Adding a pairwise difference to generic collections - implicit resolution doesn't kick in - scala

Question

Ok, so I have this:
implicit final class RichIterableLike[A, Repr <: IterableLike[A, Repr]](val it: Repr)
extends AnyVal {
def pairDiff[To](implicit num: Numeric[A], cbf: CanBuildFrom[Repr, A, To]): To = {
val b = cbf(it)
val iter = it.iterator
if (iter.hasNext) {
var pred = iter.next()
while (iter.hasNext) {
import num.mkNumericOps
val succ = iter.next()
b += succ - pred
pred = succ
}
}
b.result()
}
}
This compiles, but doesn't kick in:
val stabs = IndexedSeq(1.0, 2.0, 3.0)
stabs.pairDiff
Gives: value pairDiff is not a member of IndexedSeq[Double]
Explicit conversion works:
new RichIterableLike[Double, IndexedSeq[Double]](stabs).pairDiff
... how to fix this?
EDIT
If I apply the approach of this answer, it works:
implicit final class RichIterableLike[A, CC[~] <: Iterable[~]](val it: CC[A])
extends AnyVal {
def pairDiff[To](implicit num: Numeric[A], cbf: CanBuildFrom[CC[A], A, To]): To = {
...
}
But the question remains, what is the crucial difference that makes the implicit lookup kick in in the latter case.

Answer 1

In order for the implicit lookup to work it needs a link between A and Repr (IterableLike demands that link). You pass it in as an argument, so that argument should be typed as Repr[A]. That means you need to modify your signature so it will look something like this:
RichIterableLike[A, Repr[X] <: IterableLike[X, Repr[X]]](val it: Repr[A])
With the above signature you say:
I have an object that accepts a type parameter, I will name that object Repr and when you pass it in I would like to capture the type parameter as well. I will name that type parameter A. As an extra condition I want the type of Repr to conform to the signature of IterableLike