[swift-evolution] [Idea] How to eliminate 'optional' protocol requirements
Jordan Rose
jordan_rose at apple.com
Mon Apr 11 18:48:56 CDT 2016
> On Apr 11, 2016, at 11:41 , Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
>
>
> on Mon Apr 11 2016, Charles Srstka <cocoadev-AT-charlessoft.com <http://cocoadev-at-charlessoft.com/>> wrote:
>
>> On Apr 11, 2016, at 12:03 PM, Dave Abrahams via swift-evolution
>> <swift-evolution at swift.org> wrote:
>>
>> on Sun Apr 10 2016, Dietmar Planitzer <dplanitzer-AT-q.com> wrote:
>>
>> On Apr 10, 2016, at 11:46, Dave Abrahams via swift-evolution
>>
>> <swift-evolution at swift.org> wrote:
>>
>> on Sun Apr 10 2016, Dietmar Planitzer <swift-evolution at swift.org>
>>
>> wrote:
>>
>> I’m not sure whether you’ve read the conclusion of my
>> mail since
>> you’ve only commented on the introductory part. In the
>> conclusion I
>> wrote that a possible approach for the replacement of ObjC-style
>> optional protocol methods would be:
>>
>> 1) the default implementation of a protocol method must be
>> defined
>>
>> in
>>
>> the protocol (so just like in native Swift protocols today).
>>
>> ? They can and must be defined in protocol extensions today.
>>
>> I know.
>>
>> 2) we add a way for a protocol provider to check whether the
>>
>> protocol
>>
>> adopter has provided an “override” of the default method.
>>
>> I object to this part.
>>
>> You object why? I do understand why you object to the ObjC model since
>> there is not necessarily an implementation of the protocol method and
>> thus the protocol provider has to guard every call with an existence
>> check. But in this model here we would be guaranteed that there would
>> be an implementation of the protocol method and thus guarding the call
>> wouldn’t be necessary.
>>
>> Because it's a needless complication that will encourage protocol and
>> algorithm designers to create inefficient programs because they know the
>> user can fall back on this hack. Nobody thinks that classes need the
>> ability to check whether a given method is overridden. Why should this
>> be needed for protocols?
>>
>> Actually, Apple’s frameworks have often contained code to check whether given
>> methods are overridden, and this has allowed them to deprecate override points
>> and replace them with better API without breaking source or binary
>> compatibility. The most obvious example that comes to mind is NSDocument; when
>> they introduced the newer override points such as -readFromURL:ofType:error:
>> that used NSURLs instead of paths and allowed returning an NSError, they added
>> code in the default implementation to check whether the subclass overrode the
>> older -readFromFile:ofType: method and if it did, called that method. Otherwise,
>> it would call the modern methods. This way, older applications that were still
>> overriding -readFromFile:ofType: would continue to work correctly.
>
> I don't believe we're aiming to support this kind of evolution in Swift,
> but others understand our resilience plans better than I, so we should
> probably let them comment.
It's a useful enough pattern that I wouldn't want to rule it out entirely, but it's rare enough that it could be some strange runtime call. (Also, in many cases there's a simpler implementation, like "call the old method unilaterally".)
Jordan
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