[swift-evolution] [Pitch] Introducing role keywords to reduce hard-to-find bugs

[Xiaodi Wu]

On Wed, Jun 14, 2017 at 3:37 PM, Vladimir.S via swift-evolution <
swift-evolution at swift.org> wrote:

> On 14.06.2017 21:23, Haravikk via swift-evolution wrote:
>
>>
>> On 14 Jun 2017, at 19:08, Xiaodi Wu via swift-evolution <
>>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>
>>> On Wed, Jun 14, 2017 at 1:01 PM, David Hart via swift-evolution <
>>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>
>>>     Sorry, initially sent off-list:
>>>
>>>     I think this proposal is a great idea. But I would vote for the
>>> alternative of
>>>     only having default and implicitly deducing extend when default is
>>> not specified:
>>>
>>>
>>> This wouldn't work with the fundamental design decision that these are
>>> optional keywords, which IMO is absolutely key.
>>>
>>
>> Hmm, I'm inclined to agree with David that only the default keyword
>> really seems like it's necessary, and that extend can be implied.
>>
>>
> I'm not so sure. If they are optional, then it depends on developer if
> he/she wants to explicitly mark some func to avoid possible errors. Please
> look here :
>
> 1. First we have this
>
> protocol A {
>   func foo() {}
> }
>
> and we  write extension (possible in another file)
>
> extension A {
>   extent func bar() {} // I'm sure currently I want *additional* 'bar'
> method
> }
>
> 2. Then 'A' protocol has been changed for some reason :
>
> protocol A {
>   func foo() {}
>   func bar() {}
> }
>
> Now, if we have 'extent' - we(compiler) can detect the problem here('bar'
> was not the default implementation for A's requirement). Without 'extent' -
> 'bar' will be default implementation without our intent for this.
>
> So, in case suggested keywords are both optional - IMO we need both.
> In case 'default' is required - then yes, we need only it.
>

Yes, I think we are all in vigorous agreement about this. The question is
whether, at this point in Swift's evolution, a wide-ranging migration due
to a new required keyword will be well tolerated. If not, then the solution
is constrained.


> My preference would be to just add the default keyword, and have breaches
>> treated as warnings using the current behaviour, which we can eliminate and
>> elevate to an error in future once people have had a chance to change their
>> code.
>>
>>
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