具有抽象类型成员的具体类 [英] Concrete classes with abstract type members

问题描述

鉴于以下特征和类别.为什么要编译?这实际上可以用于某些事情吗?

Given the following traits and class. Why does this compile? Can this be actually used for something?

trait Container {
  type A
}

trait AnotherContainer[B]{
    def x(b : B) : B
}

trait Mixed extends Container with AnotherContainer[Container#A]

class Impl extends Mixed{
    def x(a : Container#A) = a 
}

new Impl().x _

scala> new Impl().x _
res0: (Container#A) => Container#A = <function>

更新:

class Baz { type T; }

实际上是一个功能，但我找不到它的动机:#1753.

Is actually a feature but I could not find the motivation for it: #1753.

推荐答案

如果对我没用，它看起来也无害.x 想要的类型不存在，因此您无法将其传递给方法.我想，无害的无用是否应该是编译时错误是一个品味问题.

It looks harmless if useless to me. The type that x wants doesn't exist, so you can't pass it to the method. Whether harmless uselessness should be a compile-time error is a matter of taste, I suppose.

如果你看看 x 实际上做了什么，它就是这样反编译的:

If you look at what x actually does, it decompiles thusly:

public java.lang.Object x(java.lang.Object);
  Code:
   0:   aload_1
   1:   areturn

这正是 identity 方法应该做的事情(加载参数而不考虑类型，返回它).你可以用更少的代码写出等价的东西:

which is exactly what the identity method should do (load the argument regardless of type, return it). You can write something equivalent with much less code:

trait AbstractType { type T }
class Useless extends AbstractType { def identity(t: AbstractType#T) = t }

除了AbstractType#T 没有任何类型，所以我们又无用了.

Except nothing has type AbstractType#T, so again we have uselessness.

除非我遗漏了什么.

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