sealed abstract class LogRow
case class TypeARow(x: String, y: Int) extends LogRow
case class TypeBRow(y: String, z: Double) extends LogRow

// Here comes the factory
object LogRow {
  def apply[T <: LogRow : ClassManifest](s: String): T = classManifest[T] match {
    case x: TypeARow => new TypeARow("xpto", 1).asInstanceOf[T]
    case x: TypeBRow => new TypeBRow("xpto", 0.5).asInstanceOf[T]
  }
}