map on a TreeMap returns a Map and not a TreeMap in Scala

Question

I am new to Scala, and I am implementing a TreeMap with a multidimensional key like this:

class dimSet (val d:Vector[Int])  extends IndexedSeq[Int] {
  def apply(idx:Int) = d(idx)
  def length: Int = d.length
}
… 
var vals : TreeMap[dimSet, A] = TreeMap[dimSet, A]()(orddimSet)

I have this method

def appOp0(t:TreeMap[dimSet,A], t1:TreeMap[dimSet,A], op:(A,A) => A, unop : (A) => A):TreeMap[dimSet,A] = {
  if (t.isEmpty) t1.map((e:Tuple2[dimSet, A]) => (e._1, unop(e._2)))
  else if (t1.isEmpty) t.map((t:Tuple2[dimSet, A]) => (t._1, unop(t._2)))
  else {
    val h = t.head
    val h1 = t1.head
    if ((h._1) == (h1._1)) appOp0(t.tail, t1.tail, op, unop) + ((h._1, op(h._2, h1._2)))
    else if (orddimSet.compare(h._1,h1._1) == 1) appOp0(t, t1.tail, op, unop) + ((h1._1, unop(h1._2)))
    else                   appOp0(t.tail, t1, op, unop) + ((h._1, unop(h._2)))
  }

}

But the map method on the TreeMaps (second and third lines) returns a Map, not a TreeMap

I tried on repl with a simplier example and I got this:

    scala> val t = TreeMap[dimSet, Double]( (new dimSet(Vector(1,1)), 5.1), (new dimSet(Vector(1,2)), 6.3), (new dimSet(Vector(3,1)), 7.1), (new dimSet(Vector(2,2)), 8.4)) (orddimSet)

scala> val tsq = t.map[(dimSet,Double), TreeMap[dimSet,Double]]((v:Tuple2[dimSet, Double]) => ((v._1, v._2 * v._2)))

<console>:41: error: Cannot construct a collection of type scala.collection.immutable.TreeMap[dimSet,Double] with elements of type (dimSet, Double) based on a collection of type scala.collection.immutable.TreeMap[dimSet,Double].
       val tsq = t.map[(dimSet,Double), TreeMap[dimSet,Double]]((v:Tuple2[dimSet, Double]) => ((v._1, v._2 * v._2)))
                                                               ^

  scala> val tsq = t.map((v:Tuple2[dimSet, Double]) => ((v._1, v._2 * v._2)))
  tsq: scala.collection.immutable.Map[dimSet,Double] = Map((1, 1) -> 26.009999999999998, (1, 2) -> 39.69, (2, 2) -> 70.56, (3, 1) -> 50.41)

I think CanBuildFrom cannot build my TreeMap as it can do with other TreeMaps, but I couldn't find why, ¿What can I do to return a TreeMap?

Thanks

Answer 1

The problem probably is that there is no implicit Ordering[dimSet] available when you call map. That call requires a CanBuildFrom, which in turn requires an implicit Ordering for TreeMap keys: see in docs.

So make orddimSet implicitly available before calling map:

implicit val ev = orddimSet
if (t.isEmpty) t1.map((e:Tuple2[dimSet, A]) => (e._1, unop(e._2)))

Or you can make an Ordering[dimSet] always automatically implicitly available, if you define an implicit Ordering in dimSet's companion object:

object dimSet {
  implicit val orddimSet: Ordering[dimSet] = ??? // you code here
}