scala 为什么不推荐使用没有参数列表的案例类？
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原文地址: http://stackoverflow.com/questions/2254710/
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Why were the case classes without a parameter list deprecated?
提问by missingfaktor
Why were the case classes without a parameter list deprecated from Scala? And why does compiler suggest to use ()as parameter list instead?
为什么 Scala 不推荐没有参数列表的 case 类？为什么编译器建议使用()作为参数列表？
EDIT :
编辑 ：
Someone please answer my second question... :|
有人请回答我的第二个问题... :|
采纳答案by retronym
It is really easy to accidentally use a no-arg case class incorrectly as a pattern.
不小心将 no-arg case 类错误地用作模式真的很容易。
scala> case class Foo
warning: there were deprecation warnings; re-run with -deprecation for details
defined class Foo
scala> (new Foo: Any) match { case Foo => true; case _ => false }
res10: Boolean = false
Instead of:
代替：
scala> (new Foo: Any) match { case _: Foo => true; case _ => false }
res11: Boolean = true
Or better:
或更好：
scala> case object Bar
defined module Bar
scala> (Bar: Any) match { case Bar => true; case _ => false }
res12: Boolean = true
UPDATEHopefully the transcript below will demonstrate why an empty parameter list is preferred to the deprecated missing parameter list.
更新希望下面的文字记录能够说明为什么空参数列表比已弃用的缺失参数列表更受欢迎。
scala> case class Foo() // Using an empty parameter list rather than zero parameter lists.
defined class Foo
scala> Foo // Access the companion object Foo
res0: Foo.type = <function0>
scala> Foo() // Call Foo.apply() to construct an instance of class Foo
res1: Foo = Foo()
scala> case class Bar
warning: there were deprecation warnings; re-run with -deprecation for details
defined class Bar
scala> Bar // You may expect this to construct a new instance of class Bar, but instead
// it references the companion object Bar
res2: Bar.type = <function0>
scala> Bar() // This calls Bar.apply(), but is not symmetrical with the class definition.
res3: Bar = Bar()
scala> Bar.apply // Another way to call Bar.apply
res4: Bar = Bar()
A case object would usually still be preferred over an empty parameter list.
与空参数列表相比，case 对象通常仍然是首选。
回答by Randall Schulz
Without parameters, every instance of the case class is indistinguishable and hence is essentially a constant. Use an object for that case.
如果没有参数，case 类的每个实例都是不可区分的，因此本质上是一个常量。在这种情况下使用一个对象。
