<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">@regexident has a long-standing proposal about how to do hashing better that would address this: <div class=""><br class=""></div><div class=""><a href="https://gist.github.com/regexident/1b8e84974da2243e5199e760508d2d25" class="">https://gist.github.com/regexident/1b8e84974da2243e5199e760508d2d25</a></div><div class=""><br class=""></div><div class="">Dave<br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On Dec 15, 2017, at 2:04 PM, T.J. Usiyan via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class="">Can we provide a 'standard' method/function that can be used to combine ordered hash values (`[Int] -> Int`)? That could make manually implementing `hashValue` less burdensome.<div class=""><br class=""></div><div class="">TJ</div></div><div class="gmail_extra"><br class=""><div class="gmail_quote">On Fri, Dec 15, 2017 at 12:08 PM, Tony Allevato via swift-evolution <span dir="ltr" class=""><<a href="mailto:swift-evolution@swift.org" target="_blank" class="">swift-evolution@swift.org</a>></span> wrote:<br class=""><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr" class=""><br class=""><br class=""><div class="gmail_quote"><span class=""><div dir="ltr" class="">On Fri, Dec 15, 2017 at 11:39 AM Howard Lovatt via swift-evolution <<a href="mailto:swift-evolution@swift.org" target="_blank" class="">swift-evolution@swift.org</a>> wrote:<br class=""></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">+1<br class="">
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.<br class="">
-- Howard.<br class=""></blockquote><div class=""><br class=""></div></span><div class="">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).</div><div class=""><br class=""></div><div class="">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.</div><div class=""><div class="h5"><div class=""><br class=""></div><div class=""> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<br class="">
> On 16 Dec 2017, at 6:24 am, Daniel Duan via swift-evolution <<a href="mailto:swift-evolution@swift.org" target="_blank" class="">swift-evolution@swift.org</a>> wrote:<br class="">
><br class="">
> +1. The proposal wasn’t explicit enough to have either supported or be against this IMO. It’s a sensible thing to spell out.<br class="">
><br class="">
> Daniel Duan<br class="">
> Sent from my iPhone<br class="">
><br class="">
>> On Dec 15, 2017, at 9:58 AM, Joe Groff via swift-evolution <<a href="mailto:swift-evolution@swift.org" target="_blank" class="">swift-evolution@swift.org</a>> wrote:<br class="">
>><br class="">
>> 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:<br class="">
>><br class="">
>> struct Foo: Hashable {<br class="">
>> // This property is "part of the value"<br class="">
>> var involvedInEquality: Int<br class="">
>> // This property isn't; maybe it's a cache or something like that<br class="">
>> var notInvolvedInEquality: Int<br class="">
>><br class="">
>> static func ==(a: Foo, b: Foo) -> Bool {<br class="">
>> return a.involvedInEquality == b.involvedInEquality<br class="">
>> }<br class="">
>> }<br class="">
>><br class="">
>> 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?<br class="">
>><br class="">
>> -Joe<br class="">
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