scala - 如何在隐式搜索期间自动剥离 Scala 特征？

Question

我正在使用类型类，但在为不相关地扩展额外(标记/指示器)特征的类型自动派生它们时遇到问题。这很难解释，但这个最小的例子应该清楚我的意思:

// Base type we are working on
trait Food {}

// Marker trait - unrelated to Food's edibility
trait Plentiful {}

// Indicator type class we want to derive
trait IsHarmfulToEat[F<:Food] {}

object IsHarmfulToEat {
  // Rule that says that if some food is harmful to eat, 
  // an enormous amount is so as well
  implicit def ignoreSupply[F1<:Food,F2<:F1 with Plentiful]
                  (implicit isHarmful: IsHarmfulToEat[F1],
                   constraint: F2=:=F1 with Plentiful): IsHarmfulToEat[F2] = 
                         new IsHarmfulToEat[F2]{}
}

// Example of food
case class Cake() extends Food {}

object Cake {
  // Mark Cake as being bad for you
  implicit val isBad: IsHarmfulToEat[Cake] = new IsHarmfulToEat[Cake] {}
}

object FoodTest extends App {
  // Our main program
  val ignoreSupplyDoesWork: IsHarmfulToEat[Cake with Plentiful] = 
    IsHarmfulToEat.ignoreSupply[Cake,Cake with Plentiful] // compiles fine

  val badCake = implicitly[IsHarmfulToEat[Cake]] // compiles fine
  val manyBadCakes = implicitly[IsHarmfulToEat[Cake with Plentiful]] 
  // ^^^ does not compile - I do not understand why
}

(如果我使 Plentiful 通用和/或向其添加自类型 Food，我会得到相同的行为。)

调查编译中的隐式日志，我发现:

Food.scala:33: util.this.IsHarmfulToEat.ignoreSupply is not a valid implicit value for IsHarmfulToEat[F1] because:
hasMatchingSymbol reported error: diverging implicit expansion for type IsHarmfulToEat[F1]
starting with method ignoreSupply in object IsHarmfulToEat
  val manyBadCakes = implicitly[IsHarmfulToEat[Cake with Plentiful]]
                               ^
Food.scala:33: util.this.IsHarmfulToEat.ignoreSupply is not a valid implicit value for IsHarmfulToEat[Cake with Plentiful] because:
hasMatchingSymbol reported error: diverging implicit expansion for type IsHarmfulToEat[F1]
starting with method ignoreSupply in object IsHarmfulToEat
  val manyBadCakes = implicitly[IsHarmfulToEat[Cake with Plentiful]]
                               ^
Food.scala:33: diverging implicit expansion for type IsHarmfulToEat[Cake with Plentiful]
starting with method ignoreSupply in object IsHarmfulToEat
  val manyBadCakes = implicitly[IsHarmfulToEat[Cake with Plentiful]]

在我看来，F1 上的类型推断正在崩溃，因为当编译器正在寻找 IsHarmfulToEat[Cake with Plentiful] 时，ignoreSupply 根本没有尝试使用正确的类型。谁能向我解释为什么会这样？和/或如何引导编译器尝试正确的类型？和/或以另一种方式实现 ignoreSupply 规则？

Answer 1

如果你使 IsHarmfulToEat 逆变以下代码编译

  trait Food

  trait Plentiful

  trait IsHarmfulToEat[-F <: Food]

  object IsHarmfulToEat {
    implicit def ignoreSupply[F <: Food]
    (implicit isHarmful: IsHarmfulToEat[F]
    ): IsHarmfulToEat[F with Plentiful] =
      new IsHarmfulToEat[F with Plentiful]{}
  }

  case class Cake() extends Food {}

  object Cake {
    implicit val isBad: IsHarmfulToEat[Cake] = new IsHarmfulToEat[Cake] {}
  }

  object FoodTest extends App {
    val ignoreSupplyDoesWork: IsHarmfulToEat[Cake with Plentiful] =
      IsHarmfulToEat.ignoreSupply[Cake]

    val badCake = implicitly[IsHarmfulToEat[Cake]]
    val manyBadCakes = implicitly[IsHarmfulToEat[Cake with Plentiful]]
  }