[swift-evolution] [Draft] Allow multiple conformances to the same protocol

[Dan Appel]

>if our protocol conforms to a protocol with associated type requirements,
then we still can't conform to our protocol multiple times.

I don't think so:

protocol A {

    associatedtype TypeA

}


protocol B<TypeB>: A {

    func convert(from: TypeB) -> TypeA

}


struct C: A {

    typealias TypeA = String

}

extension C: B<Int> {

    func convert(from int: Int) -> String {

        return String(int)

    }

}

extension C: B<Double> {

    func convert(from double: Double) -> String {

        return String(double)

    }

}

Seems to me like the above should work fine: the associated type is
declared once, but the generic protocol is conformed to multiple times.

On Sun, Jun 12, 2016 at 1:22 PM Антон Жилин <antonyzhilin at gmail.com> wrote:

> Copy of link to the proposal:
>
> https://github.com/Anton3/swift-evolution/blob/generic-protocols/proposals/NNNN-generic-protocols.md
>
> Inline:
>
> 2016-06-12 21:51 GMT+03:00 Dan Appel <dan.appel00 at gmail.com>:
>
>> Awesome that you guys started this! I've been meaning to pitch it for a
>> while. Couple notes:
>>
>> - in the motivation, you're conforming to SequenceType while defining a
>> From protocol (just a typo I think)
>>
>
> Fixed
>
>
>> - the proposal should mention Brent's comments since it doesn't have a
>> rebuttal to the "unlikely" status generic protocols were given in the
>> generic manifesto. Brent does a great job of doing that
>>
>
> Added a bold link
>
>
>> - is there a way for generic protocols to still have associated types? I
>> think that could have some interesting use cases.
>>
>
> Generic protocols can contain associated types, but no type can conform to
> multiple instances of such protocols.
>
> Actually, this limitation is more restricting than it seems, because if
> our protocol conforms to a protocol with associated type requirements, then
> we still can't conform to our protocol multiple times.
>
>
>> Dan Appel
>>
>> On Sun, Jun 12, 2016 at 11:28 AM Антон Жилин <swift-evolution at swift.org>
>> wrote:
>>
> Yes, everything that works on generic types should work for generic
>>> protocols. I'll add that.
>>>
>>> What won't work is declaring that  MyComparable : Comparable  iff  T ==
>>> Self. The same won't work for current non-generic protocols as well.
>>> Although that feature is highly requested, it is discussed in a separate
>>> proposal.
>>>
>>> - Anton
>>>
>>> 2016-06-12 21:16 GMT+03:00 Xiaodi Wu <xiaodi.wu at gmail.com>:
>>>
>>>>
>>>>
>>>> On Sun, Jun 12, 2016 at 8:01 AM, Антон Жилин <swift-evolution at swift.org
>>>> > wrote:
>>>>
>>>>> I've prepared a proper draft:
>>>>>
>>>>>
>>>>> https://github.com/Anton3/swift-evolution/blob/generic-protocols/proposals/NNNN-generic-protocols.md
>>>>>
>>>>>
>>>> When you propose this:
>>>> Syntax in protocol extensions
>>>>
>>>> protocol MyComparable<T> {
>>>>   func < (left: Self, right: T)
>>>> }extension MyComparable {
>>>>   func > (left: T, right: Self) {
>>>>     return right < left
>>>>   }
>>>> }
>>>>
>>>>
>>>> Would it be possible for me to write something like:
>>>>
>>>> ```
>>>> extension MyComparable<T : SignedNumber> { ... }
>>>> ```
>>>>
>>>
>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>
>> --
>> Dan Appel
>>
> --
Dan Appel
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