Why are my Scala types not matching?

Question

I have the following variable series:

var series: List[FlotSerie] = List(
  new FlotSerie() {
    override val label = Full("Min")
  },
  new FlotSerie() {
    override val label = Full("Max")
  },
  new FlotSerie() {
    override val label = Full("Avg")
  }
)

Unfortunately, I am getting a compiler error with the following method, which takes a new data point and updates series with a new List[FlotSeries] based upon the new data and the old series.

def updateSeries(sample: Sample): List[FlotSerie] = {
  series = series.map(serie =>
    serie match {
      case item if item.label == Full("Min") => {
        new FlotSerie() {
          override val label = item.label
          override val data = (sample.timestamp.toDouble, sample.min) :: serie.data
        }
      }
      case item if item.label == Full("Max") => {
        new FlotSerie() {
          override val label = item.label
          override val data = (sample.timestamp.toDouble, sample.max) :: serie.data
        }
      }
      case item if item.label == Full("Avg") => {
        new FlotSerie() {
          override val label = item.label
          override val data = (sample.timestamp.toDouble, sample.avg) :: serie.data
        }
      }
    }
  )
}

The Scala compiler chokes on the reassignment because it finds a type mismatch:

error: type mismatch;
found   : Unit
required: List[net.liftweb.widgets.flot.FlotSerie]
   series = series.map(serie => serie match {

What am I doing wrong here? It seems like it should be returning a List[FlotSeries] that can be assigned to series. Since the compiler finds Unit I thought of how foreach always returns Unit, I am but the match operator returns the last value of the matched expression, not Unit.

Answer 1

Assignments in Scala return Unit (aka Scala's not quite null null), unlike Ruby which returns the assigned value. Your method is attempting to return Unit instead of List[FlotSerie].

Add:

return series

to your method, or change it to return Unit.

You could also simplify your code using case classes and proper matching if it's appropriate:

  case class FlotSerie(label:Full, data:List[Tuple2[Double, Double]])
  var series: List[FlotSerie] = List( FlotSerie(Full("Min"), Nil), FlotSerie(Full("Max"), Nil), FlotSerie(Full("Avg"), Nil) )

  def updateSeries(sample: Sample): List[FlotSerie] = {
    series = series.map(serie => {
      serie.label match {
        case Full("Min") => FlotSerie(serie.label, (sample.timestamp.toDouble, sample.min) :: serie.data)
        case Full("Max") => FlotSerie(serie.label, (sample.timestamp.toDouble, sample.max) :: serie.data)
        case Full("Avg") => FlotSerie(serie.label, (sample.timestamp.toDouble, sample.avg) :: serie.data)
      }
    })
    return series
  }

I'm pretty new to Scala myself so YMMV.

Answer 2

remove series = you're assigned the map function to series and not returning anything.