[swift-evolution] SE-0185 Synthesizing Equatable and Hashable conformance

[Robert Bennett]

Yes, thanks! Here’s the full proposal for those interested: https://github.com/erica/swift-evolution/blob/c541f517dacc2030c987b6d60ad3d26d8ec5fa3a/proposals/XXXX-role-keywords.md

I think that if we want to deal with the issue of some mistake arising from conforming to Equatable and/or Hashable, it should be through that proposal, not something specific to Equatable and Hashable. This sort of issue should not count against this Equatable/Hashable proposal.

> On Aug 10, 2017, at 3:39 PM, Chris Lattner <clattner at nondot.org> wrote:
> 
>> 
>> On Aug 10, 2017, at 12:24 PM, Robert Bennett via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> I could have sworn that this sort of issue came up on this list earlier this year… Someone proposed a mechanism encompassing all protocols, not just Equatable and Hashable, to handle the issue of mistakenly believing you’re overriding a default implementation. Having trouble finding it at the moment.
> 
> Is this what you’re thinking of?
> https://github.com/apple/swift-evolution/pull/724 <https://github.com/apple/swift-evolution/pull/724>
> 
> -Chris
> 
> 
> 
>> .
>> 
>>> On Aug 10, 2017, at 3:09 PM, David Ungar via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>> If I understand it, merely adding Equatable or Hashable will cause the compiler to synthesize requirements. This syntax opens up the possibility for errors:
>>> 
>>> struct Snort: Hashable {
>>> static var hashValu /* NOTE MISSPELLING */ : Int { return 666 }
>>> }
>>> 
>>> In the above example, the programmer meant to implement hashValue but misspelled it.
>>> With the proposal as-is, the error could be covered up.
>>> 
>>> I would prefer to see a different syntax than merely adding conformance to "HashValue", in order to distinguish the two cases: explicit supplying the requirement vs synthesis.
>>> 
>>> Also, what if we want to extend this idea to other protocols? Perhaps some sort of modifier on the protocol name would be more orthogonal:
>>> 
>>> struct Foo: Synth Hashable, Equatable 
>>> 
>>> Would say that Hashable requirements get synthesized but Equatable ones do not.
>>> 
>>> Alternatively, it might be clearer, though more verbose to move the signalling inside:
>>> 
>>> struct Snort: Hashable {
>>> synth hashValue
>>> }
>>> 
>>> (I don't advocate this specific syntax, btw.) But it has the virtual of possibly making it clearer to read the code.
>>> 
>>> TL;DR: I favor the proposal but would prefer modification to make it more explicit.
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>> 
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