隐式搜索期间如何自动剥离Scala特性?

问题描述

我正在使用类型类，但对于将它们无关地扩展额外的(标记/指示符)特征的类型自动导出它们时遇到了问题.很难解释，但是这个最小的例子应该清楚我的意思:

I am working with type classes and have problems auto-deriving them for types that unrelatedly extends extra (marker/indicator) traits. It is hard to explain, but this minimal example should make it clear what I mean:

// 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 Purtiful 通用并且/或者向其中添加 Food 的自类型，我将得到相同的行为.)

(I get the same behaviour if I make Plentiful universal and/or add a self-type of Food to it.)

研究编译中的隐式日志，我发现:

Investigating the implicit-log from compilation, I find this:

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的类型推断正在中断，因为在编译器寻找 IsHarmfulToEat [Cake with Plentiful]时，没有正确地尝试使用 ignoreSupply .谁能向我解释为什么?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?

It seems to me that type inference on F1 is breaking down, as ignoreSupply is simply not tried out using the right types when the compiler is looking for a IsHarmfulToEat[Cake with Plentiful]. Can anyone explain to me why that is? And/or how to guide the compiler to try the right type? And/or to achieve the ignoreSupply rule in another way?

推荐答案

如果使 IsHarmfulToEat 与下面的代码相反，则会编译以下代码

If you make IsHarmfulToEat contravariant the following code compiles

  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]]
  }

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