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

Question

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 fina

Answer 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?.