Question

'val'和'case'如何以及为什么会影响类型系统？（特别是方差）

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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

Answer 1

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

如果在f之外看不到F，则无法解决此问题。

Answer 2

你在问什么是差异？如果你知道什么是方差，这是不言自明的。没有“val”或“case”的示例没有涉及A的外部可见成员，因此它不会引起方差错误。

Answer 3

'val'表示该字段在外部可见。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，我们设法不安全地投射f而没有明确地采取行动。这只适用于f是可见的，即如果你将它定义为val或使用case类。

Answer 4

这是一个只是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)

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T的任何子类的通道：

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

现在，想象一下，如果我们将一个历史记录跟踪添加到输出通道 - 返回到目前为止已发送的事物的列表。

//!!! 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); 
  } 
}

如果以上编译，基于字符串的输出通道的用户将遇到问题：

//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”（例如，从Any到String），类型系统不能公开类型T的值，例如我所做的“历史”字段，或者你拥有的“f”字段。

其他着名的“反对者”是功能和比较器：

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

逆变和val

4 个答案: