[swift-evolution] Variadic generics discussion

[Matthew Johnson]

Sent from my iPad

> On May 31, 2016, at 7:10 PM, Chris Lattner <clattner at apple.com> wrote:
> 
> This is very close to my priority list.  That said, I think that all of these are out of scope for swift 3 sadly.  

Happy to hear these priorities look about right to you also.  (I realized afterwards that I left off opening existentials which I would put around 5 or 6)

BTW, generalized existentials is #2 for me if we include things that already have proposals as well.  That going to be a game changer.

I've already been assuming we won't see any major new generics features in Swift 3.  

> 
> After Swift 3, the priority list will be driven by what the standard library needs to get its APIs realized in their ideal form (eg without any of the _ protocol hacks).  Conditional conformances certainly top the list, but we will have to carefully and ruthlessly prioritize things in order to get to ABI stability.

Makes sense.

> 
> -Chris
> 
>> On May 31, 2016, at 2:16 PM, Matthew Johnson <matthew at anandabits.com> wrote:
>> 
>> 
>>> On May 31, 2016, at 2:56 PM, Austin Zheng via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>> This is pretty much where my thinking about the topic has led me as well. I'll resign this topic to pursue some other, hopefully more relevant work, although anyone who wants to continue the discussion is welcome to.
>> 
>> Seems reasonable to wait until we can at least evaluate the macro approach properly.
>> 
>> Are you planning to continue work on generics?  FWIW, my top priority list for items without proposals is roughly:
>> 
>> 1. Conditional conformance (https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md#conditional-conformances-)
>> 2. Parameterized extensions (https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md#parameterized-extensions)
>> 3. Generic subscripts (https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md#generic-subscripts)
>> 4. Recursive protocol constraints (https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md#nested-generics)
>> 5. Nested generics (https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md#nested-generics)
>> 6. Default generic arguments (https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md#default-generic-arguments)
>> 7. Extensions of structural types (https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md#extensions-of-structural-types)
>> 
>> And this one seems like low hanging fruit:
>> 
>> Default implementations in protocols (https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md#default-implementations-in-protocols-)
>> 
>> How does this compare to your priorities for generics?
>> 
>>> 
>>>> On Tue, May 31, 2016 at 12:49 PM, Chris Lattner <clattner at apple.com> wrote:
>>>> 
>>>>> On May 31, 2016, at 12:17 PM, L Mihalkovic via swift-evolution <swift-evolution at swift.org> wrote:
>>>>> 
>>>>> well there is no macro system, and for the moment a clear statement from chris that this is not on the table in the short term. the code in the example looked like run-of-the-mill swift, except for the “…". so that leaves us with swift looking code that would be executed by the compiler, but with nothing particular to tell which parts to and which not. just a thought.
>>>> 
>>>> Lets be clear though: variadic generics are not in scope for Swift 3 either.  
>>>> 
>>>> I definitely don’t speak for the rest of the core team, nor have I discussed it with them…  but IMO, this whole feature seems like a better fit for a macro system than it does to complicate the generics system.  Unlike C++’s template system, our generics system inherently has runtime / dynamic dispatch properties, and I don’t think that shoehorning variadics into it is going to work out well.
>>>> 
>>>> -Chris
>>>> 
>>>>> 
>>>>> 
>>>>>> On May 31, 2016, at 7:59 PM, Austin Zheng <austinzheng at gmail.com> wrote:
>>>>>> 
>>>>>> How so? I'm interested in anything that can get us away from having to generating code at compile-time.
>>>>>> 
>>>>>> On Tue, May 31, 2016 at 10:04 AM, L. Mihalkovic <laurent.mihalkovic at gmail.com> wrote:
>>>>>>> 
>>>>>>> What's interesting about the code in the manifesto is that it looks very much like "..." is a runtime construct, as opposed to trying the get the compiler to do the heavy lifting.
>>>>> 
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