Add Countable[T]

[johnynek]

Not totally sure about this:

Think it is:

trait Countable[T] {
  def next(t: T): Option[T]
  def prev(t: T): Option[T]
}

an infinite T might never return None, but Int, Long, Double etc instances would.

[johnynek]

Haskell has one of these go from Option[T] to Option[T] which I assume is so you can start from nothing. I think that is conflating lowerbounded with countable and I don't like it, but maybe I'm wrong here.

[arkajit]

What's the use case for this? To implement bi-directional iteration over T? For basic numeric types, won't an appropriate Range get you the same effect?

[sritchie]

"for basic numeric types", exactly, but what about more complex types? Think Week.

[johnynek]

Or BatchID, or (Int, Int), or...

We want to express algorithms once that only demand the weakest properties of their inputs.