On Mon, May 15, 2017 at 7:18 PM, Tony Allevato <tony.allev...@gmail.com> wrote:
> > > On Mon, May 15, 2017 at 4:38 PM Itai Ferber <ifer...@apple.com> wrote: > >> On May 15, 2017, at 4:03 PM, Xiaodi Wu via swift-evolution < >> swift-evolution@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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