实现Ordered [A [B]]的Scala泛型给出了奇怪的编译器错误 [英] Scala generics implementing Ordered[A[B]] gives strange compiler errors

问题描述

我有奇怪的错误信息，而且我无法用 asInstanceOf [T]

I have strange error message, and I cannot fix it with asInstanceOf[T]

 trait ABase {
    type A <: ABase
    def compare(that: A): Int
  }

  case class B [A <: ABase] (someField: A) {
    //extends Ordered[B[A]] {
    def compare1(that: B [A]): Int = someField.asInstanceOf[A].compare(that.someField.asInstanceOf[A])
    /*Error: type mismatch;
 found   : A
 required: _1.A where val _1: A
    = someField.asInstanceOf[A].compare(that.someField.asInstanceOf[A])*/
    def compare2(that: B [A]): Int = someField.compare(that.someField.asInstanceOf[A])
    /*Error: type mismatch;
 found   : A
 required: B.this.someField.A
    = someField.compare(that.someField.asInstanceOf[A])*/
    def compare3(that: B [A]): Int = someField.compare(that.someField)
    /*Error: type mismatch;
 found   : that.someField.type (with underlying type A)
 required: B.this.someField.A
    = someField.compare(that.someField)*/
  }

错误本身

Error itself

错误：类型不匹配;
found：A
required：_1.A where val _1：A

Error: type mismatch; found : A required: _1.A where val _1: A

推荐答案

您可以通过将其转换为B（this） someField 类型来修复它：

You can fix it by casting to B's (this) someField type:

trait ABase {
  type A <: ABase
  def compare(that: A): Int
}

case class B [A <: ABase] (someField: A) {

  def compare1(that: B [A]): Int = someField.compare(that.someField.asInstanceOf[this.someField.A])

  def compare2(that: B [A]): Int = someField.compare(that.someField.asInstanceOf[this.someField.A])

  def compare3(that: B [A]): Int = someField.compare(that.someField.asInstanceOf[this.someField.A])
}

您还可以保留一个type属性以使其更加简洁：

You can also keep a type attribute to make it more concise:

case class B [A <: ABase] (someField: A) {

  type ThisA = this.someField.A

  def compare1(that: B [A]): Int = someField.compare(that.someField.asInstanceOf[ThisA])

  def compare2(that: B [A]): Int = someField.compare(that.someField.asInstanceOf[ThisA])

  def compare3(that: B [A]): Int = someField.compare(that.someField.asInstanceOf[ThisA])
}

这篇关于实现Ordered [A [B]]的Scala泛型给出了奇怪的编译器错误的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！