Why were the case classes without a parameter list deprecated from Scala? And why does compiler suggest to use () as parameter list instead?
EDIT :
Someone please answer my second question... :|
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问题:
回答1:
It is really easy to accidentally use a no-arg case class incorrectly as a pattern.
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:
scala> (new Foo: Any) match { case _: Foo => true; case _ => false } res11: Boolean = true Or better:
scala> case object Bar defined module Bar scala> (Bar: Any) match { case Bar => true; case _ => false } res12: Boolean = true UPDATE Hopefully the transcript below will demonstrate why an empty parameter list is preferred to the deprecated missing parameter list.
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 = 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 = 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.
回答2:
Without parameters, every instance of the case class is indistinguishable and hence is essentially a constant. Use an object for that case.