abstract-types.md

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Traits and abstract classes can have an abstract type member. This means that the concrete implementations define the actual type. Here's an example:

trait Buffer {
  type T
  val element: T
}

Here we have defined an abstract type T. It is used to describe the type of element. We can extend this trait in an abstract class, adding an upper-type-bound to T to make it more specific.

abstract class SeqBuffer extends Buffer {
  type U
  type T <: Seq[U]
  def length = element.length
}

Notice how we can use yet another abstract type U as an upper-type-bound. This class SeqBuffer allows us to store only sequences in the buffer by stating that type T has to be a subtype of Seq[U] for a new abstract type U.

Traits or classes with abstract type members are often used in combination with anonymous class instantiations. To illustrate this, we now look at a program which deals with a sequence buffer that refers to a list of integers:

abstract class IntSeqBuffer extends SeqBuffer {
  type U = Int
}


def newIntSeqBuf(elem1: Int, elem2: Int): IntSeqBuffer =
  new IntSeqBuffer {
       type T = List[U]
       val element = List(elem1, elem2)
     }
val buf = newIntSeqBuf(7, 8)
println("length = " + buf.length)
println("content = " + buf.element)

Here the factory newIntSeqBuf uses an anonymous class implementation of IntSeqBuf (i.e. new IntSeqBuffer), setting type T to a List[Int].

It is also possible to turn abstract type members into type parameters of classes and vice versa. Here is a version of the code above which only uses type parameters:

abstract class Buffer[+T] {
  val element: T
}
abstract class SeqBuffer[U, +T <: Seq[U]] extends Buffer[T] {
  def length = element.length
}

def newIntSeqBuf(e1: Int, e2: Int): SeqBuffer[Int, Seq[Int]] =
  new SeqBuffer[Int, List[Int]] {
    val element = List(e1, e2)
  }

val buf = newIntSeqBuf(7, 8)
println("length = " + buf.length)
println("content = " + buf.element)

Note that we have to use variance annotations here (+T <: Seq[U]) in order to hide the concrete sequence implementation type of the object returned from method newIntSeqBuf. Furthermore, there are cases where it is not possible to replace abstract types with type parameters.