Searching by [tag:closest] and [tag:position] I wasn't able to find an answer—but found [`Nearest`](http://reference.wolfram.com/mathematica/tutorial/UsingNearest.html) by guessing. `Nearest`, however, returns the closest number itself, not the position.

After some more looking around, I learned about `MapIndexed` and `Rules`, and came up with this:

    NearestPosition[haystack_, needle_] := Nearest[haystack, needle] /. MapIndexed[Rule, haystack];

Is there a more efficient way, in general?

(And what if it's guaranteed the list comes sorted—can that be leveraged?)

The reason I'm concerned about efficiency is that I intend to use `NearestPosition` in a `DynamicModule` where it may be triggered by every `mouseover` event. (I'm still trying to solve [this problem](http://mathematica.stackexchange.com/questions/34434/hovering-mouse-cursor-over-a-plot-to-display-a-corresponding-point-in-another).)