[swift-evolution] Strings in Swift 4

[Dave Abrahams]

on Sun Jan 22 2017, James Froggatt <swift-evolution at swift.org> wrote:

> Could we add subscript labels to the list of options? While keeping
> the range syntax is appealing, I'm concerned it may cause confusion if
> the operators are used out of context.
>
> The wording is up for debate, but something like this should be a fair alternative:
> items[from: i]
> items[upTo: i]

If we were to do that, I'd want to drop range notation altogether and
just require this for slicing:

  items[from: i, upTo: j]

The point here is to create a single unified idiom that can be used
everywhere.

> Sorry if this has been covered elsewhere (can't find the answer in
> this thread), but my first questions on discovering these operators
> (my source of confusion) would be what happens if I try the following:
> let partialRange = 0..< //is this an infinite range?

It is not a Range, but a RangeExpression with no upper bound.

> let x = items[partialRange] //shouldn't this cause an out of bounds error?

No, the upper bound is filled in by the collection

> ------------ Begin Message ------------ 
> Group: gmane.comp.lang.swift.evolution 
> MsgID: <0A458383-2415-4ED4-AD28-88393A671A34 at nondot.org> 
>
>> On Jan 20, 2017, at 9:39 PM, Brent Royal-Gordon via swift-evolution <swift-evolution at swift.org>
> wrote:
>> 
>>> On Jan 20, 2017, at 2:45 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>> on Fri Jan 20 2017, Joe Groff <swift-evolution at swift.org> wrote:
>>> 
>>>> Jordan points out that the generalized slicing syntax stomps on '...x'
>>>> and 'x...', which would be somewhat obvious candidates for variadic
>>>> splatting if that ever becomes a thing. Now, variadics are a much more
>>>> esoteric feature and slicing is much more important to day-to-day
>>>> programming, so this isn't the end of the world IMO, but it is
>>>> something we'd be giving up.
>>> 
>>> Good point, Jordan.
>> 
>> In my experiments with introducing one-sided operators in Swift 3, I
>> was not able to find a case where you actually wanted to write
>> `c[i...]`. Everything I tried needed to use `c[i..<]` instead. My
>> conclusion was that there was no possible use for postfix `...`;
>> after all, `c[i...]` means `c[i...c.endIndex]`, which means
>> `c[i..<c.index(after: c.endIndex)]`, which violates a precondition
>> on `index(after:)`.
>
> Right, the only sensible semantics for a one sided range with an open
> end point is that it goes to the end of the collection.  I see a few
> different potential colors to paint this bikeshed with, all of which
> would have the semantics “c[i..<c.endIndex]”:
>
> 1) Provide "c[i...]":
> 2) Provide "c[i..<]":
> 3) Provide both "c[i..<]” and "c[i…]":
>
> Since all of these operations would have the same behavior, it comes down to subjective questions:
>
> a) Do we want redundancy?  IMO, no, which is why #3 is not very desirable.
> b) Which is easier to explain to people?  As you say, "i..< is shorthand for i..<endindex” is nice
> and simple, which leans towards #2.
> c) Which is subjectively nicer looking?  IMO, #1 is much nicer
> typographically.  The ..< formulation looks like symbol soup,
> particularly because most folks would not put a space before ].
>
> There is no obvious winner, but to me, I tend to prefer #1.  What do other folks think?
>
>> If that's the case, you can reserve postfix `...` for future variadics features, while using
> prefix `...` for these one-sided ranges.
>
> I’m personally not very worried about this, the feature doesn’t
> exist yet and there are lots of ways to spell it.  This is something
> that could and probably should deserve a more explicit/heavy syntax
> for clarity.
>
> -Chris
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> From James
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-- 
-Dave