[swift-evolution] Arrays Returning Optionals instead of Index Out of Bounds

Rob Norback rnorback at gmail.com
Tue Jun 7 08:42:38 CDT 2016

Vladimir, thank you for pointing me to the discussion again.  I read through the entire thing. And I didn't see a consensus around [Int?].

The main argument against was that it would allow the developer to be clumsy and remain unaware of a problem in the code because the choice is not explicit enough.

The main argument for is that dictionaries already do this, so why don't we have to explicitly check if a dictionary key is within bounds?

IMO casting an array to [Int?] is an explicit choice.  One I would expect to always return an optional from a subscript.  A [Int] array should still fail fast.  

I'd like to clearly understand the downside of swift behaving this way, and why a proposal like this would not be supported.  Not to be confrontational, but simply to expand my own perspective.  Perhaps some more explicit examples of developer laziness that could cause major problems.  I find I always have to be much more careful when coding with optionals.

> On Jun 7, 2016, at 5:48 AM, Vladimir.S <svabox at gmail.com> wrote:
>> On 06.06.2016 21:02, Rob Norback wrote:
>> First of all, thank you all for bringing me up to date so quickly.  I
>> looked over the proposal and it looks awesome.
>> But as Chris mentioned, this doesn't solve the expected behavior and
>> ambiguity of ```Array<Int?>```
>> In this case I would expect the default behavior (myArray[4]) without using
>> myArray[checking: 4] should return a nil in this case.
> As Luis already noted, there was a discussion started with exactly the same suggestion : return optional for wrong index/range. During the discussion consensus was(I believe so) found that such proposal could not be supported. But, as an alternative, special explicit syntax for optional/checking results for index/range parameter was suggested. I'd recommend to read messages of these discussions.
> As for [Int?], I believe we need no any additional solution for this special case: you have to check if index is in array's bounds and if it is, get value using standard subscript method.
>> And Chris, I think it would make the most sense to have myArray[0] = nil to
>> be stored if the index is in Range, and for myArray[100] = nil to give a
>> warning of no assignment being made because index is out of range, kind of
>> like an unused variable.  Right now myArray[100] = nil gives you
>> EXC_BAD_INSTRUCTION.  Then if you assigned myArray[100] = 200, that would
>> have to simply change the Range.
>> Chris, I honestly have to think more about that ambiguity with dictionaries.
>> I can see that this would definitely need some further fleshing out, but it
>> seems to be sufficiently different from Luis' proposal to perhaps merit
>> it's own proposal.
>> It's up to you Luis, but I think this might be a bit more controversial
>> then the changes you've proposed, making it possible for your proposal
>> getting rejected.  (I want your proposal accepted, since I would use those
>> features today).  Partial acceptance of a proposal doesn't seem to be a
>> thing. (Once again please correct me if I'm wrong).
>> On Mon, Jun 6, 2016 at 11:46 AM Luis Henrique B. Sousa <lshsousa at gmail.com
>> <mailto:lshsousa at gmail.com>> wrote:
>>    Thanks Vladimir,
>>    The correct link is this one (with the additional min/max operations in
>>    the implementation):
>>    https://github.com/luish/swift-evolution/blob/proposal-lenient-collection-subscripts/proposals/nnnn-more-lenient-collections-subscripts.md
>>    Here is the pull request on the swift-evolution
>>    repo: https://github.com/apple/swift-evolution/pull/328
>>    Any help or suggestion to improve the proposal is welcome. :-)
>>    - Luis
>>    On Mon, Jun 6, 2016 at 6:10 PM, Vladimir.S via swift-evolution
>>    <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>        Please find this draft of proposal(hope this is correct link for
>>        latest version):
>>        https://github.com/luish/swift-evolution/blob/more-lenient-subscripts/proposals/nnnn-more-lenient-collections-subscripts.md
>>        The main idea is to introduce 2 new subscript methods:  [clamping:]
>>        and [checking:]
>>        There was discussion in "[Proposal] More lenient subscript methods
>>        over Collections" and (older) "[Proposal] Safer half-open range
>>        operator"
>>        On 06.06.2016 19:50, Rob Norback via swift-evolution wrote:
>>            Hi Everyone!
>>            This is my first time emailing the swift evolution list, so if
>>            this topic
>>            has already been discussed please let me know.  I looked
>>            through all the
>>            accepted and rejected proposals and it doesn't seem to be on there.
>>            The main thought is that dictionaries return optionals, so why
>>            not arrays?
>>            Or other CollectionTypes for that matter.  I would think this
>>            would be the
>>            expected behavior in this situation:
>>            var myArray:[String?] = []
>>            print(myArray[4])
>>            // EXC_BAD_INSTRUCTION, but could just be Optional(nil)
>>            Then you could do things like
>>            if let arrayValue = myArray[4] {
>>               // do something
>>            }
>>            Of course you could simply check with with the count, but
>>            considering
>>            Swift's use of optionals to represent empty variables rather
>>            than erroring
>>            out or returning an empty String, I think this functionality
>>            would be
>>            appropriate to include in the Swift standard library.
>>            And there's about 15,000 people who've looked for this
>>            functionality in the
>>            last year:
>>            http://stackoverflow.com/questions/25329186/safe-bounds-checked-array-lookup-in-swift-through-optional-bindings.
>>            Please let me know what you think.
>>            Best,
>>            Rob Norback
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