on Mon Nov 07 2016, Robert Widmann <swift-dev-AT-swift.org> wrote:
> Voting for 1. This is an ambiguity in stdlib through and through IMO.
No, this is the standard library doing the best it can with a type
checker that has mostly-unspecified semantics. It can only be
considered an ambiguity in the standard library if you presume the
semantics of choice 1, which was never specified... hence Doug's
question.
> ~Robert Widmann
>
> 2016/11/07 22:07、Mark Lacey via swift-dev <swift-dev@swift.org> のメッセージ:
>
>>
>>> On Nov 7, 2016, at 6:16 PM, Douglas Gregor via swift-dev
>>> <swift-dev@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).
>>> 2) Consider the first protocol extension to “win” because… we prefer the
>>> extension which corresponds to the associated type default (?). 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).
>>> 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.
>>
>> I can see the appeal of option 3, but IMO anything other than option
>> 1 seems pretty brittle. Presumably with that option, and with the
>> class providing a typealias for Indices, you would no longer have an
>> ambiguity and the code would compile, correct?
>>
>> Mark
>>
>>>
>>> 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
>>>
>>> _______________________________________________
>>> swift-dev mailing list
>>> swift-dev@swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-dev
>>
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--
-Dave
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