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So close, yet so far, to a worthy pimp for the Traversables...
package scalaz
import collection.generic.CanBuildFrom
class TraversableW[Coll, A](val value: Coll)(implicit ev: Coll <:< Traversable[A]) {
def as[CC[X] <: Traversable[X]](implicit cbf1: CanBuildFrom[Coll, A, CC[A]] = null, cbfDefault: CanBuildFrom[Nothing, A, CC[A]]): CC[A] = {
Option(cbf1) match {
case Some(cbf) =>
// A direct builder exists from the source collection to the target
case _ =>
// No direct builder, we have to add elements one at a time.
val builder = cbfDefault.apply
value.foreach(a => builder += a)
// def to[Result](implicit cbf: CanBuildFrom[Nothing, A, Result]): Result =
trait TraversableWs {
implicit def TraversableWTo[Coll, A](t: Coll)(implicit ev: Coll <:< Traversable[A]): TraversableW[Coll, A] = new TraversableW[Coll, A](t)
object Test extends Application {
val i = new TraversableWs {}
import i._
TraversableWTo(Map(1 -> 2)).as[Vector]
// error: could not find implicit value for parameter ev: <:<[List[Int],Traversable[A]] List(1).as[Vector]
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