Question

I’m playing with the QuickSort example at the start of Scala By Example and trying to adapt it for a generic type A, rather than just Ints.

What I’ve got working so far is

def sort[A <: Ordered[A]](xs: Array[A]) 

Which allows sort to run on all types that are reflexively ordered, like RichBoolean.

But what I’d also like to allow types A where they extend Ordered[B] where B is a superclass of A (so, for instance, anything that extends Ordered[Any]).

How can I say this?

---

What I actually got to work, thanks to agilesteel’s answer:

case class X( i : Int ) extends Ordered[X] {
  def compare( x : X ) = x.i - i
}

class Y( i : Int, j : Int ) extends X(i)

case class Z( i : Int ) extends Ordered[Any] {
  def compare( a : Any ) : Int = {
    if (! a.isInstanceOf[Z] ) 
      sys.error("whoops") 

    val z = a.asInstanceOf[Z]
    z.i - i
  }
}

object QuickSort {
  def main( args : Array[String] ) {
    val xs = Array( 3, 1, 2, 4 ) map X
    sort( xs );
    val ys = Array( 3, 1, 2, 4 ) map { i => new Y(i, -i) }
    sort[X,Y]( ys );
    val zs = Array( 3, 1, 2, 4 ) map Z
    sort[Any,Z]( zs );
  }
  def sort[B >: A, A <: Ordered[B]](xs: Array[A]) {
    def swap(i: Int, j: Int) {
      val t = xs(i); xs(i) = xs(j); xs(j) = t;
    }
    def sort1(l: Int, r: Int) {
      val pivot = xs((l + r) / 2)
        var i = 1; var j = r
      while (i <= j) {
        while (xs(i) < pivot) i += 1
        while (xs(j) > pivot) j -= 1
        if (i <= j) {
          swap(i, j)
          i += 1
          j += 1
        }
      }
      if (l < j) sort1(l, j)
      if (j < r) sort1(i, r)
    }
    sort1(0, xs.length - 1)
  }
}

I was misled by trying to use RichLong and RichBoolean as test types, since they aren’t actuallly reflexively Ordered (they extend Ordered[Long] and Ordered[Boolean] instead).

Answer 1

Something like this?

def sort[B >: A, A <: Ordered[B]](xs: Array[B])