召唤辅助以获得更高种类的类型，而不参考原始类型

我正在尝试将 Aux 模式与更高种类的类型一起使用，并且直到之后才必须指定更高种类类型的参数。这类似于所描述的SO问题here https://stackoverflow.com/questions/52581986/aux-pattern-for-higher-kinded-types但有一个显着的区别，我将采取相反的方式，即从隐式 def 回到 aux。

// The are types that I want to convert to various things
sealed trait ConversionType
trait CaseA extends ConversionType
object CaseA extends CaseA // In this case, convert to an optional
trait CaseB extends ConversionType
object CaseB extends CaseB // In this case, convert to a future etc...

trait Converter[Prefix] {
  type Paramd[_]
  def create[N](n:N): Paramd[N]
}

// Create the mechanism to convert from the cases, only doing case A for now...
object Converter {
  type Aux[Prefix, Ret[_]] = Converter[Prefix] { type Paramd[_] = Ret[_] }

  // *** Error happens here! ***
  def apply[Prefix](implicit p:Converter[Prefix]): Aux[Prefix, p.Paramd] = p

  implicit def makeOptionParamd: Aux[CaseA, Option] =
    new Converter[CaseA] {
      type Paramd[_] = Option[_]
      override def create[N](n:N): Paramd[N] = Option[N](n)
    }
}

// This seems to be fine...
val v = Converter.apply[CaseA].create("test")

我在上面提到的行上收到以下编译错误：

Error:(97, 78) type mismatch;
 found   : p.type (with underlying type Test.this.Converter[Prefix])
 required: Test.Converter.Aux[Prefix,p.Paramd]
    (which expands to)  Test.this.Converter[Prefix]{type Paramd[_] = p.Paramd[_]}
    def apply[Prefix](implicit p:Converter[Prefix]): Aux[Prefix, p.Paramd] = p

我究竟做错了什么？

你可能想要的是

object Converter {
  type Aux[Prefix, Ret[_]] = Converter[Prefix] { type Paramd[A] = Ret[A] }

  // compiles
  def apply[Prefix](implicit p:Converter[Prefix]): Aux[Prefix, p.Paramd] = p

  implicit def makeOptionParamd: Aux[CaseA, Option] =
    new Converter[CaseA] {
      type Paramd[A] = Option[A]
      override def create[N](n:N): Paramd[N] = Option[N](n)
    }
}

当你写的时候

type Paramd[_] = Ret[_]

the _左右部分是不相关的。它是一样的

type Paramd[A] = Ret[_]

type Paramd[A] = Ret[B] forSome { type B }

So Aux[Prefix, p.Paramd]你的定义相当于Converter[Prefix] { type Paramd[A] = p.Paramd[_] }, and p没有这个类型，因为p.Paramd[A] is not p.Paramd[_].