[swift-evolution] [Pitch] Adding safety to arrays

[Lucas Neiva]

AFAIK arrays use the same “subscripts” [1] feature as other collections. So this feature would have to be tagged onto subscripts in general.

I see a few problems with you suggestion:

- How do you differentiate between an out-of-bounds subscript access in an array with optional elements, e.g. `[String?]`, `[Int: String?]`
- Isn’t it conflating two different things, to use the same `?` operator here
- You’d probably have to add a new construct similar to the existing subscripts feature


[1] https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/Subscripts.html


> On 13.04.2017, at 16:34, Jeff Kelley via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Apologies if this has been suggested before, but going off of Andrew’s message, a simple syntax could be to use our existing Optional syntax:
> 
> let array = ["foo", "bar", "baz"]
> 
> let first = array[0] // type is `String`
> let third = array[2?] // type is `String?`, value is .some("baz")
> let fourth = array[3?] // type is `String?`, value is .none
> 
> Jeff Kelley
> 
> SlaunchaMan at gmail.com | @SlaunchaMan | jeffkelley.org
> 
>> On Apr 13, 2017, at 8:19 AM, David Sweeris via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>>> 
>>> On Apr 13, 2017, at 3:56 AM, Andrew Hart via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>> Recently I’ve been considering the lack of safety around array indexes. Swift is designed with safety in mind, so this example would not compile:
>>> 
>>> var myString: String? = “hello”
>>> myString.append(“ world!”)
>>> 
>>> The string is optional, not guaranteed to exist, so the last line requires a “!” to force-unwrap it.
>>> 
>>> 
>>> 
>>>     public func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
>>>         let section = self.sections[section]
>>>         
>>>         return section.items.count
>>>     }
>>> 
>>> In this example, we could provide a section number that goes beyond the bounds of the self.sections array, without any warning.
>>> 
>>> My suggestion is perhaps arrays should by default return an optional when given an index, and of course they’d support forced-unwrapping too. So you could then do this:
>>> 
>>>     let section = self.sections[section]
>>>     if section == nil {
>>>         return 0
>>>     } else {
>>>         return section!.items.count
>>>     }
>>> 
>>> Or you could do this:
>>> 
>>>     let section = self.sections[section]!
>>>     
>>>     return section.items.count
>>> 
>>> Of course this would be less convenient in a lot of cases, but this is the 1 place where apps seem to encounter a crash, crashing for the same reason that’s especially avoided across most of the rest of Swift.
>> 
>> My understanding is that we need the current behavior to meet performance goals. We’ve discussed adding a “safe” subscript before, but the discussion usually fizzles out when no clear winner for the argument label emerges.
>> 
>> - Dave Sweeris
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