Why is Seq.newBuilder returning a ListBuffer?
scala, scala-collections
Solution
The Scala Collections API is very complex and its hierarchy is rich in depth. Each level represents some sort of new abstraction. The `Seq` trait split up into two different subtraits, which give different guarantees for performance (ref.):
An `IndexedSeq` provides fast random-access of elements and a fast length operation. One representative of this `IndexedSeq` is the `Vector`.
A `LinearSeq` provides fast access only to the first element via head, but also has a fast tail operation. One representative of this `LinearSeq` is the `List`.
As the current default implementation of a `Seq` is a `List`, `Seq.newBuilder` will return a `ListBuffer`. However, if you want to use a `Vector` you can either use `Vector.newBuilder[T]` or `IndexedSeq.newBuilder[T]`:
scala> scala.collection.immutable.IndexedSeq.newBuilder[Int]
res0: scala.collection.mutable.Builder[Int,scala.collection.immutable.IndexedSeq[Int]] = scala.collection.immutable.VectorBuilder@1fb10a9f
scala> scala.collection.immutable.Vector.newBuilder[Int]
res1: scala.collection.mutable.Builder[Int,scala.collection.immutable.Vector[Int]] = scala.collection.immutable.VectorBuilder@3efe9969
Problem
Looking at ``` val sb = Seq.newBuilder[Int] println(sb.getClass.getName) sb += 1 sb += 2 val s = sb.result() println(s.getClass.getName) ``` the output is scala.collection.mutable.ListBuffer scala.collection.immutable.$colon$colon using Scala 2.10.1. I would expect `Seq.newBuilder` to return a `VectorBuilder` for example. This is returned by `CanBuildFrom`, if the result is explicitly typed to a `Seq`: ``` def build[T, C <: Iterable[T]](x: T, y: T) (implicit cbf: CanBuildFrom[Nothing, T, C]): C = { val b = cbf() println(b.getClass.getName) b += x b += y b.result() } val s: Seq[Int] = build(1, 2) println(s.getClass.getName) // scala.collection.immutable.Vector ``` in this case the builder is a `VectorBuilder`, and the result's class is a `Vector`. So I explicitly wanted to build a `Seq`, but the result is a `List` which needs more RAM, according to Scala collection memory footprint characteristics. So why does `Seq.newBuilder` return a `ListBuffer` which gives a `List` in the end?