Make traverse lazy

[julien-truffaut]

I believe we can make traverse lazy by using thunk or Eval in Applicative.map2 (please correct me if I am wrong)

def validate(i: Int): Option[Int] = if(i < 5) Some(i) else None

List(1,2,6,8).traverse(validate) // should not evaluate validate(8)

[ceedubs]

Yes, I believe this is the case, and I would vote 👍 for this happening. The question is where exactly it would need to go, since map2 is defined in terms of ap and map. I think in the current state of things, it would need to go in ap. @non how does this relate to the monoidal work you've been doing?

[ceedubs]

I think that now that Cartesian is in place, we should revisit this.

[ceedubs]

I'm working on this.

[ceedubs]

I have a good start on this in a local branch, but this week got busy. I'm hoping to submit a PR next week.