Extending a trait by an object

Question

I have a Class named A that extends a Trait X. X has an abstract variable x that has been implemented in the class A. Class A uses one of the functions in Trait X named foo. I am trying to make a companion object of Class A and put in some 'static' method inside it named Ofoo. My problem is that Ofoo uses foo. So, I tried doing something like this:

trait X{ val x:String def foo = { //Full implementation here } } case class A extends X{ val x = "barbaz" // Class uses foo } object A extends X{ def Ofoo = { //This also needs to use foo } }

I get an error saying the object A cannot be instantiated as the variable x is not defined in trait X. How should I structure A such that it can use the function foo defined in the trait X?

You should explain what you are really trying to accomplish (your real problem, not an abstraction of it). There is probably a simple solution, but it is hard to say if we don't really know what you are trying to do. — toto2
– toto2, Commented Jul 4, 2014 at 15:12

Jatin · Accepted Answer · 2014-07-04 11:47:05Z

The compiler behaves as it should. There can be 2 things: Does foo use x?

If that is the case then it is required to have an Instance with x declared as the compiler says. You could say:

object A{ def ofoo = new X{val x = "default"}.foo }

If foo does not use x then I am not sure if it should even belong to the trait X. You could then create an object X and then have foo inside it something like:

trait X{ val x:String } object X{ def foo = {} }

Hmm, I can put the function foo in an object but how will I then make sure that only class A (and its singleton object A) can access it?

Erik Kaplun · Accepted Answer · 2014-07-07 01:14:14Z

How about implementing x in A? It's just an object; it has to obey the same rules, which include implementing the abstract members of any classes/traits that are being extended.

object A extends X { val x = "hello" ... }

just because it's the companion object of a class A that already inherits from X and implements x, does not mean that object X somehow magically reuses it.

In fact, technically a class and it's companion object are not connected; their only relation exists by convention.

On the other hand, if you want to make sure the value of x is the same in both class A as well as object A, you can just define val x in object A and pull in that value in class A:

class A extends X { val x = X.x ... } object A extends X { val x = "shared-value-of-x" ... }

"In fact, technically a class and it's companion object are not connected; their only relation exists by convention." Not quite: they see each other's private methods and fields.

eruve · Accepted Answer · 2014-07-05 12:14:05Z

At any rate, x must be defined by object A or in one of its superclasses/supertraits.

Based of the previous proposals and the OP's responses, I'll assume that foo should not be accessible from elsewhere, thus:

package pack { trait X{ val x:String } // Reachable only in the same source file sealed trait Y extends X{ // if foo doesn't use x below, no need to extend X, though private[pack] def foo = { //Full implementation here println("x = "+x) } } case class A() extends X with Y{ val x = "barbaz" // Class uses foo foo } object A extends X with Y{ val x = "barbatruc" def ofoo = { //This also needs to use foo foo } } }

Actually I have a hard time to understand what the OP wants. I agree with toto2.

Collectives™ on Stack Overflow

Extending a trait by an object

3 Answers 3

2 Comments

1 Comment

Comments

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

2 Comments

1 Comment

Comments

Related