Scala 什么时候需要@uncheckedVariance，为什么在 GenericTraversableTemplate 中使用它? [英] When is @uncheckedVariance needed in Scala, and why is it used in GenericTraversableTemplate?

问题描述

@uncheckedVariance 可用于弥合 Scala 的声明站点方差注释和 Java 的不变泛型之间的差距.

@uncheckedVariance can be used to bridge the gap between Scala's declaration site variance annotations and Java's invariant generics.

scala> import java.util.Comparator    
import java.util.Comparator

scala> trait Foo[T] extends Comparator[T]
defined trait Foo

scala> trait Foo[-T] extends Comparator[T]     
<console>:5: error: contravariant type T occurs in invariant position in type [-T]java.lang.Object with java.util.Comparator[T] of trait Foo
       trait Foo[-T] extends Comparator[T]
             ^

scala> import annotation.unchecked._    
import annotation.unchecked._

scala> trait Foo[-T] extends Comparator[T @uncheckedVariance]    
defined trait Foo

这说明 java.util.Comparator 是自然逆变的，即类型参数 T 出现在参数中，从不出现在返回类型中.

This says that java.util.Comparator is naturally contra-variant, that is the type parameter T appears in parameters and never in a return type.

这提出了一个问题:为什么它还用于不从 Java 接口扩展的 Scala 集合库中?

This raises the question: why is it also used in the Scala collections library which doesn't extends from Java interfaces?

trait GenericTraversableTemplate[+A, +CC[X] <: Traversable[X]] extends HasNewBuilder[A, CC[A] @uncheckedVariance]

此注解的有效用途是什么?

What are the valid uses for this annotation?

推荐答案

问题是 GenericTraversableTemplate 被使用了两次:一次用于可变集合(其类型参数应该是不变的)，一次用于不可变集合(其中协方差不变)国王).

The problem is that GenericTraversableTemplate is used twice: once for mutable collections (where its type parameter should be invariant), and once for immutable collections (where covariance is invariably king).

GenericTraversableTemplate 的类型检查假设 A 类型参数具有协方差或不变性.但是，当我们以可变特征继承它时，我们必须选择不变性.相反，我们希望不可变子类中的协方差.

GenericTraversableTemplate's typechecks assuming either covariance or invariance for the A type parameter. However, when we inherit it in a mutable trait, we have to pick invariance. Conversely, we'd like covariance in an immutable subclass.

由于我们不能抽象 GenericTraversableTemplate 中的方差注释(还 ;-))，因此我们可以根据子类将其实例化为任何一个，我们必须求助于强制转换(@uncheckVariance 本质上是一种-投掷).为了进一步阅读，我推荐我的论文(对不起;-))或我们最近的 bitrot 论文

Since we can't abstract over the variance annotation (yet ;-)) in GenericTraversableTemplate, so that we could have instantiated it to either one depending on the subclass, we have to resort to casting (@uncheckVariance is essentially a kind-cast). For further reading, I recommend my dissertation (sorry ;-)) or our recent bitrot paper

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