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

Tue May 16 07:52:59 CDT 2017

Correct, that's not supported yet (it's in the limitations section of the
proposal). The first version is very limited in scope to support the common
cases, and additional features can be added later if needed.

Anything we would add to support omitting properties would likely also
apply to Codable, so it's better to do that in that context as well and not
just limit it to Eq/Hash.
On Tue, May 16, 2017 at 5:24 AM Xiaodi Wu <xiaodi.wu at gmail.com> wrote:

> Not in this proposal, unless something has changed. I think it's
> reasonable for the first iteration of this feature to say that if you want
> to customize how equality is determined, you write your own `==`.
>
>
> On Tue, May 16, 2017 at 05:56 T.J. Usiyan <griotspeak at gmail.com> wrote:
>
>> 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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>>>
>>>
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