2

“val”和“case”如何以及为什么会影响类型系统？（尤其是方差）

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scala> class E[-A]
defined class E

scala> class F[-A](val f: E[A] => Unit)
<console>:6: error: contravariant type A occurs in covariant position in type => (E[A]) => Unit of value f
class F[-A](val f: E[A] => Unit)
                       ^  
scala> case class C[-A](f: E[A] => Unit)
<console>:6: error: contravariant type A occurs in covariant position in type => (E[A]) => Unit of value f
   case class C[-A](f: E[A] => Unit)

scala> class F[-A](f: E[A] => Unit)    
defined class F
4

4 回答 4

4

考虑一下：

trait Equal[-A] { def eq(a1: A, a2: A): Boolean }
val e = new Equal[Option[Int]] { 
    def eq(a1: Option[Int], a2: Option[Int]) = a1 forall (x => a2 forall (x ==)) 
}

// Because Equal is contra-variant, Equal[AnyRef] is a subtype of Equal[String]
// Because T => R is contra-variant in T, Equal[AnyRef] => Unit is a supertype
// of Equal[String] => Unit
// So the follow assignment is valid
val f: Equal[AnyRef] => Unit = (e1: Equal[String]) => println(e1.eq("abc", "def"))


// f(e) doesn't compile because of contra-variance
// as Equal[Option[Int]] is not a subtype of Equal[AnyRef]

// Now let's tell Scala we know what we are doing
class F[-A](val f: Equal[A @uncheckedVariance] => Unit)

// And then let's prove we are not:
// Because F is contra-variant, F[Option[Int]] is a subtype of F[AnyRef]
val g: F[Option[Int]] = new F(f)

// And since g.f is Equal[Option[Int]] => Unit, we can pass e to it.
g.f(e) // compiles, throws exception

如果f外部不可见F，则不会发生此问题。

于 2011-02-24T00:39:56.980 回答
2

你问什么是方差？如果您知道什么是方差，这不言自明。没有 "val" 或 "case" 的示例没有涉及 A 的外部可见成员，因此它不会导致方差错误。

于 2011-02-23T23:24:42.910 回答
1

'val' 表示该字段是外部可见的。考虑：

val f: E[Any] => Unit = { ... }
val broken: F[Int] = new F[Any](f) // allowed by -A annotation
val f2: E[Int] => Unit = broken.f // must work (types match)
val f3: E[Int] => Unit = f // type error

基本上，我们设法不安全地强制转换 f 而没有明确地为它采取行动。这只适用于 f 是可见的，即如果您将其定义为 val 或使用案例类。

于 2011-02-24T00:40:28.307 回答
0

这是一个只打印到控制台的逆变“输出通道”：

class OutputChannel[-T] {   
  def write(t:T) = println(t); 
}

这是在行动： 

val out:OutputChannel[Any] = new OutputChannel[Any]
out.write(5)

还没有什么有趣的。关于逆变的一个很酷的事情是你现在可以安全地将这个输出通道分配给一个接受任何 T 子类的通道：

val out2:OutputChannel[String] = out
out2.write("five")
out2.write(55) //wont compile

现在，想象一下，如果我们向输出通道添加历史跟踪 - 以返回迄今为止已发送的内容列表。 

//!!! as you've seen code like this won't compile w/ contravariant types!!!!
class OutputChannel[-T] {   
  var history:List[T] = Nil
  def write(t:T) = { 
    history = history :+ t;  
    println(t); 
  } 
}

如果上面确实编译了，基于字符串的输出通道的用户会遇到问题：

//history(0) is an Int - runtime exception (if scala allowed it to compile)
val firstStringOutputted:String = out2.history(0)

由于逆变允许这种类型的“缩小”（即此处从 Any 到 String），类型系统不能公开类型 T 的值，例如我所做的这个“历史”字段，或您拥有的“f”字段。

其他著名的“逆向者”是 Function 和 Comparators：

val strHashCode:String => Int = { s:Any => s.hashCode }  //function which works with any object
val strComp:Comparator<String> = new HashCodeComparator()   //comparator object which works with any object
于 2011-02-24T02:45:58.103 回答