斯卡拉泛型：如何声明一个类型必须是一个案例类？ [英] Scala generics: How to declare that a type must be a case class?

问题描述

我有几个使用 count 字段的案例类。默认情况下它是1，并且我在代码中减少了对重复数据进行分组并对其进行求和的总和，以查找每个对象的数量。例如：

I have several case classes with a count field. It's 1 by default, and I have a reduce in my code that groups duplicates and sums that value to find the number of each object. E.g.:

case class Person(name: String, count = 1)
personList.groupBy(_.name).reduce((x,y) => x.copy(count = x.count + 1))

我在几个case类中有这个逻辑，因为我的逻辑比上面的例子复杂一点，所以我想创建一个泛型合并函数。

I have this logic in several case classes, and since my logic is a bit more complicated than the example above I want to create a generic merging function.

我使用 count 字段创建了一个密封特质。然后我改变了我的case类以扩展它，例如：

So I've created a sealed trait with a count field. I've then changed my case classes to extend from this, e.g.:

case class Person(name: String, override val count) extends Countable

到目前为止，那么好。

然而，我无法弄清楚如何声明我的 merge 函数，以便它只接受扩展 Countable 。因此，它无法找到 copy 方法。

However, I can't work out how to declare my merge function so that it only accepts case classes that extend Countable. Because of that, it can't find the copy method.

以下是我的：

Here's what I have:

  def merge[T <: Countable](f: T => Seq[String])(ms: Seq[T]): Vector[T] = 
    ms.groupBy(x => f(x).mkString("_")).mapValues(_.reduce { (x,y) => 
     x.copy(count = x.count + 1)        // can't find `copy`
  }).values.toVector

是否有一个类型类型可以包含它，意味着类型有一个 copy 方法（或者是一个case类）使用Scala 2.11.7？

Is there a typeclass that I can also include that means a type has a copy method (or is a case class) using Scala 2.11.7?

更新：

Countable 特质是：

Countable trait is:

sealed trait Countable {

  def timesSeen: Long = 1
}

推荐答案

你是如何定义你可数特质的。
以下代码片段适用于我：

How did you defined you Countable trait. Following snippet works fine for me:

  trait Countable[Z] {
    def count: Int
    def copy: Z
  }

  case class Person(name: String, override val count: Int) extends Countable[Person] {
    override def copy: Person = this
  }

  def merge[T <: Countable[T]](f: T => Seq[String])(ms: Seq[T]): Vector[T] = {
    val r = ms.groupBy(x => f(x).mkString("_")).mapValues(_.reduce { (x, y) =>
      x.copy
    }).values.toVector
    r
  }

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