Trouble using Implicit Ordered with PriorityQueue (Scala) - scala

Question

I'm trying to create a data structure that has a PriorityQueue in it. I've succeeded in making a non-generic version of it. I can tell it works because it solves the A.I. problem I have.
Here is a snippet of it:
class ProntoPriorityQueue { //TODO make generic
implicit def orderedNode(node: Node): Ordered[Node] = new Ordered[Node] {
def compare(other: Node) = node.compare(other)
}
val hashSet = new HashSet[Node]
val priorityQueue = new PriorityQueue[Node]()
...
I'm trying to make it generic, but if I use this version it stops solving the problem:
class PQ[T <% Ordered[T]] {
//[T]()(implicit val ord: T => Ordered[T]) {
//[T]()(implicit val ord: Ordering[T] {
val hashSet = new HashSet[T]
val priorityQueue = new PriorityQueue[T]
...
I've also tried what's commented out instead of using [T <% Ordered[T]]
Here is the code that calls PQ:
//the following def is commented out while using ProntoPriorityQueue
implicit def orderedNode(node: Node): Ordered[Node] = new Ordered[Node] {
def compare(other: Node) = node.compare(other)
} //I've also tried making this return an Ordering[Node]
val frontier = new PQ[Node] //new ProntoPriorityQueue
//have also tried (not together):
val frontier = new PQ[Node]()(orderedNode)
I've also tried moving the implicit def into the Node object (and importing it), but essentially the same problem.
What am I doing wrong in the generic version? Where should I put the implicit?
Solution
The problem was not with my implicit definition. The problem was the implicit ordering was being picked up by a Set that was automatically generating in a for(...) yield(...) statement. This caused a problem where the yielded set only contained one state.

Answer 1

What's wrong with simply defining an Ordering on your Node (Ordering[Node]) and using the already-generic Scala PriorityQueue?
As general rule, it's better to work with Ordering[T] than T <: Ordered[T] or T <% Ordered[T]. Conceptually, Ordered[T] is an intrinsic (inherited or implemented) property of the type itself. Notably, a type can have only one intrinsic ordering relationship defined this way. Ordering[T] is an external specification of the ordering relationship. There can any be any number of different Ordering[T].
Also, if you're not already aware, you should know that the difference between T <: U and T <% U is that while the former includes only nominal subtype relations (actual inheritance), the latter also includes the application of implicit conversions that yield a value conforming to the type bound.
So if you want to use Node <% Ordered[Node] and you don't have a compare method defined in the class, an implicit conversion will be applied every time a comparison needs to be made. Additionally, if your type has its own compare, the implicit conversion will never be applied and you'll be stuck with that "built-in" ordering.
Addendum
I'll give a few examples based on a class, call it CIString that simply encapsulates a String and implements ordering as case-invariant.
/* Here's how it would be with direct implementation of `Ordered` */
class CIString1(val s: String)
extends Ordered[CIString1]
{
private val lowerS = s.toLowerCase
def compare(other: CIString1) = lowerS.compareTo(other.lowerS)
}
/* An uninteresting, empty ordered set of CIString1
(fails without the `extends` clause) */
val os1 = TreeSet[CIString1]()
/* Here's how it would look with ordering external to `CIString2`
using an implicit conversion to `Ordered` */
class CIString2(val s: String) {
val lowerS = s.toLowerCase
}
class CIString2O(ciS: CIString2)
extends Ordered[CIString2]
{
def compare(other: CIString2) = ciS.lowerS.compareTo(other.lowerS)
}
implicit def cis2ciso(ciS: CIString2) = new CIString2O(ciS)
/* An uninteresting, empty ordered set of CIString2
(fails without the implicit conversion) */
val os2 = TreeSet[CIString2]()
/* Here's how it would look with ordering external to `CIString3`
using an `Ordering` */
class CIString3(val s: String) {
val lowerS = s.toLowerCase
}
/* The implicit object could be replaced by
a class and an implicit val of that class */
implicit
object CIString3Ordering
extends Ordering[CIString3]
{
def compare(a: CIString3, b: CIString3): Int = a.lowerS.compareTo(b.lowerS)
}
/* An uninteresting, empty ordered set of CIString3
(fails without the implicit object) */
val os3 = TreeSet[CIString3]()

Answer 2

Well, one possible problem is that your Ordered[Node] is not a Node:
implicit def orderedNode(node: Node): Ordered[Node] = new Ordered[Node] {
def compare(other: Node) = node.compare(other)
}
I'd try with an Ordering[Node] instead, which you say you tried but there isn't much more information about. PQ would be declared as PQ[T : Ordering].