我正在使用类型类,并且有些问题会自动导出它们,这些类型无关地扩展了额外的(标记/指标)特征。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/或添加自我类型的Food
,我会得到相同的行为。)
从编译中调查隐式日志,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的类型推断正在打破,因为当编译器正在寻找ignoreSupply
时,IsHarmfulToEat[Cake with Plentiful]
根本没有尝试使用正确的类型。任何人都可以向我解释为什么会这样吗?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?
如果你使IsHarmfulToEat
逆变,下面的代码编译
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]]
}