[swift-evolution] Variadic generics discussion

Tue May 31 15:34:28 CDT 2016

> On May 31, 2016, at 3:25 PM, Austin Zheng <austinzheng at gmail.com> wrote:
> 
> I have a proposal for #6 in the pipe, but there are actually some subtleties I have to work out (it's not as simple as just slapping a generic type signature on a let constant).

Cool.  Looking forward to reviewing a draft when it’s ready.

> 
> I think #5 is just considered a 'bug' and doesn't need a proposal (it might actually be finished already; I saw some commits recently); same with #4. #7 is not very useful without variadic generics (it pretty much exists to allow tuples to conform to protocols, and tuples are inherently variadic).
> 

Good to know 4 and 5 are considered bugs.  I know #4 is important for the standard library so I suppose that will ensure it is a priority soon enough.

I included #7 because it would still be nice to have for a number of reasons.  Maybe there is a way to pull it off for a handful of types that are known to the compiler.

> I wanted to take a stab at #2.

Are you still thinking about this one or did you decide not to pursue it?

> The core team has talked so much about #1 that I'd be surprised if they don't already have an idea as to how they want to do it, plus it's complicated for a number of reasons to get right. In such a case having the community push forward an alternate proposal would just be giving everyone more unneeded work.

Agree here as well.  I’ve avoided generics proposals mostly because I thought the core team was leading the charge on all them.  It now appears like that may not have been the right assumption across the board.  I wish we had a bit more visibility on this...

> 
> #3 seems semantically straightforward. AFAIK there's nothing a subscript can do that a getter and setter method can't do together, and methods can already be generic. A proposal shouldn't be hard to put together.

Agree.  Someone just needs to jump in and write it up.  :-)  If it had a chance of making it into Swift 3 I would do it right away, but it’s hard to tell...

> 
> Let me know your thoughts.
> 
> Best,
> Austin
> 
> 
> 
> 
> On Tue, May 31, 2016 at 1:16 PM, Matthew Johnson <matthew at anandabits.com <mailto:matthew at anandabits.com>> wrote:
> 
>> On May 31, 2016, at 2:56 PM, Austin Zheng via swift-evolution <swift-evolution at swift.org <mailto: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- <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 <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 <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 <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 <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 <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 <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- <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 <mailto:clattner at apple.com>> wrote:
>> 
>>> On May 31, 2016, at 12:17 PM, L Mihalkovic via swift-evolution <swift-evolution at swift.org <mailto: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 <mailto: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 <mailto: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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