[swift-evolution] [Pitch] Normalize Slice Types for Unsafe Buffers

Dave Abrahams dabrahams at apple.com
Fri Dec 9 12:27:37 CST 2016

on Thu Dec 08 2016, Xiaodi Wu <xiaodi.wu-AT-gmail.com> wrote:

> On Thu, Dec 8, 2016 at 6:53 PM, Ben Cohen via swift-evolution <
> swift-evolution at swift.org> wrote:
>> On Dec 8, 2016, at 4:35 PM, Jordan Rose via swift-evolution <
>> swift-evolution at swift.org> wrote:
>> Um, Sequence doesn’t have a subscript (or indexes). Sequences 
>> are
>> single-pass. So if this is important, it needs to stay a 
>> Collection.
>> Just because something fulfills one of the requirements of a 
>> Collection
>> does not mean it should be one. It needs to tick all the boxes 
>> before its
>> allowed to be elevated.
>> But it’s still allowed to have subscripts (UnsafePointer has 
>> subscripting
>> but isn’t a collection) or be multi-pass (strides are multiples 
>> but are
>> only sequences). That’s OK
>> In this case, yes it’s multi-pass, yes it has a subscript, but 
>> no it isn’t
>> a collection because it doesn’t meet the requirements for 
>> slicing i.e. that
>> indices of the slice be indices of the parent.
>> (relatedly… it appears this requirement is documented on the 
>> concrete
>> Slice type rather than on Collection… which is a documentation 
>> bug we
>> should fix).
> If this is indeed a requirement for Collection, then my vote 
> would be for
> Nate's option #1 and Andy's option #2, to give 
> UnsafeRawBufferPointer a
> Slice type that fulfills the requirement. It's the smallest 
> change,
> preserves the use of integer indices, and preserves what Andy 
> stated as the
> desired use case of making it easy for users to switch out code 
> written for
> [UInt8].
> I'm not sure I fully understand yet why Dave finds the idea of 
> Collection
> conformance fishy, 

Because the memory can easily be already bound to another type 
UInt8, and there's no obvious reason why UInt8 should be 
privileged as a
type you can get out of a raw buffer without binding the memory.

> but I'm comfortable with a type that's clearly labeled as unsafe 
> not
> being fully footgun-proof.
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