Why and when should trait and object have same name?

user_2437292 0
Viewed 848

What is the reason that 

  trait Dog {
    def bark = "woof"
  }
  object Dog extends Dog

works, but the following does not

trait Dog {
  def bark = "woof"
}
class Dog extends Dog

I read somewhere that in the first case mentioned above the object is not a companion object. What is the scenario when object and trait should have the same name ?

companion-object traits scala
2 Answers

From introduction to Chapter 2 (Identifiers, Names & Scopes):

There are two different name spaces, one for types and one for terms.

Section 5.5 (Object definitions) contains almost exactly your example (with Point instead of Dog), with the following comment:

Note that the double use of the name Point is legal, since the class definition defines the name Point in the type name space, whereas the object definition defines a name in the term namespace.

(emphasis mine)

Your second example with class Dog extends Dog does not work, because both the class Dog and trait Dog end up in the namespace for types, and this results in a conflict.

Companion objects are used whenever you would use static methods in e.g. Java (except that companion objects are much nicer, because they are, well, objects, and can be passed around as ordinary values).

The situation object Foo extends Foo appears somewhat contrived and rather uncommon. Have you an actual example where it is used? If not, then it's just a corner case that is formally valid, simply because it's not explicitly prohibited.

EDIT: object AnsiColor extends AnsiColor seems to be one of the most "important" applications of the object X extends X pattern.

user_27077920

The reason the second example fails here, is not because Dog is not a companion object, but because you define a new duplicate class Dog. I always view traits and classes as one category, and objects as another - and object is an singleton instance of either trait or class.

Why you can't create classes with the same name? It's impossible to distinguish two entities if that's allowed.

Why you can create (class/trait) and object with the same name? You can distinguish them, because class or trait is type and object is a value.

When should you name your objects the same name as classes? If you wan't to define companion object, so your implicits for the object would work. Or if you want to define convenient apply method on an object. Otherwise I don't bother myself at all with either object is companion or not, but often create objects with the class name for convenience.

user_15738250
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