[swift-evolution] [Proposal] Explicit Synthetic Behaviour
Vladimir.S
svabox at gmail.com
Wed Sep 13 12:19:28 CDT 2017
On 13.09.2017 19:08, Ondrej Barina via swift-evolution wrote:
> Maybe something like this as middle ground.
>
> protocol Equatable {
> @syntetic static func ==(_ lhs: Self, _ rhs: Self) -> Bool
> }
>
> protocol itself contains default implementation, but without real body. Instead the
> function is marked that the real body is generated by compiler.
> There is explicit mentions of default impl (by compiler magic), but it does not
> affects users as they would still use protocol in normal way:
>
> struct Foo: Equatable { .... }
Yes, I also thought about this. And personally for me it is also good solution, while
`struct S: Equatable {/*nothing*/}` will *still* lead to compiler's error or at least
warning about not implemented requirements.
So, I'll be explicit regarding my intention: do I want requirements to be
auto-generated or I want to do this manually.
But still. If you see
struct S: Equatable, Codable {
// a lot of lines
}
you can't say right now if requirements for Equatable and/or Codable was implemented
manually or will be auto-generated without checking all the code of a type. This
knowledge can help to faster solve issues related to comparison/archiving.
So for me the best solution is still 'deriving'-like keyword, which adds clarity and
show intention without any boilerplate code:
struct S: Equatable, deriving Codable {
// all clear:
// manually implemented Equatable
// auto-generated Codable
// a lot of lines
}
Vladimir.
>
> Ondrej B.
>
> On Wed, Sep 13, 2017 at 4:14 PM, Haravikk via swift-evolution
> <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>
>
>> On 13 Sep 2017, at 03:26, Xiaodi Wu <xiaodi.wu at gmail.com
>> <mailto:xiaodi.wu at gmail.com>> wrote:
>>
>> On Tue, Sep 12, 2017 at 11:43 AM, Haravikk via
>> swift-evolution<swift-evolution at swift.org <mailto:swift-evolution at swift.org>>wrote:
>>
>>
>>> On 12 Sep 2017, at 12:08, Xiaodi Wu <xiaodi.wu at gmail.com
>>> <mailto:xiaodi.wu at gmail.com>> wrote:
>>>
>>>> On Mon, Sep 11, 2017 at 06:03 Haravikk via swift-evolution
>>>> <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>>
>>>> See, this is another flawed assumption; you are assuming that
>>>> omitting a custom implementation of == is always intentional rather
>>>> than an oversight, which is not guaranteed. This is one of my gripes
>>>> with the retroactive change to Equatable, as it is
>>>> currently*impossible* to omit an implementation.
>>>
>>>
>>> Again, this applies equally to the addition of _any_ default
>>> implementation. And again, such changes don’t even require Swift Evolution
>>> approval.
>>
>> So what? Because the Swift Evolution process is currently deficient we
>> should just give up on discussing problems with features and the language
>> altogether?
>>
>>
>> I don't claim that it's a deficiency; I claim it's reflective of Swift's
>> opinionated take on default implementations. Are you, after all, saying that
>> you have a problem with the addition of _any_ default implementation to an
>> existing protocol? If so, this conversation isn't about synthesis/reflection at
>> all.
>
> No, and you should know that by now. I suggest actually reading some of what I
> have written as I am sick of repeating myself.
>
>>>>>> And precisely what kind of "evidence" am I expected to give? This
>>>>>> is a set of features that*do not exist yet*, I am trying to argue
>>>>>> in favour of an explicit end-developer centric opt-in rather than
>>>>>> an implicit protocol designer centric one. Yet no-one seems
>>>>>> interested in the merits of allowing developers to choose what they
>>>>>> want, rather than having implicit behaviours appear potentially
>>>>>> unexpectedly.
>>>>>
>>>>> Both options were examined for Codable and for Equatable/Hashable.
>>>>> The community and core team decided to prefer the current design. At
>>>>> this point, new insights that arise which could not be anticipated
>>>>> at the time of review could prompt revision. However, so far, you
>>>>> have presented arguments already considered during review.
>>>>
>>>> And so far all I have heard about this is how it was "decided";
>>>> no-one seems interested in showing how any of these concerns were
>>>> addressed (if at all), so as far as I can tell they were not, or they
>>>> were wilfully ignored.
>>>
>>>
>>> They were addressed by being considered.
>>
>> And yet no-one can apparently summarise what those "considerations" might
>> be, suggesting that they were either *not* considered at all, or that the
>> "consideration" was so weak that no-one is willing to step forward to
>> defend it. Either way it is not sufficient by any reasonable measure.
>>
>> If I were to run over your foot in my car, would you be happy to accept
>> that I "considered" it first?
>>
>>
>> How do you mean? People wrote in with their opinions. Then, taking into account
>> the community's response, the proposal was approved.
