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// a.scala
// Mon Jul 11 14:18:26 PDT 2011
object ExistentialsConsideredHarmful {
class Animal(val name: String)
object Dog extends Animal("Dog")
object Sheep extends Animal("Sheep")
trait Tools[A] {
def shave(a: A): A
}
def tools[A](a: A): Tools[A] = null // dummy
case class TransportBox[A <: Animal](animal: A, tools: Tools[A]) {
def label: String = animal.name
}
// 1.
def carry[A <: Animal](box: TransportBox[A]): Unit = {
println(box.animal.name+" got carried away")
}
val aBox =
if (math.random < 0.5)
TransportBox(Dog, tools(Dog))
else
TransportBox(Sheep, tools(Sheep))
// 2.
//aBox.tools.shave(aBox.animal)
// Use pattern match to avoid opening the existential twice
aBox match {
case TransportBox(animal, tools) => tools.shave(animal)
}
abstract class BoxCarrier[R <: Animal](box: TransportBox[R]) {
def speed: Int
def talkToAnimal: Unit = println("The carrier says hello to"+box.animal.name)
}
// 3.
//val bc = new BoxCarrier(aBox) {
// Use pattern match to avoid opening the existential twice
// Type annotation on bc is required ... possible compiler bug?
// val bc : BoxCarrier[_ <: Animal] = aBox match {
val bc = aBox match {
case tb : TransportBox[a] => new BoxCarrier(tb) {
def speed: Int = 12
}
}
}
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