我有一个复杂的玩具算法,我希望纯粹在类型级别上表示:根据饮食要求选择当天菜肴的修改。对卷积表示歉意,但我认为我们需要每一层才能达到我想要使用的最终界面。
我的代码有一个问题,如果我们对 Aux
表达类型约束-基于另一个泛型类型的模式生成类型,它无法进行类型推断。
这些是餐食,实际上会有很多种披萨和很多基础餐:
trait Pizza
trait CheeselessPizza extends Pizza
饮食要求:
sealed trait DietaryRequirement
trait Vegan extends DietaryRequirement
每日一道菜类型类:
sealed trait DishOfTheDay[Meal]
object DishOfTheDay {
implicit val dishOfTheDay: DishOfTheDay[Pizza] = null
}
这将每天改变膳食,独立于程序的其余部分。
ModifiedMeal 类型类,它接受膳食和饮食要求,并生成满足要求的副餐。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
}
这是我们最终的类型类,它为我们进行计算:
// 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
}
问题是调用此方法无法编译,但它应该。
如果复制整个程序但删除 <: Meal
根据生成的膳食的要求,它进行编译。这又是整个事情,但是“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
}
但我们不想要这个,因为它允许我们生成不是当日菜肴的子类型的菜肴。
有谁知道为什么要继承在Aux
中模式导致失败,或者我如何使用中间隐式构造程序来尝试解决问题?
最佳答案
尝试用证据替换泛型上的绑定(bind):
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
}
关于scala - 具有继承类型的 Aux 模式推理失败,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/52660723/