具有继承类型的辅助模式无法推理

Question

我有一个复杂的玩具算法，我希望纯粹在类型级别表示：根据饮食要求为当天的一道菜选择修改。为卷积道歉，但我认为我们需要每一层才能到达我想要使用的最终界面。

我的代码存在问题，如果我们在基于另一个泛型类型的Aux-pattern-generated 类型上表达类型约束，则类型推断失败。

这些是餐点，实际上会有很多种比萨饼和多种基础餐点：

trait Pizza
trait CheeselessPizza extends Pizza

饮食要求：

sealed trait DietaryRequirement
trait Vegan extends DietaryRequirement

当日菜式分类：

sealed trait DishOfTheDay[Meal]

object DishOfTheDay {
  implicit val dishOfTheDay: DishOfTheDay[Pizza] = null
}

这将改变每天的膳食，独立于程序的其余部分。

ModifiedMeal 类型类，它接受膳食和饮食要求并生成满足要求的submeal。子类型在这里很重要：

// <: Meal is important
sealed trait ModifiedMeal[Meal, D <: DietaryRequirement] { type Mod <: Meal }

object ModifiedMeal {

  type Aux[Meal, D <: DietaryRequirement, Mod0 <: Meal] = ModifiedMeal[Meal, D] { type Mod = Mod0 }

  // Only one instance so far, Vegan Pizza = CheeselessPizza
  implicit val veganPizzaModifiedMeal: ModifiedMeal.Aux[Pizza, Vegan, CheeselessPizza] = null

}

这是为我们进行计算的最终类型类：

// Given a dietary requirement, give us a dish of the day which satisfies it
// if one exists
trait DishOfTheDayModification[Req <: DietaryRequirement] { type Out }

object DishOfTheDayModification {

  type Aux[Req <: DietaryRequirement, Out0] = DishOfTheDayModification[Req] { type Out = Out0 }

  // Find the dish of the day, then find a ModifiedMeal of it
  // <: Meal is important here so we pick up ONLY pizzas and not some other meal
  implicit def dishOfTheDayModification[Meal, Req <: DietaryRequirement, Mod <: Meal](
    implicit d: DishOfTheDay[Meal],
    impl: ModifiedMeal.Aux[Meal, Req, Mod]
  ): DishOfTheDayModification.Aux[Req, Mod] = null

}

这是测试：

object MealTesting {
  def veganDishOfTheDay[Mod](implicit d: DishOfTheDayModification.Aux[Vegan, Mod]): Mod = ???

  // Does not compile but it should
  veganDishOfTheDay: CheeselessPizza
}

问题是调用这个方法不能编译，但它应该。

如果你复制整个程序但从生成的餐中删除<: Meal 要求，它会编译。这又是整个事情，但“工作”：

trait Pizza
trait CheeselessPizza extends Pizza

sealed trait DietaryRequirement
trait Vegan extends DietaryRequirement

sealed trait DishOfTheDay[Meal]

object DishOfTheDay {
  implicit val dishOfTheDay: DishOfTheDay[Pizza] = null
}

sealed trait ModifiedMeal[Meal, D <: DietaryRequirement] { type Mod }

object ModifiedMeal {

  type Aux[Meal, D <: DietaryRequirement, Mod0] = ModifiedMeal[Meal, D] { type Mod = Mod0 }

  implicit val veganPizzaModifiedMeal: ModifiedMeal.Aux[Pizza, Vegan, CheeselessPizza] = null

}

trait DishOfTheDayModification[Req <: DietaryRequirement] { type Out }

object DishOfTheDayModification {

  type Aux[Req <: DietaryRequirement, Out0] = DishOfTheDayModification[Req] { type Out = Out0 }

  implicit def dishOfTheDayModification[Meal, Req <: DietaryRequirement, Mod](
    implicit d: DishOfTheDay[Meal],
    impl: ModifiedMeal.Aux[Meal, Req, Mod]
  ): DishOfTheDayModification.Aux[Req, Mod] = null

}

object MealTesting {
  def veganDishOfTheDay[Mod](implicit d: DishOfTheDayModification.Aux[Vegan, Mod]): Mod = ???

  // DOES compile
  veganDishOfTheDay: CheeselessPizza
}

但我们不希望这样，因为它允许我们生成不是当天菜肴的子类型的菜肴。

有谁知道为什么Aux 模式中的继承会导致失败，或者我可以如何使用中间隐式构造程序来尝试解决问题？

Answer 1

尝试用证据替换泛型上的绑定：

trait Pizza
trait CheeselessPizza extends Pizza

sealed trait DietaryRequirement
trait Vegan extends DietaryRequirement

sealed trait DishOfTheDay[Meal]

object DishOfTheDay {
  implicit val dishOfTheDay: DishOfTheDay[Pizza] = null
}

sealed trait ModifiedMeal[Meal, D <: DietaryRequirement] { type Mod <: Meal }

object ModifiedMeal {
  type Aux[Meal, D <: DietaryRequirement, Mod0 /*<: Meal*/] = ModifiedMeal[Meal, D] { type Mod = Mod0 }

  //implicit val veganPizzaModifiedMeal: ModifiedMeal.Aux[Pizza, Vegan, CheeselessPizza] = null

  def mkAux[Meal, D <: DietaryRequirement, Mod](implicit ev: Mod <:< Meal): Aux[Meal, D, Mod] = null

  implicit val veganPizzaModifiedMeal: ModifiedMeal.Aux[Pizza, Vegan, CheeselessPizza] = mkAux
}

trait DishOfTheDayModification[Req <: DietaryRequirement] { type Out }

object DishOfTheDayModification {
  type Aux[Req <: DietaryRequirement, Out0] = DishOfTheDayModification[Req] { type Out = Out0 }

  implicit def dishOfTheDayModification[Meal, Req <: DietaryRequirement, Mod /*<: Meal*/](implicit 
    d: DishOfTheDay[Meal],
    impl: ModifiedMeal.Aux[Meal, Req, Mod],
    ev: Mod <:< Meal
  ): DishOfTheDayModification.Aux[Req, Mod] = null
}

object MealTesting {
  def veganDishOfTheDay[Mod](implicit d: DishOfTheDayModification.Aux[Vegan, Mod]): Mod = ???

  veganDishOfTheDay: CheeselessPizza
}

Answer 2

您的原始方法非常接近没有问题，只需对 1 dishOfTheDayModification 签名进行细微调整，即可使用 Scala v2.12 成功编译。

作为参考，在原始的DishOfTheDayModification 对象定义中是这样的：

// Find the dish of the day, then find a ModifiedMeal of it
// <: Meal is important here so we pick up ONLY pizzas and not some other meal
implicit def dishOfTheDayModification[Meal, Req <: DietaryRequirement, Mod <: Meal](
  implicit
     // vvvvvv - Here's the problem
     d: DishOfTheDay[Meal],
     impl: ModifiedMeal.Aux[Meal, Req, Mod]
  ): DishOfTheDayModification.Aux[Req, Mod] = null

将顺序改为：

implicit def dishOfTheDayModification[Meal, Req <: DietaryRequirement, Mod <: Meal](
  implicit
     impl: ModifiedMeal.Aux[Meal, Req, Mod],
     d: DishOfTheDay[Meal]
): DishOfTheDayModification.Aux[Req, Mod] = null

允许编译器在解析d之前为impl成功统一Meal和Mod。