scala - 使用通用案例类了解删除

Question

对于以下案例类别:

scala> case class Foo[T](name: String) {}
defined class Foo

scala> val foo = Foo[Int]("foo")
foo: Foo[Int] = Foo(foo)

为什么 Scala 让我(正如我认为的那样)匹配 Foo[Int]？ Int 不是被删除了吗？

scala> foo match { 
     |   case _: Foo[Int] => "foo"
     |   case _        => "bar"
     | }
res2: String = foo

但是当包含另一个模式匹配情况时它会显示编译时错误？

scala> foo match { 
     |  case _: Foo[String] => "string"
     |  case _: Foo[Int]    => "int"
     |  case _              => "other"
     | }
<console>:12: warning: non-variable type argument String in type pattern Foo[String] is unchecked since it is eliminated by erasure
               case _: Foo[String] => "string"
                       ^
    <console>:12: error: pattern type is incompatible with expected type;
     found   : Foo[String]

 required: Foo[Int]
               case _: Foo[String] => "string"
                       ^

Answer 1

  class SuperFoo;
  case class Foo[T](name: String) extends SuperFoo {}
  val foo: SuperFoo = Foo[Int]("foo")
  foo match {
    case _: Foo[String] => "foo"
    case _        => "bar"
  }  //> res0: String = foo + warning

在您的情况下，编译器知道 foo 的确切类型。