隐式参数优先级

Question

我试图将泛型类型转换为HList:

trait ToHList[T] {
type Out <: HList
def apply(value: T): Out
}
trait LowPriorityToHList {
implicit def default[T]: ToHList.Aux[T, T :: HNil] =
new ToHList[T] {
override type Out = T :: HNil
override def apply(value: T): T :: HNil = value :: HNil
}
}
object ToHList extends LowPriorityToHList {
type Aux[T, Out0] = ToHList[T] { type Out = Out0 }
def apply[T](implicit toHList: ToHList[T]): Aux[T, toHList.Out] = toHList
implicit def toHList[T, Repr <: HList, N <: Nat](implicit
gen: Generic.Aux[T, Repr],
len: Length.Aux[Repr, N],
lt: LT[Nat._0, N]): ToHList.Aux[T, Repr] =
new ToHList[T] {
override type Out = Repr
override def apply(value: T): Repr = gen.to(value)
}
}
object Main extends App {
println(ToHList.apply[Int].apply(1)) // expected 1 :: HNil
println(ToHList.apply[(Int, Int)].apply((1, 2))) // expected 1 :: 2 :: HNil
}

我打算优先考虑ToHList.toHList而不是ToHList.default，但是该代码导致以下编译错误：

[error] ToHList.scala:39:24: ambiguous implicit values:
[error]  both method toHList in object ToHList of type [T, Repr <: shapeless.HList, N <: shapeless.Nat](implicit gen: shapeless.Generic.Aux[T,Repr], implicit len: shapeless.ops.hlist.Length.Aux[Repr,N], implicit lt: shapeless.ops.nat.LT[shapeless.Nat._0,N])ToHList.Aux[T,Repr]
[error]  and method default in trait LowPriorityToHList of type [T]=> ToHList.Aux[T,T :: shapeless.HNil]
[error]  match expected type ToHList[(Int, Int)]
[error]   println(ToHList.apply[(Int, Int)].apply((1, 2))) // expected 1 :: 2 :: HNil

我想优先考虑ToHList.toHList而不是ToHList.default。如何修复此错误？

Answer 1

如果toHList和default都适用，则它们具有相同的优先级，因此会产生歧义。事实上，尽管default被定义为低优先级的超级性状，但它比toHList更具体。请参阅"为什么会发生这种隐含的模糊行为？"中的详细信息？

因此，没有理由将default置于低优先级的超级性状中，这不会产生预期的影响。但如果将toHList和default放在同一个对象中，则default将更具体地获胜。从expected 1 :: 2 :: HNil看来，你希望toHList获胜，反之亦然。您可以使用shapeless.LowPriority

object ToHList {
type Aux[T, Out0] = ToHList[T] { type Out = Out0 }
def apply[T](implicit toHList: ToHList[T]): Aux[T, toHList.Out] = toHList
implicit def toHList[T, Repr <: HList, N <: Nat](implicit
gen: Generic.Aux[T, Repr],
len: Length.Aux[Repr, N],
lt: LT[Nat._0, N]
): ToHList.Aux[T, Repr] =
new ToHList[T] {
override type Out = Repr
override def apply(value: T): Repr = gen.to(value)
}
implicit def default[T](implicit 
lowPriority: LowPriority
): ToHList.Aux[T, T :: HNil] =
new ToHList[T] {
override type Out = T :: HNil
override def apply(value: T): T :: HNil = value :: HNil
}
}

或者，在这种特定情况下，您可以使用shapeless.Refute、shapeless.OrElse

implicit def default[T](implicit
orElse: OrElse[Refute[Generic[T]], Generic.Aux[T, HNil]]
): ToHList.Aux[T, T :: HNil] =
new ToHList[T] {
override type Out = T :: HNil
override def apply(value: T): T :: HNil = value :: HNil
}