Mapping over a list of existential types

问题:

I have a list of existientially-typed objects I want to map over. Something like this:

sealed abstract class IntBox(val v: Int)
case object IB1 extends IntBox(1)
case object IB2 extends IntBox(2)

case class Adder[A  <: IntBox](ib: A, v: Int) {
  def add(i: A) = v + i.v
}

val adders: List[Adder[_ <: IntBox]] = List(Adder(IB1, 0), Adder(IB2, 0))

adders.map(adder => adder.add(adder.ib))

However, I'm getting an error like

found: adder.ib.type
required: _$1

I feel it's something like because the map somehow unifies the different IntBoxs into one unobtainable anonymous type...

Can I get what I want without casting (i.e. adder.asInstanceOf[Adder[adder.ib.type]]...?

回答1:

The type is not unobtainable, you can get it using type pattern matching:

adders.map{ case a: Adder[t] => a.add(a.ib) }

Here, the existential type parameter is bound to type variable t, and the compiler can correctly infer additional properties of t that it cannot infer for the whole list.

Refer to section 8.3.1 of the Specification for more details.

---

Full code with trait replaced by class to make it compile

sealed abstract class IntBox(val v: Int)
case object IB1 extends IntBox(1)
case object IB2 extends IntBox(2)

case class Adder[A  <: IntBox](ib: A, v: Int) {
  def add(i: A) = v + i.v
}

val adders: List[Adder[_ <: IntBox]] = List(Adder(IB1, 0), Adder(IB2, 0))

adders.map{ case a: Adder[t] => a.add(a.ib) }