[swift-evolution] Strings in Swift 4

[Dave Abrahams]

on Tue Jan 31 2017, Xiaodi Wu <xiaodi.wu-AT-gmail.com> wrote:

> But that's not getting to the biggest hitch with your proposal. If
> subscript were lenient, then `arr[lenient: 42...]` would also have to give
> you a result even if `arr.count == 21`.
>
> This is not at all what Dave Abrahams was proposing, though (unless I
> totally misunderstand). He truly doesn't want an infinite range. He wants
> to use a terser notation for saying: I want x to be the lower bound of a
> range for which I don't yet know (or haven't bothered to find out) the
> finite upper bound. It would be plainly clear, if spelled as `arr[from:
> 42]`, that if `arr.count < 43` then this expression will trap, but if
> `arr.count >= 43` then this expression will give you the rest of the
> elements.

I think you do misunderstand.  Notionally, 0... is an infinite range.
The basic programming model for numbers in swift is (to a first
approximation), program as if there's no overflow, and we'll catch you
by trapping if your assumption is wrong.  It doesn't make sense for the
semantics of 0... to depend on the deduced type of 0 or the
representable range of Int

for example, 

    for x in zip(n..., someArray) {

    }

How many iterations should this give you?  If it doesn't process all of
someArray, I want a trap.

-- 
-Dave