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

[Chris Lattner]

> 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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