Scala 中的个案继承 [英] Case to case inheritance in Scala

问题描述

我有一个抽象类，我扩展并创建了许多案例类.现在我想复制那些只是改变第一个参数的案例类的实例，所以我使用案例类的 copy 方法.

I have an abstract class which I extend and make numerous case classes. Now I want to copy instances of those case classes just changing first parameter, so I use case class' copy method.

由于我必须对从通用抽象类扩展而来的所有案例类都这样做，而不是对所有案例类都这样做，因此我尝试使其通用并将抽象类设为案例类.

Since I have to do this for all case classes that have been extended from common abstract class, rather than doing it for all, I tried to make it general and made the abstract class a case class.

然后 Scala 给了我这个:

Then Scala gives me this:

case class Octopus 有 case 祖先 Organism，但禁止 case-to-case 继承.为了克服这个限制，使用提取器在非叶节点上进行模式匹配.

case class Octopus has case ancestor Organism, but case-to-case inheritance is prohibited. To overcome this limitation, use extractors to pattern match on non-leaf nodes.

代码:

abstract class Organism(legs: Int)
case class Octopus(override val legs: Int, weight: Double, ...)
case class Frog(override val legs: Int, ...)
def clone(o: Organism) = o.copy(legs = -1)

这就是我想做的.但是如果我不能使 clone 方法工作，那么我将不得不对 Octopus 和 & 进行复制.青蛙.

This is what I want to do. But if I can't make the clone method work, then I will have to do copying for both Octopus & Frog.

任何建议，以减少这种冗长?

Any suggestions, to decrease this verbosity?

推荐答案

您不能对案例类的 copy 方法进行一般抽象.我建议使用 Shapeless 或 Monocle 的镜片:

You can't abstract over case class' copy methods generically. I'd suggest using Lenses from Shapeless or Monocle:

trait Organism { def legs: Int }
// monocle @Lenses uses a macro to generate lenses
@Lenses case class Octopus(override val legs: Int, weight: Double, ...)
  extends Organism
@Lenses case class Frog(val legs: Int, ...) extends Organism

def clone[O <: Organism](o: O, legsLens: Lens[O, Int]): O =
  legsLens.set(-1)(o)

val myOctopus = Octopus(8, 2.4, ...)
val myFrog = Frog(2, ...)

// use the generated Lenses
val cloneOctopus: Octopus = clone(myOctopus, Octopus.legs)
clone(myFrog, Frog.legs)

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