为什么Scala的不可变集的类型不协变？ [英] Why is Scala's immutable Set not covariant in its type?

问题描述

编辑：根据原始答案重新编写该问题

scala.collection.immutable.Set 类的类型参数不是协变的。为什么是这样？

  import scala.collection.immutable._ 
 
 def foo（s：Set [CharSequence]） ：单位= {
 println（s）
} 
 
 def bar（）：单位= {
 val s：Set [String] = Set（ Hello ，世界）； 
 foo; //不会编译，无论是否在$ val声明
中明确声明了类型
} 
  

解决方案

Set 的set参数是不变的，这是因为set作为函数的概念。以下签名应稍作澄清：

 特征集[A]扩展（A =>布尔值）{
 def apply（e：A）：布尔值
} 
  

如果 Set 在 A 中是协变的，因此 apply 方法将无法采用参数由于函数的差异性，请键入 A 。 Set 在 A 中可能是 contravariant ，但是当您要做这样的事情：

  def元素：可迭代[A] 
   immutable.Map 的类型参数之一也是不变的。
 
EDIT: Re-written this question based on original answer

The scala.collection.immutable.Set class is not covariant in its type parameter. Why is this? 
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
}

解决方案
Set is invariant in its type parameter because of the concept behind sets as functions.  The following signatures should clarify things slightly:
trait Set[A] extends (A=>Boolean) {
  def apply(e: A): Boolean
}
If Set were covariant in A, the apply method would be unable to take a parameter of type A due to the contravariance of functions.  Set could potentially be contravariant in A, but this too causes issues when you want to do things like this:
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.Map is also invariant in one of its type parameters.

                        
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