Can scala compiler enforce to implement abstract trait methods when mixin in?

Question

I have a trait with abstract methods and concrete implemented methods, so something like this:

trait MyTrait extends BaseClass {
    def myAbstractMethod: MyReturnType
    def myConcreteMethod = { /*implementation*/ }
}

Now I mixin the trait:

class MyClass extends BaseClass with MyTrait {

}

The BaseClass does not implement the abstract method. I expected the scala compiler to enforce that the abstract method must be implemented (just like a Java interface) when I mix in the trait. But there is no compiler error.

My particular case is more complicated. I was not able to test what happens at runtime, yet.

Why doesn't the scala compiler enforce the implementation of the abstract method?
Can I make the scala compiler enforce the implementation of the abstract method?
Must I add abstract or override somewhere?
What happens at runtime when I try to create and use instances of MyClass?

I've just encountered this when I had other errors in the same file/project (but not in other projects in the workspace). I'm using Eclipse 4.5 (Mars) with Scala 4.2.0. I've reported this as a bug: assembla.com/spaces/scala-ide/support/tickets/… — Eyal Roth

mericano1 mericano1 · Accepted Answer · 2012-12-08T18:32:22

You should definitely get a compiler error...

scala> :paste
// Entering paste mode (ctrl-D to finish)

trait MyTrait extends BaseClass {
    def myAbstractMethod: MyReturnType
    def myConcreteMethod = { /*implementation*/ }
}

class MyClass extends BaseClass with MyTrait {    
}


// Exiting paste mode, now interpreting.

<console>:14: error: class MyClass needs to be abstract, since method myAbstractMethod in trait MyTrait of type => MyReturnType is not defined
       class MyClass extends BaseClass with MyTrait {


 ^

Can scala compiler enforce to implement abstract trait methods when mixin in?

1 Answers