>
> I mean because not once have you summarised what these alleged "considerations"
> were; if they exist then you should be able do so, yet all I am hearing is "it
> was considered", which frankly is not an argument at all as it is entirely
> without substance.
>
> If it was genuinely considered then someone should be able to say what points
> were considered and what conclusions were reached and why. And even if there
> *was* an earlier decision, that doesn't necessarily make it right. We are
> discussing it now, and it is clear that any decision that has been made has been
> made poorly at best.
>
> And if you're talking about the discussion on Equatable/Hashable specifically,
> I'm afraid your memory of the "considerations" is radically different to mine; as
> the concerns I raised were essentially ignored, as not a single person gave a
> justification more substantial than "but, but Codable!" which frankly isn't a
> justification at all.
>
>>>>>> Therefore, your argument reduces to one about which default
>>>>>> implementations generally ought or ought not to be
>>>>>> provided--that is, that they ought to be provided only when
>>>>>> their correctness can be guaranteed for all (rather than almost
>>>>>> all) possible conforming types. To which point I sketched a
>>>>>> rebuttal above.
>>>>>
>>>>> If a protocol defines something, and creates a default
>>>>> implementation based only upon those definitions then it must by
>>>>> its very nature be correct. A concrete type may later decided to
>>>>> go further, but that is a feature of the concrete type, not a
>>>>> failure of the protocol itself which can function correctly
>>>>> within the context it created. You want to talk evidence, yet
>>>>> there has been no example given that proves otherwise; thus far
>>>>> only Itai has attempted to do so, but I have already pointed out
>>>>> the flaws with that example.
>>>>>
>>>>> The simple fact is that a default implementation may either be
>>>>> flawed or not within the context of the protocol itself; but a
>>>>> reflective or synthetic implementation by its very nature goes
>>>>> beyond what the protocol defines and so is automatically flawed
>>>>> because as it does not rely on the end-developer to confirm
>>>>> correctness, not when provided implicitly at least.
>>>>>
>>>>>
>>>>> Again, if it applies generally, it must apply specifically. What is
>>>>> "automatically flawed" about the very reasonable synthesized default
>>>>> implementation of ==?
>>>>
>>>> It makes the assumption that every equatable property of a type is
>>>> necessarily relevant to its equality.
>>>
>>>
>>> No necessarily, only provisionally and rebuttably. If it’s not the case,
>>> override the default.
>>
>> So… entirely unlike standard default implementations
>> which*cannot* "provisionally" assume something is relevant at all,
>>
>>
>> Why not?
>
> Because they can only act upon properties/methods that they themselves (or a
> parent protocol) define. FFS, what is so unclear about that? Or are you arguing
> on this subject without every having actually used a protocol before?
>
>> thereby making them entirely different from synthesised/reflective
>> implementations!
>>
>> I'm sorry, but you keep trying to argue that they're the same, but then
>> admitting that they're not. You can't have it both ways.
>>
>>
>> Well, certainly, synthesized default implementations differ from
>> non-synthesized ones in key respects. However, they do not differ in terms of
>> the user experience of conforming to the protocol and having to override the
>> default.
>
> Except that that's not true at all, is it?
>
> Synthesised default implementations go much further in how they attempt (and
> potentially fail) to implement those defaults, and in the specific case of
> Equatable/Hashable they are fully implementing a protocol without a single
> property of method being raised as a requirement; they are utterly different at a
> fundamental level, no amount of mental contortion changes that fact.
>
>>>> Consider for example if a type stores a collection index for
>>>> performance reasons; this isn't an intrinsic part of the type, nor
>>>> relevant to testing equality, yet this default implementation will
>>>> treat it as such because it*knows nothing about the concrete type's
>>>> properties*. If a protocol does not define a property then any action
>>>> taken upon such a property is necessarily based upon an assumption;
>>>> just because it might be fine some of the time, does not make it any
>>>> less flawed.
>>>>
>>>> The big difference here between explicit and implicit synthetic
>>>> implementations is where this assumption originates; if a method is
>>>> synthesised implicitly then the assumption is made by the protocol
>>>> designer alone, with no real involvement by the end developer. If I
>>>> explicitly opt-in to that default however I am signalling to the
>>>> protocol that it is okay to proceed. In the former case the
>>>> assumption is unreasonable, in the latter it is explicitly
>>>> authorised. It is a difference between "I want to make the decision
>>>> on what's correct" and "I am happy for you (the protocol designer) to
>>>> decide".
