哪些事情在case类将被删除后Scala 2.9？ [英] Which things around case classes will be removed after Scala 2.9 exactly?

问题描述

  scala>我们知道， case类Foo（）
定义类Foo 
 
 scala> case class Bar（）extends Foo（）
< console>：9：warning：case class`class Bar'有case ancestor`class Foo'。案例间继承具有潜在的危险错误，这些错误不太可能被修复。强烈建议您使用提取器在非叶节点上进行模式匹配。 
 case class Bar（）extends Foo（）
 ^ 
定义类Bar 
  

$ b b

或不带参数列表的case类（不确定）：

  scala> case类Foo 
< console>：1：warning：没有参数列表的case类已被弃用; 
使用case对象或case类与`（）'作为参数列表。 
 case class Foo 
 ^ 
< console>：7：warning：没有参数列表的case类已被弃用; 
使用case对象或case类与`（）'作为参数列表。 
 case class Foo 
 ^ 
定义类Foo 
  

$ p

解决方案

Scala中的每个类必须至少有一个非隐式参数部分。如果你不包括一个，编译器会为你添加它。

  scala> class X 
 defined class X 
 
 scala> new X（）
 res4：X = X @ 68003589 
 
 scala>类Y 
定义类Y 
 
 scala>案例类别也不例外。新的Y（）
 res5：Y = Y @ 467f788b 
  

。但是与模式匹配的交互是一个混乱和错误的来源，这激发了贬低。

  scala> case类A 
< console>：1：warning：不带参数列表的case类已被弃用; 
使用case对象或case类与`（）'作为参数列表。 
 case class A 
 ^ 
定义类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所说，与一个case对象。

我不知道一个时间线删除对空参数列表case类的支持。案例类继承在2.9.0中几乎被删除，但是已经推迟到下一个主要版本。

进一步阅读：

I know that the are some changes planned regarding case classes, like disallowing inheritance between them:

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

or case classes without parameter list (not sure about that):

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

What else is currently @deprecated?

解决方案

Every class in Scala must have at least one non-implicit parameter section. If you don't include one, the compiler will add it for you.

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

Case classes are no exception. But the interaction with pattern matching is a source of confusion and bugs, which motivates the deprecation.

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

As Dean suggests, it's usually better to model this with a case object.

I'm not aware of a timeline for removing support for empty-param list case classes. Case class inheritance was almost removed in 2.9.0, but that has been deferred until the next major release.

Further Reading:

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

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