[swift-evolution] Pitch: Automatically deriving Equatable/Hashable for more value types

[Xiaodi Wu]

On Mon, May 15, 2017 at 7:18 PM, Tony Allevato <tony.allevato at gmail.com>
wrote:

>
>
> On Mon, May 15, 2017 at 4:38 PM Itai Ferber <iferber at apple.com> wrote:
>
>> On May 15, 2017, at 4:03 PM, Xiaodi Wu via swift-evolution <
>> swift-evolution at swift.org> wrote:
>>
>> This is nice. Thanks for taking the time to write it up. I do have some
>> concerns/questions:
>>
>> Do the rules you spell out align with those for Codable? I think it is
>> very important that these are paralleled as closely as possible, and that
>> any deviations are explicitly called out in the text with reasoning as to
>> why it must deviate. Knowing when something is synthesized is difficult
>> enough with one set of rules--two is certainly one too many.
>>
>> To spell out the rules of Codable conformance clearly, for reference:
>>
>> For example, is it permitted to extend a type in the same module in order
>> to obtain synthesized Codable conformance? How about for a type in a
>> different module? The same rules should then apply for Equatable and
>> Hashable synthesis.
>>
>> Yes, Codable conformance can be added in an extension both intra-module,
>> and inter-module (i.e. you can add Codable conformance via extensions in
>> your own module, or to types in other modules). If there is a situation
>> where this is not possible, that’s likely a bug.
>> [For reference, it is actually easier to allow this than to prevent it. I
>> had to do very little extra work to support this because of how this is
>> organized in the compiler.]
>>
>
> To the best of my knowledge, the Equatable/Hashable synthesis I added uses
> the same rules as Codable, since I based my implementation on it.
>
> This is slightly different than what we initially discussed in this
> thread, which was that we should not support synthesized conformance in
> extensions in other modules. But after implementing it, my feeling is that
> if it falls out naturally and prohibiting it would be more work, then we
> shouldn't do that unless we have good reason to, and we should do it
> consistently with other derivations. So I'm using the same model.
>
>
>
>> Furthermore, does Codable ignore computed properties? If not, then
>> neither should Equatable and Hashable.
>>
>> Yes. Derived conformance for Codable ignores all computed properties
>> (including lazy properties and their associated storage). This is also some
>> relatively easy default behavior; you can iterate all properties matching
>> this requirement via `NominalTypeDecl.getStoredProperties`
>> (getStoredProperties(/*skipInaccessible=*/true) will skip the storage
>> associated with lazy vars).
>> [The thought process here is that accessing computed vars (and more so
>> lazy vars) will generally have side effects. We don’t want to trigger side
>> effects on encoding/checking for equality/hashing, and in general, those
>> types of properties will not affect equality/hash value/encoded
>> representation.]
>>
>
> Yes, I'm using the same getStoredProperties call to find the struct
> members to apply it to (thanks Itai for the early pointers!), so Eq/Hash
> should be synthesized for structs under the same conditions as Codable.
>
>
>>
>> There are also some complicated rules with generics, if I recall, that
>> may force something to be a computed property. It would be worth exploring
>> if such rules make ignoring computed properties counterintuitive. For
>> instance, if a user has to redesign the type, changing a stored property to
>> a computed property just to satisfy certain rules of the language, and all
>> of a sudden the definition of equality has silently changed as a
>> consequence, then that could end up being very hard to debug. If we find
>> that this is a plausible issue, then it might be worth considering refusing
>> to synthesize Equatable conformance for a type with any computed
>> properties--obviously limiting, but better limiting than surprising. To be
>> clear, I'm not suggesting that we do make this limitation, just that I
>> don't know that the consequences have been adequately explored for not
>> including computed properties.
>>
>> I’m not sure about this — someone else will have to weigh in. I don’t
>> think I’ve ever encountered a situation like this while working on Codable.
>> That being said, if there’s a limiting factor here that we encounter, we
>> should absolutely be consistent between all implementations of derived
>> conformance.
>>
>
> The concern that changing a stored property to a computed property would
> silently change the behavior of Eq/Hash is definitely something we should
> be aware of and we should see if it's something that people run into
> frequently once they start using the synthesis. Nothing obvious comes to
> mind as a way of preventing or warning about it, though—I'd have to think
> more on it.
>
>
>> It would be helpful to document these rules somewhere, so noted.
>>
>
> +1.
>

Highly agree with all your responses; also, delighted to hear that the
implementation work has fallen into place so naturally.
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