Scala - why cannot use trait from object

Question

I have created a simple application with singleton object which contains local traits:

object Singleton {

  trait FirstTrait {
    val func1 = (i: Int) => i * 2
  }

  trait SecondTrait {
    val func2 = (s: String) => s
  }

  trait ThirdTrait {
    val func3 = () => println("Func 3")
  }
}

And now, in Main object, I would like to do something like this:

object Main extends App {    
  val singleton = Singleton.FirstTrait//cannot do this
}

But I cannot do this (compile error). Why? Why I have not an access into this local trait? If I change Singleton object into:

object Singleton {

  trait FirstTrait {
    val func1 = (i: Int) => i * 2
  }

  trait SecondTrait {
    val func2 = (s: String) => s
  }

  trait ThirdTrait {
    val func3 = () => println("Func 3")
  }

  object FirstObject extends FirstTrait {
    println(func1)
  }
}

Main works well and program compiles. But I call another singleton object from Singleton, not a trait. I understand that trait cannot be instanced, but I think it is not a solution for it, because I have also a simple ScalaTest test name, which looks like

"Singleton" should "test it" in Singleton.FirstTrait{...}

and here I have an access into FirstTrait. So why I cannot use it in normal code? I cannot understand it well. Maybe I am an idiot, but if someone could explain it to me well, I would be greatful.

Leo C · Accepted Answer · 2018-12-12 18:30:56Z

5

It's a trait so you'll need to "instantiate" it properly:

val singleton = new Singleton.FirstTrait {}
// singleton: Singleton.FirstTrait = $anon$1@5e97da56

Note that technically a trait cannot be instantiated. The above expression is an instantiation of the anonymous class that extends the trait.

edited Dec 12, 2018 at 18:30

answered Dec 12, 2018 at 17:58

Leo C

22.5k3 gold badges28 silver badges42 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Developus Over a year ago

Ok, I understand. So in Scala the trait can be instantiate like normal class? new keyword and object in one are not logical for me. Could you tell more?

Leo C Over a year ago

That's right, and since it's a trait you'll need to implement its abstract members, if any. Here's a relevant link which might be of interest.

Alexey Romanov Over a year ago

Not quite, this isn't instantiating the trait, but an anonymous class extending the trait.

Leo C Over a year ago

@Alexey Romanov, good point. It's indeed an instantiation of the anonymous class that extends the trait.

Alexey Romanov · Accepted Answer · 2018-12-12 18:05:51Z

1

Singleton.FirstTrait is a type, not a value. You can't write

val singleton = Singleton.FirstTrait

any more than you can write

val singleton = Int
val singleton = String

etc. Traits and classes can have companion objects (objects with the same name), but FirstTrait obviously doesn't have one.

You can use it as a type, e.g.

def foo(x: Singleton.FirstTrait) = {}
val x: Singleton.FirstTrait = new Singleton.FirstTrait {}

answered Dec 12, 2018 at 18:05

Alexey Romanov

172k38 gold badges329 silver badges509 bronze badges

2 Comments

Developus Over a year ago

Ok, I got it. So in my second example I have an companion object for a trait so I can use it like a value.

Alexey Romanov Over a year ago

It's just an object (not a companion object because it's called FirstObject, not FirstTrait), and you can use any object as a value.

Collectives™ on Stack Overflow

Scala - why cannot use trait from object

2 Answers 2

4 Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

4 Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related