Scala模式匹配与类型推断

Question

Option("foo") match {
  case x: List[String] => println("A")
  case _ => println("B")
}

Answer 1

代码是注释的，所以让我们花点时间细细品味一下：

  /** If we can absolutely rule out a match we can fail early.
   *  This is the case if the scrutinee has no unresolved type arguments
   *  and is a "final type", meaning final + invariant in all type parameters.
   */

例如，请注意None不是final。我知道，对吧？

如果您曾经尝试过scalac-ypatmat-debug，下面的注释可能会有所帮助：

https://github.com/scala/scala/pull/650

可达性几乎触手可及：

https://issues.scala-lang.org/browse/si-6146

trait Foo[+A]
final class Fuzz[+A] extends Foo[A]
final object Fooz extends Foo[Nothing]
object Futz extends Foo[Nothing]

//error
Fooz match {
  case x: List[_] => println("A")
  case _ => println("B")
}
//no error
Futz match { ... }