on Mon Nov 07 2016, Douglas Gregor <swift-dev-AT-swift.org> wrote:
> Hi all,
>
> While working on the type checker, I came across an interesting case for
> associated type inference
> with the ‘Indices’ type of RandomAccessCollection. At issue is a simple model
> of
> RandomAccessCollection where the Index type is Int:
>
> class ReferenceCollection : RandomAccessCollection {
> typealias Index = Int
>
> var startIndex: Int {
> return 0
> }
>
> var endIndex: Int {
> return 1
> }
>
> subscript(index: Int) -> String {
> return ""
> }
>
> func index(after i: Int) -> Int {
> return 1
> }
>
> func index(before i: Int) -> Int {
> return 0
> }
> }
>
> What’s the inferred associated Indices? The RandomAccessIterator protocol has
> a default:
>
> protocol RandomAccessCollection {
> associatedtype Indices : _RandomAccessIndexable, BidirectionalCollection
> = DefaultRandomAccessIndices<Self>
> var indices: Indices { get }
> }
>
> which will kick in if nothing else can be inferred. There is also an
> implementation for this
> defaulted case in a protocol extension from which we can infer Indices:
>
> extension RandomAccessCollection where Indices ==
> DefaultRandomAccessIndices<Self> {
> public var indices: DefaultRandomAccessIndices<Self> { }
> }
>
> Those line up, which is easy, but there is *another* protocol
> extension of RandomAccessIterator from which we can infer Indices:
>
> extension RandomAccessCollection
> where Index : Strideable,
> Index.Stride == IndexDistance,
> Indices == CountableRange<Index> {
>
> public var indices: CountableRange<Index> {
> return startIndex..<endIndex
> }
> }
>
> Note that both DefaultRandomAccessIndices<ReferenceCollection> and
> CountableRange<Int> would be
> valid inferences for Indices. We have three options:
>
> 1) Consider type inference to be ambiguous, because there is no natural
> ordering between the two
> protocol extensions (they have incompatible same-type constraints on
> the associated type Indices).
That seems reasonable, but I would like to have a way to *create* such a
natural ordering.
> 2) Consider the first protocol extension to “win” because… we prefer
> the extension which corresponds to the associated type default
> (?).
Up until now, specific extensions have never behaved like (or at least,
have never been intended to behave like) distinguishable entities in the
user model; I'm wary of entering that world, though I know it has been
discussed w.r.t. conditional conformances.
> This would be consistent with a world where we don’t have
> associated type inference at all. (It also matches Swift 3.0.1’s
> behavior).
?? This statement makes no sense to me. If there's no associated type
inference, what would it mean for this extension to "win?"
> 3) Consider the second protocol extension to “win” because…the other
> protocol extension corresponds to the associated type default, and
> could therefore be considered to be a lowest-common-denominator
> implementation only there to provide the most basic defaults.
>
> For reference, Swift 3.0.1 picked
> DefaultRandomAccessIndices<ReferenceCollection>, current Swift master
> picks CountableRange<Int>, and my work-in-progress to improve the type
> checker calls it ambiguous, hence the question :)
>
> - Doug
>
> _______________________________________________
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> swift-dev@swift.org
> https://lists.swift.org/mailman/listinfo/swift-dev
>
--
-Dave
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