Why is `Nil` defined as a` case object`

Question

In the scala source, I found:

case object Nil extends List[Nothing] { ... } 

I can’t understand why it is declared as a case object and not an object ?

I found this question [ The difference between an object of an object and an object ] useful, and I believe that this reason is the key:

default serialization implementation

because we often send a list of data to another player, so Nil has to be serializable, right?

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With the answers provided (thanks), I'm trying to write code to verify it:

 trait MyList[+T] object MyNil extends MyList[Nothing] val list: MyList[String] = MyNil list match { case MyNil => println("### is nil") case _ => println("### other list") } 

You can see that MyNil not a case object , but I can still use it according to the pattern. Here is the result:

 ### is nil 

Am I misunderstanding something?

Answer 1

In general, for immutable data, the question should never be “why is it an object-object (or class)”, but rather “Can I make a case object?”. With a few exceptions (mainly due to inheritance), data elements in Scala must be immutable and must be implemented through case classes / objects. Given that implementing Nil and :: as a case object and a case class (respectively) is standard practice for which there are no drawbacks.

Answer 2

As mentioned in the comments on this related question, the one thing you get is a more beautiful .toString result

 scala> MyNil.toString res0: String = MyNil$@51aa572b scala> case object MyNil2 extends MyList[Nothing] defined module MyNil2 scala> MyNil2.toString res2: String = MyNil2 scala> Nil.toString res1: String = List()