Why were the case classes without a parameter list deprecated?

Question

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... :|

Answer 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 = <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.

Answer 2

Without parameters, every instance of the case class is indistinguishable and hence is essentially a constant. Use an object for that case.