[swift-users] [swift-evolution] How does "Sequence.joined" work?

John McCall rjmccall at apple.com
Tue Aug 8 15:25:02 CDT 2017

> On Aug 8, 2017, at 3:24 PM, Daryle Walker via swift-evolution <swift-evolution at swift.org> wrote:
>> On Aug 8, 2017, at 12:35 AM, Félix Cloutier <felixcloutier at icloud.com <mailto:felixcloutier at icloud.com>> wrote:
>> All this means is that `joined()` does not create an array that contains the new result. It's only as magic as the COW semantics on arrays.
> So you’re saying the COW semantics for Array and other standard library types have secret references/pointers that work even for “let”-mode objects, and the Sequence variants the various forms of “joined” need use a Sequence/Collection of those secret references?

Why would .joined() need some sort of special reference instead of just storing the collection values as normal values?


>>> Le 7 août 2017 à 21:12, Daryle Walker via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> a écrit :
>>> I was looking at random items at SwiftDoc.org <http://swiftdoc.org/>, and noticed the “FlattenBidirectionalCollection” structure. It helps implement some versions of “joined” from Sequence (probably when the Sequence is also a BidirectionalCollection). The directions for the type state that “joined” does not create new storage. Then wouldn’t it have to refer to the source objects by reference? How; especially how does it work without requiring a “&” with “inout” or how it works with “let”-mode objects? Or am I misunderstanding how it works behind the covers?
>>> (If there is a secret sauce to have one object refer to another without “&”/“inout”/“UnsafeWhateverPointer”, I like to know. It may help with implementing an idea. The idea involves extending the language, so “compiler magic” that the user can’t access is OK; I’d just claim to use the same sauce in my proposal.)
>> Daryle Walker
> Mac, Internet, and Video Game Junkie
> darylew AT mac DOT com 
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