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

Mon May 15 19:18:01 CDT 2017

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.

>
> On Mon, May 15, 2017 at 17:21 Tony Allevato via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>> Yes—the PR of the proposal is here:
>> https://github.com/apple/swift-evolution/pull/706
>>
>> It needs to be updated slightly—I'll remove the references to the
>> "multiplicative hash function" recommendation because I ended up using the
>> existing _mixInt and xor, which is how the standard library implements its
>> Collection hashValues. (The proposal probably really doesn't need to state
>> anything about the hash function used, and its entirely an implementation
>> detail.)
>>
>>
>> On Mon, May 15, 2017 at 3:18 PM Andrew Bennett <cacoyi at gmail.com> wrote:
>>
>>> Nice work Tony! Is this proposal up for PR on swift-evolution as well?
>>>
>>> On Tue, 16 May 2017 at 7:30 am, Tony Allevato <tony.allevato at gmail.com>
>>> wrote:
>>>
>>>> Just to update everyone on the thread—it took a little longer than I'd
>>>> hoped to get the kinks out, but I finally have the implementation up as a
>>>> PR: https://github.com/apple/swift/pull/9619
>>>>
>>>> Hopefully there's still enough time to get the proposal reviewed, make
>>>> any changes needed, and get this into Swift 4!
>>>>
>>>>
>>>> On Tue, May 9, 2017 at 10:27 PM Brent Royal-Gordon <
>>>> brent at architechies.com> wrote:
>>>>
>>>>> On May 9, 2017, at 3:53 PM, Tony Allevato via swift-evolution <
>>>>> swift-evolution at swift.org> wrote:
>>>>>
>>>>> Likewise, proposing a new public addition to the standard library
>>>>> would inspire far more design discussion than I believe we have time for if
>>>>> we want this to make Swift 4. :)
>>>>>
>>>>>
>>>>> Agreed. What I would do here is add an `_combineHashes` function (or
>>>>> `Hashable` extension method, or whatever is most convenient) to the
>>>>> standard library in Swift 4, have your compiler magic feature use it, and
>>>>> defer the name-and-interface discussion until Swift 5.
>>>>>
>>>>> --
>>>>> Brent Royal-Gordon
>>>>> Architechies
>>>>>
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