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.