[swift-evolution] Shouldn't ".withUnsafeBufferPointer" and ".withUnsafeMutableBufferPointer" be parts of protocols?

[Dave Abrahams]

on Sat Jan 28 2017, Brent Royal-Gordon <brent-AT-architechies.com> wrote:

>> On Jan 28, 2017, at 11:48 AM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> The way to handle Deque is to add this requirement to Collection when
>> we get the language features to express it:
>> 
>
>>  protocol Collection {
>> 
>>    associatedtype Segments : Collection 
>>    where Segments.Element : Collection,
>>      Segments.Element.Element == Element
>>     = EmptyCollection<EmptyCollection<Element>>
>> 
>>    var segments: Segments? {get}
>>    ...
>>  }
>> 
>>  extension Collection 
>>  where Segments == EmptyCollection<EmptyCollection<Element>> {
>>    var segments: Segments? { return nil }
>>  }
>
> Couldn't that be be expressed more accurately with `Never` if it were a subtype-of-all-types?
>
> 	 protocol Collection {
>
> 	   associatedtype Segments : Collection 
> 	   where Segments.Element : Collection,
> 	     Segments.Element.Element == Element
> 	    = Never
>
> 	   var segments: Segments? {get}
> 	   ...
> 	 }
>
> 	 extension Collection 
> 	 where Segments == Never {
> 	   var segments: Segments? { return nil }
> 	 }

Maybe, if we had the language feature, we could do that... but we don't.

> Or you could say that there is always at least *one* segment:
>
> 	 protocol Collection {
>
> 	   associatedtype Segments : Collection 
> 	   where Segments.Element : Collection,
> 	     Segments.Element.Element == Element
> 	    = CollectionOfOne<Self>
>
> 	   var segments: Segments {get}
> 	   ...
> 	 }
>
> 	 extension Collection 
> 	 where Segments == CollectionOfOne<Self> {
> 	   var segments: Segments { return CollectionOfOne(self) }
> 	 }

That's not actually helpful, because segmented algorithms need to be
able to use the presence of segments to stop recursing :-)

-- 
-Dave