[swift-evolution] Standard library 'Data' type pre-proposal

Austin Zheng austinzheng at gmail.com
Wed May 11 13:29:02 CDT 2016

Hi Dmitri,

Thanks for the feedback! I'm glad that we could start a conversation on the
lists, and happy to see people offering their unvarnished opinions.

I think conditional conformances upon Array<UInt8> is definitely an avenue
worth exploring. I'm not sure what the performance implications are - Zach
brought up use cases in which the ability for a data type to be backed by
non-contiguous storage was important. More generally, I wanted to open up
discussion as to what people wanted from a native Data type.

It seems like a DataProtocol-like protocol may be a good idea. Array<UInt8>
could conform through conditional conformances to provide an implementation
for people wanting a simple contiguous buffer that could be punned to an
array or other linear collection, while a more robust dispatch_data_t-like
conforming Swift stdlib type could be provided for more demanding use
cases. This actually seems to be a good fit - if you only care about a data
buffer as an arbitrary collection of bytes, the abstract protocol interface
gives you flexibility, while if you have requirements that require a
specific representation of data in memory you should use a concrete type.


On Wed, May 11, 2016 at 11:01 AM, Dmitri Gribenko <gribozavr at gmail.com>

> On Wed, May 11, 2016 at 2:37 AM, Austin Zheng via swift-evolution
> <swift-evolution at swift.org> wrote:
> > Hello swift-evolution,
> >
> > I've been thinking about a standard library 'Data' type for a while,
> > analogous to NSData in the same way Swift's Arrays and Dictionaries are
> > analogous to NSArrays and NSDictionaries. A first-class container for
> binary
> > data that is available to every Swift user, conforms to Swift semantics,
> and
> > is safer and easier to work with than UnsafeBufferPointer seems like a
> > natural fit for the standard library.
> Hi Austin,
> This is an interesting territory!
> One thing that I would like to suggest for us to consider is
> justifying why Data needs to be a separate type from Array<Int8> and
> Array<UInt8>.  We can add conditional extensions to Array of Int8 and
> UInt8 if we find that existing NSData/dispatch_data_t usecases need a
> few special APIs that won't make sense on arrays in general.
> For example, something that I would imagine people want to do with
> "data buffer" types is being able to make an unaligned or type punned
> load or store.  For example, in Java, this is one of the primary
> usecases for a type similar in spirit, java.nio.ByteBuffer
> (https://docs.oracle.com/javase/7/docs/api/java/nio/ByteBuffer.html).
> Another usecase that is a crossover between Array and Data, allow
> Array to (unsafely) adopt ownership of an existing initialized unsafe
> buffer pointer.  We had quite a few requests for this.  Do you think
> this is an interesting usecase?  Does it overlap with this discussion?
> Dmitri
> --
> main(i,j){for(i=2;;i++){for(j=2;j<i;j++){if(!(i%j)){j=0;break;}}if
> (j){printf("%d\n",i);}}} /*Dmitri Gribenko <gribozavr at gmail.com>*/
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