[swift-evolution] [Proposal] More lenient subscript methods over Collections (was: [Proposal] Safer half-open range operator)

Thorsten Seitz tseitz42 at icloud.com
Fri Apr 29 09:46:08 CDT 2016

Some alternatives to 'safe:'



> Am 29.04.2016 um 00:20 schrieb Luis Henrique B. Sousa via swift-evolution <swift-evolution at swift.org>:
> Thanks Vladimir, your considerations and suggestions are totally valid, I'm going to change the document accordingly. 
> Also as a non-native English speaker I think that other words could fit better, such as 'tolerant' or 'permissive' -- but I dunno if they would look great as a label. We will come up with the right keyword for it.
> In relation to bad code, it could be a valid argument if my initial proposal was under discussion instead, where the default 'fail fast' behaviour would be "camouflaged" and bugs would be more difficult to catch. In this new proposal we have such features explicitly defined, where the user will be familiar with what it does and what results to expect for. I don't see a way that it could drive to bad written code.
> - Luis
>> On Thu, Apr 28, 2016 at 2:37 PM, Vladimir.S <svabox at gmail.com> wrote:
>> I support this proposal. Probably we all should select the best labels (truncate/lenient or other). As not native English speaker, I don't feel like 'lenient' is well-known word or often-used word in software development. But all this just a details we need to discuss.
>> What I think could be improved - is a motivation section. IMO the main purpose of proposed features is not to "eliminate the need for validations, reduce the number of fatal errors in runtime" but to allow us to have more clean code when *such validations just don't required*, when we just *don't care* about details.
>> I.e. in situations, when we'll use [max(-1, a.startIndex) ..< min(5, a.endIndex)] and bounds checking manually to have the same result as in proposed subscripts.
>> I.e. it is just a very handy addition to standard methods for collections, just like we can get first element by index but we have handy property '.first' for this purpose. Btw, it does not raise error, but returns T?. I think you can add notes regarding analogues with .first / .last properties(and probably with other) in proposal text.
>> Someone can argue, that by using these subscripts, coders can write 'bad' code - but I can't accept such an argument - 'bad' coders already can write 'bad' code with other features of Swift and at the end they can implement these subscripts in their project and write 'bad' code. Should we stop to introduce handy and explicit feature for 'good' coders because of this?
>>> On 28.04.2016 15:11, Luis Henrique B. Sousa via swift-evolution wrote:
>>> As we have discussed throughout this thread, the initial proposal was
>>> modified to include alternative subscript methods instead of modifying the
>>> default operator/subscript behaviour.
>>> The first draft is
>>> here: https://github.com/luish/swift-evolution/blob/more-lenient-subscripts/proposals/nnnn-more-lenient-collections-subscripts.md
>>> I've also put this as a gist so that you can leave comments with respect to
>>> the proposal document itself. Any suggestion or help is very welcome.
>>> https://gist.github.com/luish/832c34ee913159f130d97a914810dbd8
>>> Regards,
>>> - Luis
>>> On Mon, Apr 11, 2016 at 1:23 PM, Luis Henrique B. Sousa <lshsousa at gmail.com
>>> <mailto:lshsousa at gmail.com>> wrote:
>>>     This proposal seeks to provide a safer ..< (aka half-open range
>>>     operator) in order to avoid **Array index out of range** errors in
>>>     execution time.
>>>     Here is my first draft for this proposal:
>>>     https://github.com/luish/swift-evolution/blob/half-open-range-operator/proposals/nnnn-safer-half-open-range-operator.md
>>>     In short, doing that in Swift causes a runtime error:
>>>     leta =[1,2,3]
>>>     letb =a[0..<5]
>>>     print(b)
>>>     > Error running code:
>>>     > fatal error: Array index out of range
>>>     The proposed solution is to slice the array returning all elements that
>>>     are below the half-open operator, even though the number of elements is
>>>     lesser than the ending of the half-open operator. So the example above
>>>     would return [1,2,3].
>>>     We can see this very behaviour in other languages, such as Python and
>>>     Ruby as shown in the proposal draft.
>>>     This would eliminate the need for verifications on the array size
>>>     before slicing it -- and consequently runtime errors in cases when the
>>>     programmer didn't.
>>>     Viewing that it is my very first proposal, any feedback will be helpful.
>>>     Thanks!
>>>     Luis Henrique Borges
>>>     @luishborges
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>>> swift-evolution at swift.org
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