Scala abstract inheritance

Question

So, what I want to do is to create an abstract inheritance model using traits. I think example code works best so I created this small showcase to present my problem.

trait Animals{
 val owned: Seq[Animal]
 type Animal <: TAnimal
 trait TAnimal {
  def name : String
 }
}

So far so good. Now I have another trait "Dogs". The dogs are chipped so they have and identification Number. Also I want to implement the sequence containing all the dogs I have (lets say I have 5 dogs with random names and random identNo, for simplicities sake).

trait Dogs extends Animals{
 type Dog <: TDog
 val owned = ???
 trait TDog extends TAnimal {
  def identNo : Int
 }
}

The Problem is, since Animal or Dog are only types I cannot create concrete instances of them. I think I can use something like Seq.fill but I'm not able to create a matching expression.

4lex1v · Accepted Answer · 2013-11-29 12:17:28Z

2

This is called a Cake Pattern. And you don't need to write all this stuff in Dog trait, you can define it like this:

trait Animals{
 type Animal <: TAnimal
 def owned: Seq[Animal]
 trait TAnimal {
  def name : String
 }
}

trait Dogs extends Animals{
 type Animal <: TDog
 trait TDog extends TAnimal {
  def identNo : Int
 }
}

Then "at the end of the world" you assemble your cake with some concrete implementation:

trait ReservoirDogs extends Dogs {
  case class Animal(name: String, identNo: Int) extends TDog
  val owned = List(Animal("Pink", 1), Animal("Blue", 2))
}

Now you can mix it in:

scala> val dogs = new Dogs with ReservoirDogs {}
dogs: Dogs with ReservoirDogs = $anon$1@6f6f6727

scala> val dogs = new Animals with ReservoirDogs {}
dogs: Animals with ReservoirDogs = $anon$1@11c8ce34

This is what Cake Pattern is all about

answered Nov 29, 2013 at 12:17

4lex1v

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1 Comment

norathem Over a year ago

Thanks, that really helped. Didn't knew about the Cake Pattern so far. Btw. I really like your example =)

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