[swift-evolution] Swift 3 Generics

Douglas Gregor dgregor at apple.com
Tue Dec 15 22:45:03 CST 2015

Hi Matthew,

Sent from my iPhone

> On Dec 15, 2015, at 7:25 PM, Matthew Johnson <matthew at anandabits.com> wrote:
> Dmitri, Slava and Doug thank you for taking the time to respond.  I apologize for not thanking you sooner.  
> Since the core team has a loosely shared understanding of the goals but hasn't had time to write it down yet I thought it might be helpful if I put together a summary of the features that seem to me like obvious candidates for completing the current generic system.  If I have missed anything significant please point that out. 

This is helpful, thank you!

Lots of comments below. An overarching theme here is that I'm trying to scope down the list to what we need to achieve ABI stability and an expressive library  fewer features means we're more likely to succeed with better quality. 

> I'm placing a "**" next to items that I beleive I have seen mentioned by the core team as desired features.  This is based on memory and may be innacurate or may be based on desires expressed, but not necessarily pinned to the Swift 3 timeline.  
> I'm hoping the core team might be able to indicate which items are likely to be part of the work you're doing on Swift 3, which items you might be interested in the community contributing to, and which items are unliekly to be considered during the Swift 3 timeframe even with community involvement.  This could help interested contributors start thinking about what they might be interested in working on while we wait for a more complete document describing the vision.
> - ** generic typealias

This isn't high on my list. It's a good feature, and I want it someday, but introducing generic typealiases won't have any impact on the ABI and therefore can wait IMO. 

> - allow protocols to specify a non-covarying Self in static method declarations (a possible solution to the problem of conforming non-final classes in Cocoa to protocols containing factory methods)

Maybe. This isn't terribly high on my list, but in also behind on the discussion. 
> - extensions
> 	- ** allow same type constraints

I'm assuming you mean an extension like 

  extension Array where Element == String  {}

Yes, it's something we want to support 

> 	- ** allow protocol conformance in constrained extensions

This is something like:

  extension Array : Equatable where Element : Equatable {}

It is very very high priority for Swift 3.

> 	- allow protocol conformance in protocol extensions

This is something like:

  extension CollectionType : Equatable where Generator.Element : Equatable {}

This is probably not a priority for Swift 3. It seems very cool, and I've advocated for similar features in the past (in more static generics systems), but the potential for ambiguities with such definitions is very high and the runtime cost for asking questions such as "is this T Equatable?" can be very high when this feature is in play. 

> - associated type constraints
> 	- use of Self as a superclass constraint

This wasn't on my list at all. Care to elaborate on why this is important?

> 	- ** where clause constraining inidividual associated types
> 	- ** where clause relating multiple assocated types

Yes to both. We need the to properly express some relationships in the standard library types, such as a SubSequence's Element type matching the Element type of the  sequence itself. 

> - existentials for protocols with associated types
> 	- protocol existentials don't conform to the protocol itself
> 	- ** fully bound: protocol<GeneratorType where Element == Int> 
> 	- ** fully constrained: protocol<GeneratorType where Element: CustomStringConvertible>
> 	- partially bound / constrained - allow access to members only mentioning constrained associated types
> 	- ** unbound - allow access to members not mentioning Self or associated types
> 	- composition: protocol<P1, P2 where P1.Associated: P3, P2.Associated == Int, P1.Other == P2.Other>

This one is tough. It is a very, very common user request and existentials are painfully under implemented in Swift. On the other hand, it's an additive feature that isn't likely (IMO!) to be important for ABI stabilization in the language or library. So, I would put it out of scope for Swift 3. 

> - nesting (per Slava's email)
> 	- ** Generic types nested inside generic functions
> 	- ** Generic types nested inside generic types
> 	- ** Generic functions nested inside generic functions which capture values or outer generic types

I think Slava and I disagree on this one ;)

I don't consider this critical for Swift 3. The compiler will greatly improve simply by making this work (because the dumb assumptions that block this feature likely trigger additional bugs), but we don't need to allow it for ABI stability. There are a handful of places in the standard library where we've had to promote a type that would normally be nested to the top level to work around this, but we could live with that as a smallish ABI wart in the long term and introduce generic typealiases to clean up the syntax once those features become available. 

> Some things that I expect are out of scope for Swift 3 but am mentioning for completeness / confirmation of that:
> - variadic generics
> - higher kinded types
> - higher rank types

Agreed that all are out of scope for Swift 3. The first is a feature of particular interest to me... But absolutely is out of scope. I also want to eventually use variadic generics to extend arbitrary length tuples of Equatable types to make them Equatable, for example ;)

A couple features you didn't mention that I consider in scope for Swift 3 generics:

- generic subscripts (the standard library might need these), e.g.,

  subscript<C : SequenceType where C.Generator.Element == Index> (indices: C) -> [Index]

- recursive protocol constraints, where an associated type can be stated to conform to its enclosing protocol (directly or indirectly), eg,

  protocol SequenceType {
    typealias SubSequence : SequenceType

- operators defined in types (only indirectly related to generics):

  extension Array where Element : Equatable {
    func ==(lhs: Array, rhs: Array) -> Bool { ... }

- "real" default implementations in protocols. It looks like a syntactic simplification from
Putting the definition in an extension, but putting the default implementation in the protocol itself has an important effect on resilience: one should be able to add requirements to a protocol without breaking ABI so long as those requirements have a default implementation. 

Several of these (recursive constraints, where clauses placing requirements on associated types in protocols, etc) are partly aimed at eliminated many of the underscored protocols in the standard library, because we don't want them to be part of the library ABI. 

I suspect I'll remember other small things, but that's the "big" list... and it's size perhaps illustrates why we need to choose carefully to maintain focus. 

  - Doug

> Thanks,
> Matthew
>>> On Dec 11, 2015, at 11:24 PM, Douglas Gregor <dgregor at apple.com> wrote:
>>> On Dec 10, 2015, at 3:45 PM, Matthew Johnson via swift-evolution <swift-evolution at swift.org> wrote:
>>> One of the stated focus areas for Swift 3 is to complete the generics system.  
>>> How far along is the design for the “complete” generics system?  
>> There’s a loose shared understanding of the pieces we need among the compiler and standard library developers that have been co-evolving the generics system, but it’s not written down in any single place.
>>> Is there appetite among the core team to involve the community in evaluating planned features or submitting proposals to complement existing plans?
>> Yes, absolutely. I feel like we (the core team) need to articulate our vision here—what we feel we need to accomplish (in features, in the standard library API, in the implementation) in Swift 3 vs. what we believe we can introduce later on, how the pieces all fit together, etc.—to help facilitate those discussions.
>>> Also, is there any documentation other than https://github.com/apple/swift/blob/master/docs/Generics.rst describing in detail what the complete vision for the generics system is and what new features will be added in Swift 3 (as well as any generics features that have been decided against for Swift or version 3 specifically)?
>> No, that document is the best overall documentation for the vision of the generics system, despite being mostly untouched for more than two years and lacking newer features (protocol extensions, anyone?).
>> So, we need to write up a document describing our vision here. It’s going to take a little time, both because it’s a nontrivial task and because the likely principal authors are also engaged in other large Swift 3 tasks (e.g., https://github.com/apple/swift-evolution/blob/master/proposals/0005-objective-c-name-translation.md)
>> 	- Doug
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