>>>>
>>>> Right now, when I conform to Equatable, it is a declaration of "I
>>>> will implement this", but with this retroactive implicit change it is
>>>> now a declaration of "implement this for me", these are two entirely
>>>> different things. Consider; what if I'm working on a piece of code
>>>> that requires types to be Equatable, but one of the types I'm using
>>>> currently isn't, so I quickly throw Equatable conformance onto it and
>>>> go back to what I was doing, with the intention of completing
>>>> conformance later. With this change that type may now receive a
>>>> default implementation that is wrong, and I've lost the safety net
>>>> that currently exists.
>>>
>>>
>>> Right now, it still wouldn’t compile, so I don’t see why you would do
>>> that. In the future, if you want to make it not compile, there is nothing
>>> stopping you from conforming to a non-existent “NotYetEquatable”. This was
>>> something that you asked about earlier and it was answered.
>>
>> So your solution is to intentionally write invalid code to work around the
>> fact that a feature is being implemented badly?
>>
>>
>> You stated a use case where you *want* the compiler to stop your code from
>> compiling by stating a conformance to Equatable without implementing its
>> requirements. You then stated that the major problem you have with synthesized
>> `==` is that the compiler will now use a default implementation that you might
>> forget about instead of stopping compilation. Therefore, I demonstrated how you
>> could continue to have the compiler stop your code from compiling. It's not my
>> solution that is intentionally writing invalid code; your stated aim was to be
>> able to do so.
>
> My stated aim was nothing of the sort.
>
> I was pointing out that right now conforming to Equatable means something
> entirely different from what it will mean in future if this idiotic change makes
> it into release. Please actually read what I write before deciding for yourself
> what my 'stated aim' is.
>
> I am *not* asking for workarounds to circumvent a ridiculously flawed change to
> the language, I am arguing why it is flawed and must be changed. If I wanted a
> workaround I'd do what I'm now seriously considering, which is ditching Swift
> completely, as I will not use a language if I can no longer trust the team
> developing it or the decisions that they make.
>
>>>> A non-synthesised/reflective implementation cannot strictly be
>>>> incorrect, because as long as it is implemented properly it will
>>>> always be correct within the context of the protocol itself. It may
>>>> not go quite as far as an end developer might want, but that is
>>>> because they want to add something onto the protocol, not because the
>>>> protocol is wrong.
>>>>
>>>> A synthesised/reflective implementation differs because if it goes
>>>> too far it is wrong not only within the context of the concrete type,
>>>> but also the protocol itself, it is simply incorrect.
>>>
>>>
>>> Again, this is an assertion that misses the mark. If the default
>>> implementation is unsuitable for a type, it’s unsuitable whether it
>>> “doesn’t go quite as far” or “goes too far.”
>>
>> Because not going quite far enough is not a failure of the protocol, as
>> protocols by their very nature can only go as far as what they define. If a
>> protocol Foo defines two properties, a method which uses those two
>> properties correctly, then the method is correct. A developer of a concrete
>> type might want to add more information or tailor the behaviour, but that
>> doesn't make the default implementation incorrect, it's just considering
>> the type only within the context of being an instance of Foo.
>>
>> Going too far is the opposite; it's the protocol designer messing around
>> with stuff they do not define at all. It's only ever right by chance, as
>> it's operating within the context of the concrete type, about which the
>> protocol does not know anything with certainty.
>>
>>
>> Yes, you have defined "not going far enough" and "going too far" based on
>> whether an implementation uses only protocol requirements or not. However, you
>> haven't at all demonstrated why this distinction is at all meaningful in terms
>> of the issue you describe with a user conforming to a protocol. If there is a
>> default implementation, either it returns the expected result for the
>> conforming type or it does not--those are the only two choices. Are you arguing
>> that, empirically, the default implementation for Equatable will more often be
>> unsuitable for conforming types? If so, what's your evidence?
>
> What's yours? If this issue was as "considered" as you constantly claim then
> where is the evidence that there is no meaningful distinction? Surely such
> evidence exists, or else the issue hasn't been considered at all, has it?
>
> Frankly I am sick of being asked to provide evidence when you are seemingly
> unwilling to do anything in return, especially when you have conveniently ignored
> every single example that I have already given.
>
> It cuts both ways; you claim that "going too far" and "not going far enough" are
> the same thing? Well prove it.
>
>>> You state but do not give any rationale for the claim that the former is
>>> not wrong in some context while the latter is always wrong.
>>>
>>> By this line of argumentation, you’d be perfectly content if instead we
>>> simply had the default implementation of == as “return true” because it
>>> would be somehow not wrong.
>>
>> Only if return true were a reasonable default to give in the context of the
>> protocol, which it clearly is not, as it's not performing any kind of
>> comparison of equality.
>>
>>
>> Sure it is; `return true` satisfies all the semantic requirements for equality:
>> reflexivity, symmetry, transitivity; and, in the context of the protocol which
>> only provides for this one facility (determination of equality or inequality),
>> any two instances that compare equal _are_ completely interchangeable "within
>> the context of the protocol itself," as you would say.
