[swift-evolution] Refining SE-0185: Should providing a custom == suppress the default hashValue?

[Howard Lovatt]

I think that is an advanced use, rather than a common use. I would prefer that to be something you manually code. 

-- Howard.

> On 16 Dec 2017, at 7:08 am, Tony Allevato <tony.allevato at gmail.com> wrote:
> 
> 
> 
>> On Fri, Dec 15, 2017 at 11:39 AM Howard Lovatt via swift-evolution <swift-evolution at swift.org> wrote:
>> +1
>> I think the simple solution of if you provide either == or hashValue you have to provide both is the best approach. Good catch of this bug.
>> -- Howard.
> 
> That would be a significant usability hit to a common use case. There are times where a value is composed of N fields where N is large-ish, and equality is dependent on the values of all N fields but the hash value only needs to be "good enough" by considering some subset of those fields (to make computing it more efficient).
> 
> That still satisfies the related relationship between == and hashValue, but a user wanting to explicitly implement a more efficient hashValue should *not* necessarily be required to explicitly write the same == that would be synthesized for them in that case.
> 
>  
>> 
>> > On 16 Dec 2017, at 6:24 am, Daniel Duan via swift-evolution <swift-evolution at swift.org> wrote:
>> >
>> > +1. The proposal wasn’t explicit enough to have either supported or be against this IMO. It’s a sensible thing to spell out.
>> >
>> > Daniel Duan
>> > Sent from my iPhone
>> >
>> >> On Dec 15, 2017, at 9:58 AM, Joe Groff via swift-evolution <swift-evolution at swift.org> wrote:
>> >>
>> >> SE-0185 is awesome, and brings the long-awaited ability for the compiler to provide a default implementation of `==` and `hashValue` when you don't provide one yourself. Doug and I were talking the other day and thought of a potential pitfall: what should happen if you provide a manual implementation of `==` without also manually writing your own `hashValue`? It's highly likely that the default implementation of `hashValue` will be inconsistent with `==` and therefore invalid in a situation like this:
>> >>
>> >> struct Foo: Hashable {
>> >> // This property is "part of the value"
>> >> var involvedInEquality: Int
>> >> // This property isn't; maybe it's a cache or something like that
>> >> var notInvolvedInEquality: Int
>> >>
>> >> static func ==(a: Foo, b: Foo) -> Bool {
>> >>   return a.involvedInEquality == b.involvedInEquality
>> >> }
>> >> }
>> >>
>> >> As currently implemented, the compiler will still give `Foo` the default hashValue implementation, which will use both of `Foo`'s properties to compute the hash, even though `==` only tests one. This could be potentially dangerous. Should we suppress the default hashValue derivation when an explicit == implementation is provided?
>> >>
>> >> -Joe
>> >> _______________________________________________
>> >> swift-evolution mailing list
>> >> swift-evolution at swift.org
>> >> https://lists.swift.org/mailman/listinfo/swift-evolution
>> > _______________________________________________
>> > swift-evolution mailing list
>> > swift-evolution at swift.org
>> > https://lists.swift.org/mailman/listinfo/swift-evolution
>> _______________________________________________
>> swift-evolution mailing list
>> swift-evolution at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-evolution
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20171216/49e7df06/attachment.html>