Scala，使用通用特征扩展对象 [英] Scala, Extend object with a generic trait

问题描述

我正在使用Scala，我想扩展一个具有特征的(单例)对象，该特征可以提供数据结构和一些方法，例如:

I'm using Scala and I want to extend a (singleton) object with a trait, which delivers a data structure and some methods, like this:

trait Tray[T] {
  val tray = ListBuffer.empty[T]

  def add[T] (t: T) = tray += t
  def get[T]: List[T] = tray.toList
}

然后我想将特征混合到一个对象中，如下所示:

And then I'll would like to mix-in the trait into an object, like this:

object Test with Tray[Int]

但是add和get中存在类型不匹配:

But there are type mismatches in add and get:

Test.add(1)
// ...

我如何才能使它正常工作?还是我的错误是什么?

How can I'll get this to work? Or what is my mistake?

推荐答案

问题是您正在使用add和get方法上的T遮盖特征的type参数.请参阅我的答案此处以获取有关该问题的更多详细信息.

The problem is that you're shadowing the trait's type parameter with the T on the add and get methods. See my answer here for more detail about the problem.

这是正确的代码:

trait Tray[T] {
  val tray = ListBuffer.empty[T]

  def add (t: T) = tray += t      // add[T] --> add
  def get: List[T] = tray.toList  // get[T] --> add
}

object Test extends Tray[Int]

请注意在对象定义中使用extends-请参阅规范，解释为什么单独的with在这里不起作用.

Note the use of extends in the object definition—see section 5.4 of the spec for an explanation of why with alone doesn't work here.

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