依赖于 trait 中 case 类的方法 [英] rely on methods of case class in trait
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问题描述
有没有办法依赖在 trait 中的 case 类中定义的方法?例如,复制:以下不起作用.不过我不知道为什么.
Is there a way to rely on methods defined in case class in a trait? E.g., copy: the following doesn't work. I'm not sure why, though.
trait K[T <: K[T]] {
val x: String
val y: String
def m: T = copy(x = "hello")
def copy(x: String = this.x, y: String = this.y): T
}
case class L(val x: String, val y: String) extends K[L]
给出:
error: class L needs to be abstract, since method copy in trait K of type
(x: String,y: String)L is not defined
case class L(val x: String, val y: String) extends K[L]
^
推荐答案
我想在 trait 中具有名称为 copy 的方法指示编译器不在 case class 中生成方法副本 - 因此在您的示例中方法副本未在您的情况下实现班级.下面是在 trait 中实现的方法复制的简短实验:
I suppose that having method with name copy in trait instructs compiler to not generate method copy in case class - so in your example method copy is not implemented in your case class. Below short experiment with method copy implemented in trait:
scala> trait K[T <: K[T]] {
| val x: String
| val y: String
| def m: T = copy(x = "hello")
| def copy(x: String = this.x, y: String = this.y): T = {println("I'm from trait"); null.asInstanceOf[T]}
| }
defined trait K
scala> case class L(val x: String, val y: String) extends K[L]
defined class L
scala> val c = L("x","y")
c: L = L(x,y)
scala> val d = c.copy()
I'm from trait
d: L = null
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