Scala Covariance and Lower Type Bounds Explanation

Question

I am trying to get my head around covariance in respect with methods creating new immutable types using lower bounds

class ImmutableArray[+T](item: T, existing:          


        

Answer 1

It works because the append method returns a broader class than the original one. Let's conduct a little experiment.

    scala> case class myIntClass(a:Int)
    defined class myIntClass

    scala> case class myIntPlusClass(a:Int, b:Int)
    defined class myIntPlusClass

   scala> class ImmutableArray[+T](item: T, existing: List[T] = Nil){
         | 
         | private val items = item :: existing
         | 
         | def append[S >: T](value: S) = new ImmutableArray[S](value,items)
         | def getItems = items
         | }
    defined class ImmutableArray

    scala> val ia = new ImmutableArray[myIntClass](myIntClass(3))
    ia: ImmutableArray[myIntClass] = ImmutableArray@5aa91edb

    scala> ia.getItems
    res15: List[myIntClass] = List(myIntClass(3))

    scala> ia.append(myIntPlusClass(3,5))
    res16: ImmutableArray[Product with Serializable] = ImmutableArray@4a35a157

    scala> res16.getItems
    res17: List[Product with Serializable] = List(myIntPlusClass(3,5), myIntClass(3))

    scala> res16
    res18: ImmutableArray[Product with Serializable] = ImmutableArray@4a35a157

So you can add a derived class here, but it only works due to the fact that the base type of the resulting array is demoted to a lowest common denominator (in this case, Serializable).

If we try to force the derived type on the resulting array, it won't work:

scala> ia.append[myIntPlusClass](myIntPlusClass(3,5))
:23: error: type arguments [myIntPlusClass] do not conform to method append's type parameter bounds [S >: myIntClass]
              ia.append[myIntPlusClass](myIntPlusClass(3,5))

Trying to do the same making append return an array of derived types won't work, because T is not a subclass of S:

scala> class ImmutableArray[+T](item: T, existing: List[T] = Nil){
     |           
     |          private val items = item :: existing
     |          
     |          def append[S <: T](value: S) = new ImmutableArray[S](value,items)
     |          def getItems = items
     |          }
:21: error: type mismatch;
 found   : List[T]
 required: List[S]
                def append[S <: T](value: S) = new ImmutableArray[S](value,items)