On Fri, Dec 9, 2016, at 11:50 AM, Dave Abrahams via swift-evolution wrote: > > on Fri Dec 09 2016, Andrew Trick <swift-evolution@swift.org> wrote: > > >> On Dec 9, 2016, at 10:27 AM, Dave Abrahams via swift-evolution > > <swift-evolution@swift.org> wrote: > >> > >> > >> on Thu Dec 08 2016, Xiaodi Wu <xiaodi.wu-AT-gmail.com > >> <http://xiaodi.wu-at-gmail.com/>> wrote: > >> > > > >>> On Thu, Dec 8, 2016 at 6:53 PM, Ben Cohen via swift-evolution < > >>> swift-evolution@swift.org> wrote: > >>> > >>>> > >>>> On Dec 8, 2016, at 4:35 PM, Jordan Rose via swift-evolution < > >>>> swift-evolution@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 than > >> 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. > > > > I strongly disagree with that statement. The overwhelmingly common use > > case for raw buffers is to view them as a sequence of UInt8 *without* > > binding the type. Generally, at the point that you're dealing with a > > raw buffer it's impossible to (re)bind memory because you don't know > > what type it holds. > > Oh, you can't just rebind to UInt8 because that's not defined as > universally compatible with all data. OK, sorry.
If you're dealing with raw bytes to begin with then I'd hope you're working with types that can be safely expressed as a collection of bytes. And since this is an unsafe type, I don't think there's any problem with having the possibility of trying to express e.g. [NSObject] as a raw byte buffer. > > The reason it's so important to have an UnsafeRawBufferPointer data > > type is precisely so that users don't need mess about with binding > > memory. It's easy to get that wrong even when it's possible. > > > > The only reason that UInt8 is special is that when users create > > temporary typed buffers for bytes (e.g. they sometimes want a growable > > array or just don't want to bother with manual allocation) they always > > use UInt8 as the element type. > > > > That said, we could easily divide these concerns into two types as > > you suggested. A raw buffer, which doesn't have any special UInt8 > > features, and a RawBytes collection that handles both buffer slicing > > and UInt8 interoperability. > > But, now that I think of it, that wouldn't really solve any problems, > would it? Agreed. If we have a separate RawBytes type, then I'm not really sure what the point of UnsafeRawBufferPointer is anymore. The whole point of that type is it's a collection of bytes rather than just being a tuple `(baseAddress: UnsafeRawPointer, count: Int)`. -Kevin Ballard _______________________________________________ swift-evolution mailing list swift-evolution@swift.org https://lists.swift.org/mailman/listinfo/swift-evolution
