class - 在 Scala 2.9 之后，case 类的哪些内容将被删除？

Question

我知道有一些关于案例类的计划更改，例如禁止它们之间的继承:

scala> case class Foo()
defined class Foo

scala> case class Bar() extends Foo()
<console>:9: warning: case class `class Bar' has case ancestor `class Foo'.  Case-to-case inheritance has potentially dangerous bugs which are unlikely to be fixed.  You are strongly encouraged to instead use extractors to pattern match on non-leaf nodes.
       case class Bar() extends Foo()
                  ^
defined class Bar

或没有参数列表的案例类(不确定):

scala> case class Foo
<console>:1: warning: case classes without a parameter list have been deprecated;
use either case objects or case classes with `()' as parameter list.
       case class Foo
                     ^
<console>:7: warning: case classes without a parameter list have been deprecated;
use either case objects or case classes with `()' as parameter list.
       case class Foo
                     ^
defined class Foo

目前@deprecated 还有什么？

Answer 1

Scala 中的每个类都必须至少有一个非隐式参数部分。如果您不包含一个，编译器会为您添加它。

scala> class X
defined class X

scala> new X()
res4: X = X@68003589

scala> class Y
defined class Y

scala> new Y()
res5: Y = Y@467f788b

案例类也不异常(exception)。但是与模式匹配的交互是混淆和错误的来源，这促使弃用。

scala> case class A
<console>:1: warning: case classes without a parameter list have been deprecated;
use either case objects or case classes with `()' as parameter list.
       case class A
                   ^
defined class A

scala> val a = A()
a: A = A()

scala> (a: Any) match { case A => 1; case _ => 2 }
res0: Int = 2

scala> val companion = A
companion: A.type = A

scala> (companion: Any) match { case A => 1; case _ => 2 }
res0: Int = 1

正如 Dean 建议的那样，通常最好用案例对象对此进行建模。

我不知道取消对空参数列表案例类的支持的时间表。案例类继承在 2.9.0 中几乎被删除，但已推迟到下一个主要版本。

进一步阅读:

Why can't the first parameter list of a class be implicit?