CODEWITHSUNDEEP
scalacovariancecontravariance
Asad Iqbal
Asad Iqbal

Reputation: 3321

method variance: Return a list of abstract supertype after inserting concrete elements

I am writing insertInOrder function for a superclass, but cannot access the value age from the method:

abstract class Parent(age: Int)
case class Child1(age: Int) extends Parent(age)
case class Child2(age: Int) extends Parent(age)

def insertInOrder [U >: Parent] (x:  U, acc: List[U] ): List[U]  ;
// value age is not a member of type parameter U

I tried this: 

def insertInOrder [U >: Parent, U <: Nothing] (x:  U, acc: List[U] ): List[U]  = {
// U is already defined as type U

What should I do?

Upvotes: 0

Views: 58

Answers (3)

Eugene Zhulenev
Eugene Zhulenev

Reputation: 9734

In your example age defined in abstract Parent is not accessible, you need to add val in front of it to make accessible from Parent. However you'll have problems with case class overrides for Child. Also you have incorrect type bound for U. That's how I suggest you to fix code:

  trait Parent {
    def age: Int
  }
  case class Child1(age: Int) extends Parent
  case class Child2(age: Int) extends Parent

  def insertInOrder [U <: Parent] (x: U) = {
    println(x.age)
  }

  insertInOrder(Child1(10))

Upvotes: 1

Gabriele Petronella
Gabriele Petronella

Reputation: 108101

abstract class Parent(val age: Int)

will generate the getter you're looking for.

Upvotes: 1

Denis Makarenko
Denis Makarenko

Reputation: 2938

No getter/setter is created for age constructor parameter in the Parent non-case class. So add 'val' (maybe 'protected val' if it shouldn't be exposed outside of derived classes) to it.

A few examples and a nice summary: https://www.safaribooksonline.com/library/view/scala-cookbook/9781449340292/ch04s03.html

Upvotes: 1

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