为什么 Scala 的不可变 Set 的类型不是协变的？
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Why is Scala's immutable Set not covariant in its type?
提问by oxbow_lakes
EDIT: Re-written this question based on original answer
编辑：根据原始答案重写此问题
The scala.collection.immutable.Setclass is not covariant in its type parameter. Why is this?
该scala.collection.immutable.Set班是不是在它的类型参数不变性。为什么是这样？
import scala.collection.immutable._
def foo(s: Set[CharSequence]): Unit = {
println(s)
}
def bar(): Unit = {
val s: Set[String] = Set("Hello", "World");
foo(s); //DOES NOT COMPILE, regardless of whether type is declared
//explicitly in the val s declaration
}
采纳答案by Daniel Spiewak
Setis invariant in its type parameter because of the concept behind sets as functions. The following signatures should clarify things slightly:
Set由于集合作为函数背后的概念，它的类型参数是不变的。以下签名应该稍微澄清一下：
trait Set[A] extends (A=>Boolean) {
def apply(e: A): Boolean
}
If Setwere covariant in A, the applymethod would be unable to take a parameter of type Adue to the contravariance of functions. Setcould potentially be contravariantin A, but this too causes issues when you want to do things like this:
如果Set是协变的A，则由于函数的逆变，该apply方法将无法采用类型参数A。 Set可能是逆变的A，但是当您想要执行以下操作时，这也会导致问题：
def elements: Iterable[A]
In short, the best solution is to keep things invariant, even for the immutable data structure. You'll notice that immutable.Mapis also invariant in one of its type parameters.
简而言之，最好的解决方案是保持事物不变，即使对于不可变的数据结构也是如此。您会注意到immutable.Map它的类型参数之一也是不变的。
回答by Seth Tisue
at http://www.scala-lang.org/node/9764Martin Odersky writes:
在http://www.scala-lang.org/node/9764Martin Odersky 写道：
"On the issue of sets, I believe the non-variance stems also from the implementations. Common sets are implemented as hashtables, which are non-variant arrays of the key type. I agree it's a slightly annoying irregularity."
“关于集合的问题，我相信非方差也源于实现。公共集合被实现为哈希表，它是键类型的非变异数组。我同意这是一个有点烦人的不规则性。”
So, it seems that all of our efforts to construct a principled reason for this were misguided :-)
因此，似乎我们为此构建原则性原因的所有努力都被误导了:-)
回答by Jorge Ortiz
EDIT: for anyone wondering why this answer seems slightly off-topic, this is because I (the questioner) have modified the question.
编辑：对于任何想知道为什么这个答案似乎有点离题的人，这是因为我（提问者）修改了这个问题。
Scala's type inference is good enough to figure out that you want CharSequences and not Strings in some situations. In particular, the following works for me in 2.7.3:
Scala 的类型推断足以确定在某些情况下您需要 CharSequences 而不是 Strings。特别是，以下内容在 2.7.3 中对我有用：
import scala.collections.immutable._
def findCharSequences(): Set[CharSequence] = Set("Hello", "World")
As to how to create immutable.HashSets directly: don't. As an implementation optimization, immutable.HashSets of less than 5 elements are not actually instances of immutable.HashSet. They are either EmptySet, Set1, Set2, Set3, or Set4. These classes subclass immutable.Set, but not immutable.HashSet.
至于如何直接创建 immutable.HashSets：不要。作为实现优化，少于 5 个元素的 immutable.HashSets 实际上并不是 immutable.HashSet 的实例。它们是 EmptySet、Set1、Set2、Set3 或 Set4。这些类是 immutable.Set 的子类，但不是 immutable.HashSet。
