[swift-evolution] Pitch: Automatically deriving Equatable/Hashable for more value types

[Matthew Johnson]

> On May 8, 2017, at 4:17 PM, Tony Allevato <tony.allevato at gmail.com> wrote:
> 
> 
> 
> On Mon, May 8, 2017 at 2:11 PM Matthew Johnson <matthew at anandabits.com <mailto:matthew at anandabits.com>> wrote:
>> On May 8, 2017, at 4:02 PM, Tony Allevato <tony.allevato at gmail.com <mailto:tony.allevato at gmail.com>> wrote:
>> 
>> On Sat, May 6, 2017 at 4:17 PM Chris Lattner <clattner at nondot.org <mailto:clattner at nondot.org>> wrote:
>> On May 5, 2017, at 11:33 AM, Tony Allevato via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>> 
>>> 
>>> Can the opt-in conformance be declared in an extension?  If so, can the extension be in a different module than the original declaration?  If so, do you intend any restrictions, such as requiring all members of the type declared in a different module to be public?  My initial thought is that this should be possible as long as all members are visible.
>>> 
>>> Declaring the conformance in an extension in the same module should definitely be allowed;
>> 
>> Please follow the precedent of the Codable proposal as closely as possible.  If you’d like this to be successful for Swift 4, you should look to scope it as narrowly as possible.  Because it is additive (with opt-in), it can always be extended in the future.
>> 
>>> I believe this would currently be the only way to support conditional conformances (such as the `Optional: Hashable where Wrapped: Hashable` example in the updated draft), without requiring deeper syntactic changes.
>> 
>> This proposal doesn’t need to cover all cases, since it is just sugaring a (very common) situation.  Conditional conformances to Hashable could be written manually.
>> 
>>> I'm less sure about conformances being added in other modules,
>> 
>> It is a bad idea, it would break resilience of the extended type.
>> 
>>> But after writing this all out, I'm inclined to agree that I'd rather see structs/enums make it into Swift 4 even if it meant pushing classes to Swift 4+x.
>> 
>> Agreed, keep it narrow to start with.
>> 
>> Also, I don’t know how the rest of the core team feels about this, but I suspect that they will be reticent to take an additive proposal at this late point in the schedule, unless someone is willing to step up to provide an implementation.
>> 
>> That someone is me :)  I have a branch where it's working for enums (modulo some weirdness I need to fix after rebasing a two-month-old state), and adapting that logic to structs should hopefully be straightforward after that. Going with the more narrowly-scoped proposal and making conformances explicit simplifies the implementation a great deal as well (I was previously blocked with recursive types when it was implicit).
>> 
>> Thanks for the feedback—after consideration, I've pulled classes out of the proposal completely (even non-final) and mentioned the other limitations so we'd have a record of what was discussed in this thread.
>> 
>> I've created a PR for the proposal text, in the event that the core team is interested in moving this forward: https://github.com/apple/swift-evolution/pull/706 <https://github.com/apple/swift-evolution/pull/706>
> Thanks for continuing to push this forward Tony!  The current proposal looks like the right approach for getting this into Swift 4.  
> 
> I only have one question which I will present with an example:
> 
> protocol Foo: Equatable {}
> protocol Bar: Hashable {}
> 
> struct FooType: Foo {}
> struct BarType: Bar {}
> 
> Do FooType and BarType receive synthesis?
> 
> Great question! Yes they should. It's "explicit" transitively since the answer to "does FooType/BarType conform to Equatable/Hashable?" is still "yes". (And I've confirmed that my prototype handles this case.)
> 
> This is especially helpful since Hashable extends Equatable, so a user only needs to list conformance to the former to get correctly synthesized implementations of both, which helps to guarantee that they're implemented consistently with respect to each other.

Great.  This is what I was hoping to hear.  :)

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>> -Chris

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