[swift-evolution] [Pitch] Make Optional, Array, and Dictionary conditionally Equatable

[John McCall]

> On Nov 22, 2017, at 1:08 PM, Douglas Gregor via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
> 
> Sent from my iPhone
> 
> On Nov 22, 2017, at 9:48 AM, Chris Lattner <clattner at nondot.org <mailto:clattner at nondot.org>> wrote:
> 
>> IMO this is obvious and you should put it in.
>> 
>> The process serves a purpose: to ensure the evolution of the language is going in the right place, both directionally in an details.  It is obvious that we’re going to take this, and the details are clear, therefore doing an evolution cycle for this would just waste everyone’s time.
> 
> I’ve been leaning this way as well. We can treat this small addition as an amendment to SE-0143 so the change is documented appropriately. 

Agreed.  I think bringing it up here and letting it "pass by acclamation" is the appropriate process; doing a separate proposal would be a waste.

John.

> 
>> 
>> That said, when you get to less obvious introductions and start doing more major consolidation and simplification of the stdlib, those changes may be worthy of discussion to ensure the details are right.
> 
> Right. All of the consolidation of the various Slice and lazy types is big enough to warrant a proposal, for example. 
> 
>   - Doug
> 
>> 
>> -Chris
>> 
>>> On Nov 21, 2017, at 10:51 PM, Douglas Gregor via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>> Hi all,
>>> 
>>> We’re having a bit of a debate <https://github.com/apple/swift/pull/12910> over the question of whether SE-0143 “Conditional Conformances” <https://github.com/apple/swift-evolution/blob/master/proposals/0143-conditional-conformances.md> actually proposes any standard library changes at all, or whether they should all be brought up separately. So, I’ll pitch the pieces that I’d love to put into 4.1 to see if they’re as obvious as I think they should be :)
>>> 
>>> Proposal: make Optional, Array, ArraySlice, ContiguousArray, and Dictionary conform to Equatable when their type parameters are Equatable (and Set always conform to Equatable). Specifically, add to the standard library:
>>> 
>>> 	extension Optional: Equatable where Wrapped: Equatable { /*== already exists */ }
>>> 	extension Array: Equatable where Element: Equatable { /*== already exists */ }
>>> 	extension ArraySlice: Equatable where Element: Equatable { /*== already exists */ }
>>> 	extension ContiguousArray: Equatable where Element: Equatable { /*== already exists */ }
>>> 	extension Dictionary: Equatable where Value: Equatable { /*== already exists */ }
>>> 	extension Set: Equatable { /*== already exists */ }
>>> 
>>> Motivation: we need these for ==/!= to properly compose. It’s a highly-requested feature and an obvious “first use” of conditional conformances for the standard library that is unlikely to break any code.
>>> 
>>> Implementation: https://github.com/apple/swift/pull/13046 <https://github.com/apple/swift/pull/13046>
>>> 
>>> Thoughts?
>>> 
>>> 
>>> 	- Doug
>>> 
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