scala - Class Extends a Trait With/Without Parentheses

Question

Looking at this code from Scala in Depth...

scala> trait Foo {
     |  def someMethod(): Int = 5
     | }
defined trait Foo

scala> class Main() extends Foo {
     |   }
defined class Main

Then, I created a new Main (without the parentheses).

scala> class Main extends Foo {}
defined class Main

What is the meaning the parentheses in class Main()? How about without parentheses?

Answer 1

5.3 of the spec:

If no formal parameter sections are given, an empty parameter section () is assumed.

This is different from methods, which can be parameterless.

When you write

class A

you are really writing

class A()

Moreover,

scala> class A(implicit i: Int)
defined class A

scala> new A
<console>:9: error: could not find implicit value for parameter i: Int
              new A
              ^

scala> new A()(1)
res1: A = A@32185a4a

scala> new A(1)
<console>:9: error: too many arguments for constructor A: ()(implicit i: Int)A
              new A(1)
              ^