: error: could not find implicit value for parameter e:
  EnumerableAdt[Foo]
              implicitly[EnumerableAdt[Foo]]
                        ^

Baz.type但显然是单身人士.我们可以尝试Witness手动将实例放在范围内以获得乐趣:

implicit val barSingleton = Witness[Bar.type]
implicit val bazSingleton = Witness[Baz.type]

不知怎的,它现​​在有效:

scala> implicitly[EnumerableAdt[Foo]].values
res1: Set[Foo] = Set(Bar, Baz)

我不明白为什么这些实例在这种情况下会起作用,而Witness.apply宏方法(我们用来创建它们)生成的实例则不然.这里发生了什么？是否有一个方便的解决方法,不需要我们手动枚举构造函数？

Answer 1

这与最近的无形2.1.0-SNAPSHOT一样.