> On Nov 8, 2016, at 1:58 PM, Dave Abrahams via swift-dev <swift-dev@swift.org>
> wrote:
>
>
> on Mon Nov 07 2016, Douglas Gregor <swift-dev-AT-swift.org
> <http://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.
One such way is to drop the same-type requirement (Indices ==
DefaultRandomAccessIndices<Self>) from the first extension, making it an
unconstrained extension and, therefore, more general than the second
(constrained) extension. I think that’s the best solution here. The downside is
that a concrete type like ‘ReferenceCollection’ will have the subscript
operators from both RandomAccessCollection extensions. That’s a problem I think
we should solve more generally, perhaps with some name-shadowing rule or
keyword to say “only use this declaration to satisfy a requirement and for
nothing else”.
I’ll go ahead with this solution for now.
>
>> 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?"
If there’s no associated type inference, one would get the associated type
default, DefaultRandomAccessIndices<Self>.
- Doug
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