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

Daryle Walker darylew at mac.com
Tue Aug 8 14:24:25 CDT 2017

> On Aug 8, 2017, at 12:35 AM, Félix Cloutier <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?

>> 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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