How can I ensure that the dynamic type of my custom Scala collection is preserved during a map()?

问题

I read the very interesting article on the architecture of the Scala 2.8 collections and I've been experimenting with it a little bit. For a start, I simply copied the final code for the nice RNA example. Here it is for reference:

abstract class Base
case object A extends Base
case object T extends Base
case object G extends Base
case object U extends Base

object Base {
  val fromInt: Int => Base = Array(A, T, G, U)
  val toInt: Base => Int = Map(A -> 0, T -> 1, G -> 2, U -> 3)
}

final class RNA private (val groups: Array[Int], val length: Int)
    extends IndexedSeq[Base] with IndexedSeqLike[Base, RNA] {

  import RNA._

  // Mandatory re-implementation of `newBuilder` in `IndexedSeq`
  override protected[this] def newBuilder: Builder[Base, RNA] =
    RNA.newBuilder

  // Mandatory implementation of `apply` in `IndexedSeq`
  def apply(idx: Int): Base = {
    if (idx < 0 || length <= idx)
      throw new IndexOutOfBoundsException
    Base.fromInt(groups(idx / N) >> (idx % N * S) & M)
  }

  // Optional re-implementation of foreach, 
  // to make it more efficient.
  override def foreach[U](f: Base => U): Unit = {
    var i = 0
    var b = 0
    while (i < length) {
      b = if (i % N == 0) groups(i / N) else b >>> S
      f(Base.fromInt(b & M))
      i += 1
    }
  }
}

object RNA {

  private val S = 2 // number of bits in group
  private val M = (1 << S) - 1 // bitmask to isolate a group
  private val N = 32 / S // number of groups in an Int

  def fromSeq(buf: Seq[Base]): RNA = {
    val groups = new Array[Int]((buf.length + N - 1) / N)
    for (i <- 0 until buf.length)
      groups(i / N) |= Base.toInt(buf(i)) << (i % N * S)
    new RNA(groups, buf.length)
  }

  def apply(bases: Base*) = fromSeq(bases)

  def newBuilder: Builder[Base, RNA] =
    new ArrayBuffer mapResult fromSeq

  implicit def canBuildFrom: CanBuildFrom[RNA, Base, RNA] =
    new CanBuildFrom[RNA, Base, RNA] {
      def apply(): Builder[Base, RNA] = newBuilder
      def apply(from: RNA): Builder[Base, RNA] = newBuilder
    }
}

Now, here's my problem. If I run this, everything's fine:

val rna = RNA(A, G, T, U)
println(rna.map(e => e)) // prints RNA(A, G, T, U)

but this code transforms the RNA to a Vector!

val rna: IndexedSeq[Base] = RNA(A, G, T, U)
println(rna.map(e => e)) // prints Vector(A, G, T, U)

This is a problem, as client code unaware of the RNA class may transform it back to a Vector instead when only mapping from Base to Base. Why is that so, and what are the ways to fix it?

P.-S.: I've found a tentative answer (see below), please correct me if I'm wrong.

回答1:

If the static type of the rna variable is IndexedSeq[Base], the automatically inserted CanBuildFrom cannot be the one defined in the RNA companion object, as the compiler is not supposed to know that rna is an instance of RNA.

So where does it come from? The compiler falls back on an instance of GenericCanBuildFrom, the one defined in the IndexedSeq object. GenericCanBuildFroms produce their builders by calling genericBuilder[B] on the originating collection, and a requirement for that generic builder is that it can produce generic collections that can hold any type B — as of course, the return type of the function passed to a map() is not constrained.

In this case, RNA is only an IndexedSeq[Base] and not a generic IndexedSeq, so it's not possible to override genericBuilder[B] in RNA to return a RNA-specific builder — we would have to check at runtime whether B is Base or something else, but we cannot do that.

I think this explains why, in the question, we get a Vector back. As to how we can fix it, it's an open question…

Edit: Fixing this requires map() to know whether it's mapping to a subtype of A or not. A significant change in the collections library would be needed for this to happen. See the related question Should Scala's map() behave differently when mapping to the same type?.

回答2:

On why I think it's not a good idea to statically type to a weaker type than RNA. It should really be a comment (cause it's more an opinion but that would be harder to read). From your comment to my comment:

Why not? As a subclass of IndexedSeq[Base], RNA is able to do everything IndexedSeq[Base] does, as per the Liskov substitution principle. Sometimes, all you know is that it's an IndexedSeq, and you still expect filter, map and friends to keep the same specific implementation. Actually, filter does it — but not map

filter does it because the compiler can statically guarantee it. If you keep elements from a particular collection, you end up with a collection from the same type. map cannot guarantee that, it depends on the function that is passed.

My point is more on the act of specifying explicitly a type and expecting more than what it can deliver. As a user of the RNA collection, I may write code that depends on certain properties of this collection such as efficient memory representation.

So let's assume I state in val rna: IndexedSeq[Base] that rna is just an IndexedSeq. A few lines later I call a method doSomething(rna) where I expect the efficient memory representation, what would be the best signature for that? def doSomething[T](rna: IndexedSeq[Base]): T or def doSomething[T](rna: RNA): T?

I think it should be the latter. But if that's the case, then the code won't compile because rna is not statically an RNA object. If the method signature should be the former, then in essence I'm saying that I don't care about the memory representation efficiency. So I think the act of specifying a weaker type explicitly but expecting a stronger behavior is a contradiction. Which is what you do in your example.

Now I do see that even if I did:

val rna = RNA(A, G, T, U)
val rna2 = doSomething(rna)

where somebody else wrote:

def doSomething[U](seq: IndexedSeq[U]) = seq.map(identity)

I would like to have rna2 be a RNA object but that won't happen... It means that this somebody else should write a method that takes a CanBuildFrom if they want to have callers get more specific types:

def doSomething[U, To](seq: IndexedSeq[U])
   (implicit cbf: CanBuildFrom[IndexedSeq[U], U, To]) = seq.map(identity)(cbf)

Then I could call: val rna2: RNA = doSomething(rna)(collection.breakOut)

来源：https://stackoverflow.com/questions/5660656/how-can-i-ensure-that-the-dynamic-type-of-my-custom-scala-collection-is-preserve