How do you use Swift 2.0 popFirst() on an Array?

Question

Swift 2.0 popLast() works on an Array, like this:

var arr = [1,2,3]
let i = arr.popLast()

Now arr is [1,2] and i is 3 (wrapped in an Optional).

But although there is also a popFirst(), it doesn't compile. I'm pretty sure it used to, but it doesn't now:

var arr = [1,2,3]
let i = arr.popFirst() // compile error

What's going on here? When is this method actually usable?

Answer 1

Oddly, popFirst works only on Array derivatives such as slices. So, for example, this compiles:

let arr = [1,2,3]
var arrslice = arr[arr.indices]
let i = arrslice.popFirst()

Now arrslice is [2,3] (as a slice), i is 1 (wrapped in an Optional), and arr is untouched.

So that's how to use it.

But I do not understand why this odd restriction is imposed on the use of popFirst(). I see how it is imposed (in the Swift header) but I don't see why.

EDIT Thinking about it some more, I'm guessing it has to do with efficiency. When you call popFirst() on a slice, you get what amounts to a sparse array: there is now no index 0. So we didn't have to slide all the elements down one slot; instead, we simply incremented the startIndex.