Overloading constructor in generic scala class - scala

Question

I'm trying to overload a constructor in a generic scala class but it's not compiling.
Here's my code:
class V[T](m: Map[T,Double]) {
def this(dt: Seq[Double]) = this(dt.zipWithIndex.map(_.swap).toMap)
}
And the error messages I get:
ERROR: called constructor's definition must precede calling constructor's definition : line 6
ERROR: overloaded method constructor V with alternatives:
(dt: Seq[Double])V[T] <and> (m: Map[T,Double])V[T] cannot be applied to
(scala.collection.immutable.Map[Int,Double]) : line 6
As far as I understand constructor overloading in scala, I think I'm following the proper syntax and the restriction that the call to this should precede everything else.
So what am I doing wrong and how can I fix this?

Answer 1

With
def this(dt: Seq[Double]) = this(dt.zipWithIndex.map(_.swap).toMap)
You're creating a new map Map[Int,Double]; Int being the type of the index created by zipWithIndex.
If T were Int, then you can use the constructor (m:Map[T,Double].
However: T is not yet bound to a type since you're defining the class. Nor will the type matching bind T to Int at this point.
Therefore the type matching fails.
Solutions:
How to fix it depends on what you're trying to do.
If it were the case that T <: Int, then bounding the type-param with <: Int could resolve your problem; however it seems a bit unlikely that T is a subclass of Int...
If it is always true that T : Int, then drop the generic T.
If T is to remain generic and unbounded then that leaves you with making a special case for when T : Int; senia's solution looks good for that.

Answer 2

You can fix it with companion object:
scala> :paste
// Entering paste mode (ctrl-D to finish)
class V[T](m: Map[T,Double])
object V{
def apply(dt: Seq[Double]) = new V[Int](dt.zipWithIndex.map(_.swap)(collection.breakOut))
}
// Exiting paste mode, now interpreting.
defined class V
defined module V
scala> V(Seq(1.,2.,3.))
res0: V[Int] = V#1130e2ea