[swift-evolution] [Draft] Rename Sequence.elementsEqual

[Xiaodi Wu]

On Mon, Oct 16, 2017 at 14:55 Kevin Nattinger via swift-evolution <
swift-evolution at swift.org> wrote:

> On Oct 16, 2017, at 11:23 AM, David Sweeris via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>
> On Oct 16, 2017, at 10:42, BJ Homer via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> On Oct 16, 2017, at 8:20 AM, Thorsten Seitz via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>
> Am 16.10.2017 um 07:19 schrieb Xiaodi Wu <xiaodi.wu at gmail.com>:
>
> What useful generic algorithms would this protocol support that are not
> already possible?
>
>
> It would allow expressing generic algorithms depending on an order.
>
> -Thorsten
>
>
> We can already express generic algorithms that depend on an order—any
> generic algorithm that works on a Sequence works on something that is
> ordered. A Swift Set has an undefined order right now, but a generic
> algorithm working on any arbitrary Sequence likely doesn’t care about
> *what* the order, just that an order exists. And a Swift Set does indeed
> have an order. If you have a generic algorithm that only works on inputs
> sorted in a particular manner, then you’ve likely either documented that or
> added a “sortedBy” parameter. Otherwise, you probably just want to be able
> to iterate through everything.
>
> Let’s assume, though, that you wanted to write an algorithm that works
> only on MeaningfullyOrdered inputs.
>
> func extractInfo<T: MeaningfullyOrdered>(_ input: T) { }
> extractInfo(someArray)
>
> What stops the caller from simply wrapping the Set in an Array?
>
> extractInfo(Array(someSet))
>
> The Array constructed here is going to reflect the arbitrary ordering
> provided by Set, but as far as the type system is concerned, the input is
> an Array, which is certainly meaningfully-ordered. Have we gained anything
> by requiring the caller to wrap the input in an array? We’ve made the call
> site a bit more awkward, and we’ve lost a bit of performance. We certainly
> need to be able to convert Sets in to Arrays; to eliminate that would be
> massively source-breaking, and it’s not clear that allowing that conversion
> is actively harmful, so it’s unlikely to change in Swift 5.
>
>
> Should/could we just rename `Set` to `UniquedArray` or something like
> that? This is starting to feel a bit like the access control debate.
>
>
> So essentially convert Set to OrderedSet and not offer a theoretical
> unordered Set? I think that would be acceptable (if we apply it to
> dictionaries as well), BUT that doesn't address the more general case of
> other, potentially custom unordered Sequences.
>

Think of it this way: all Swift stdlib types that model unordered sequences
are in fact ordered—no attempt is made to support the modeling of unordered
sequences as unordered Swift types, custom or built-in.


>
> - Dave Sweeris
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