Why aren't type parameters allowed in Scala auxiliary constructors?

Question

Say I'm defining a simple 2D point class in Scala, and I want to be able to construct it with various types:

class Point(x:Float, y:Float) {
    this(x:Double, y:Double) = this(x.toFloat, y.toFloat)
    this(x:Int, y:Int) = this(x.toFloat, y.toFloat)
    // etc...
}

I want to boil this down using a template, such as:

class Point(x:Float, y:Float) {
    this[T](x:T, y:T) = this(x.toFloat, y.toFloat)
}

I know this won't work anyway, since T could be a type for which toFloat isn't defined, but the compiler error I get is:

no type parameters allowed here

Is this just unsupported in Scala? If so, why, and is there any simple way to get around this?

Answer 1

Scala's class constructors (unlike Java's) can't take type parameters, only the class itself can. As to why Scala made this design choice, I assume the main reason is simplicity.

If you want a secondary "builder" method that is generic, the natural thing to do is define it on the companion object. For example,

object Point {
  def build[T : Numeric](x: T, y: T) = {
    val n = implicitly[Numeric[T]]
    new Point(n.toFloat(x), n.toFloat(y))
  }
}

class Point(val x:Float, val y:Float)

val p = Point.build(1, 2) // Companion object's builder
p.x + p.y

Here I've used the Numeric typeclass to get a generic toFloat method.