It’s a problem with Apple documentation. swiftdoc.org does list the associated
types, but is no longer maintained :(.
-- Howard.
> On 16 Dec 2017, at 1:17 pm, Nevin Brackett-Rozinsky via swift-users
> <swift-users@swift.org> wrote:
>
> In Xcode if I write,
>
> extension Collection {
> var secondIndex: Index { return index(after: startIndex) }
> }
>
> and then option-click on “Index”, it shows “associatedtype Index :
> Comparable” followed by a description. Is this documented somewhere?
>
> When I look at the documentation for Collection on developer.apple.com there
> is no mention of an Index associated type. I also don’t see it in The Swift
> Programming Language.
>
> If someone didn’t already know Collection.Index exists, how would they be
> expected to learn of it? And if they didn’t know it must be Comparable how
> would they learn that?
>
> Also, are there any semantic requirements on Collection.Index, for example is
> it required that “idx < index(after: idx)” must evaluate to true for any
> valid index “idx”?
>
> • • •
>
> On a somewhat related note, the “indices” property of Collection has a
> discussion section visible by option-clicking in Xcode or by looking at the
> documentation for it on developer.apple.com.
>
> And that discussion recommends against iterating over “indices” when mutating
> a collection, because “indices” could hold a strong reference to the
> collection and thus create an unexpected copy. Instead one should manually
> advance from startIndex to endIndex in a while loop.
>
> Now, this is at least documented, but perhaps it should be made more
> prominent? It seems to me that if “for i in c.indices” represents a
> performance pitfall then it should be regarded as an attractive nuisance.
>
> Nevin
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