如何确保在map()期间保留自定义Scala集合的动态类型？

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

现在,这是我的问题.如果我运行这个,一切都很好：

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

但是这段代码将RNA转换为Vector！

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

这是一个问题,因为不知道RNA类的客户端代码可能会在仅从Base映射到Base时将其转换回Vector.为什么会这样,有什么方法可以解决它？

P.-S.：我找到了一个试探性的答案(见下文),如果我错了,请纠正我.