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

[T.J. Usiyan]

Is there any mechanism to mark a property as not participating in derived
conformances? One instance might be that I have a memoization/cache related
property that is stored but should not be considered when equating two
instances.

TJ

On Tue, May 16, 2017 at 3:51 AM, Xiaodi Wu via swift-evolution <
swift-evolution at swift.org> wrote:

> 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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