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

[Robert Bennett]

Sorry, I think I misunderstood your earlier email. I believe we are in agreement.

> On Aug 10, 2017, at 5:54 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
> 
> Yes, but to be clear, this is an objection that is equally applicable to any change where a protocol requirement is given a default implementation.
> 
> Unless I’m mistaken, ordinarily, the addition of such a default implementation isn’t even considered an API change and doesn’t require Swift Evolution approval.
>> On Thu, Aug 10, 2017 at 16:41 David Ungar <dungar at apple.com> wrote:
>> As long as I've been clear that the adoption of *this* proposal would transform a misspelling from a bug that the compiler catches to a bug that the compiler does not catch, I feel that my objection has been heard.
>> 
>> Thank you all,
>> 
>> - David
>> 
>> 
>>> On Aug 10, 2017, at 1:51 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>> 
>>> Right. The objection raised is applicable to the overriding of any default implementation. However. _this_ proposal under review is about the synthesis of a default implementation, and we shouldn’t try to invent new syntax to address an orthogonal issue—and only partially at that.
>>> On Thu, Aug 10, 2017 at 14:45 Robert Bennett via swift-evolution <swift-evolution at swift.org> wrote:
>>>> 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> 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
>>>>> 
>>>>> -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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>>>>>>> swift-evolution at swift.org
>>>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>>> 
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>>>> 
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>> 
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