Issue with scala object companion and trait

Question

i am trying to do something like this in scala so that the Category class receives its attributes by parameters but i get the following error:

object creation impossible, since method apply in trait ModelCompanion of type => asd.Category is not defined
  object Category extends ModelCompanion[Category] {
         ^
one error found

Code here:

object asd {

  trait ModelCompanion[M <: Model[M]] {
    def apply: M
  }

  trait Model[M <: Model[M]] {
    var id: Int = 0
  }

  object Category extends ModelCompanion[Category] {
    def apply(name: String): Category = new Category(name)
  }

  class Category(var name: String) extends Model[Category] {
  // Do something with name
  }

}

I am new to scala so if you could give me some guidance on this I would be very grateful.

Answer 1

ModelCompanion defines an abstract method apply without any arguments (or argument lists). In Category you define an apply method that takes an argument of type String . That's not an implementation of the abstract method because it doesn't accept the same number and types of arguments. Therefore Category does not provide a suitable definition of ModelCompanion 's abstract apply method and therefore can not be instantiated.

Depending on the behavior you want, you should either change the definition of ModelCompanion.apply to def apply(name: String): M or introduce another type argument and use that as the argument type.

Answer 2

Shortly:

def apply:M
//and 
def apply(name:String):M
//are not the same methods

//if you try define it with override modifier
override def apply(name: String): Category = new Category(name)

//it will expose this fact to you with error:
//method apply overrides nothing.
//Note: the super classes of object Category 
//      contain the following, non final members named apply:
//def apply: ammonite.$sess.cmd8.asd.Category

//You need to define `ModelCompanion` with appriopriate `apply`

trait ModelCompanion[M <: Model[M]] {
  def apply(name:String): M
}

// or override properly current (argumentless) one  
object Category extends ModelCompanion[Category] { 
  override def apply: Category = new Category("Category" + Random.nextInt())
}