>
> The purpose of Equatable is to identify types that can be compared for equality;
> returning true does not satisfy that aim because no such comparison is occurring,
> so your example is intentionally ridiculous. Even a less contrived example such
> as comparing memory addresses doesn't fulfil the purpose of Equatable, which is
> all about comparing equality of different instances that might still be the same.
>
>>>>> Put another way, what the proposal about synthesizing
>>>>> implementations for Equatable and Hashable was about can be
>>>>> thought of in two parts: (a) should there be default
>>>>> implementations; and (b) given that it is impossible to write
>>>>> these in Swift, should we use magic? Now, as I said above,
>>>>> adding default implementations isn't (afaik) even considered an
>>>>> API change that requires review on this list. Really, what
>>>>> people were debating was (b), whether it is worth it to
>>>>> implement compiler-supported magic to make these possible. Your
>>>>> disagreement has to do with (a) and not (b).
>>>>
>>>> Wrong. The use of magic in this case produces something else
>>>> entirely; that's the whole point. It is*not the same*, otherwise
>>>> it wouldn't be needed at all. It doesn't matter if it's compiler
>>>> magic, some external script or a native macro, ultimately they
>>>> are all doing something with a concrete type that is currently
>>>> not possible.
>>>>
>>>> And once again;*I am not arguing against a default implementation
>>>> that cuts boilerplate*, I am arguing against it being implicit.
>>>> What I want is to be the one asking for it, because it is not
>>>> reasonable to assume that just throwing it in there is always
>>>> going to be fine, because it quite simply is not.
>>>>
>>>>
>>>> If you have to ask for it, then it's not a default. You *are* against
>>>> a default implementation.
>>>
>>> A default implementation is an implementation that I, as the concrete
>>> type developer, do not have to provide myself. If you want default to
>>> mean only "automatic" then your attempt to pigeon-hole what I am
>>> arguing is incorrect, because what I am arguing is then neither about
>>> default implementations nor the means of actually implementing it, but
>>> something else entirely.
>>>
>>> But as far as I'm concerned it still absolutely still a default
>>> implementation whether it is requested or not; the difference is I, as
>>> the end developer, am able to refine what type of defaults that I want.
>>>
>>>
>>> The word “default” indicates something that arises in the absence of a
>>> user indication otherwise.
>>
>> Then this proposal is just for a different mechanism for "indicating
>> otherwise".
>>
>> You keep trying to argue that a synthesised/reflective default
>> implementation is the same as a normal default implementation, yet you seem
>> to be consistently forgetting that even if that is true without this
>> proposal, that the very proposal itself is to change that, effectively
>> causing a category of default implementation to become explicitly
>> opted-into, rather than implicitly. They're still implementations that will
>> be provided automatically, just only when they are permitted to do-so.
>>
>>
>> So to be clear, you are *against* them being the *default*: you wish them to be
>> the *otherwise*.
>
> You seem to be insisting upon a narrow definition of default; what I want is
> control over which types of default implementations are provided. Just because
> they must be opted-into explicitly does not stop them being "default", as they
> are still implementations that I myself do not need to implement. The difference
> is that I want to actually *want* them rather than have provided through
> potentially flimsy assumptions made by a protocol designer. Just because there's
> an extra step doesn't make them any less automatic, otherwise having to conform
> to a protocol in the first place would also prevent them from being defaults.
>
> Asking *for* something is more like a middle-ground between the two; the
> synthetic implementations are still possible defaults, they just aren't provided
> unless you allow them, while omitting the necessary keyword/attribute prevents
> them being used.
>
>>>> On 9 Sep 2017, at 23:17, Gwendal Roué <gwendal.roue at gmail.com
>>>> <mailto:gwendal.roue at gmail.com>> wrote:
>>>>
>>>> All right, I'll be more positive: our science, IT, is a
>>>> *constructive* science, by *essence*. If there is a problem, there
>>>> must be a way to show it.
>>>> It you can't, then there is no problem.
>>>
>>> You mean just as I have asked for examples that prove
>>> non-synthetic/reflective default implementations are as dangerous as
>>> synthetic/reflective ones? Plenty have suggested this is the case yet
>>> no reasonable examples of that have been given either.
>>>
>>> However, examples highlighting problems with the synthesised behaviour
>>> are simple:
>>>
>>> structFoo :Equatable{vardata:String}// Currently an error, won't
>>> be in future
>>>
>>>
>>> Or something a bit more substantial:
>>>
>>> structKeyPair :Equatable{
>>> staticvarcount:Int=0
>>>
>>> varcount:Int
>>> letkey:String// This is the only property that should be equatable
>>> varvalue:String
>>>
>>> init(key:String, value:String) {
>>> letcount =KeyPair.count&+1
>>> KeyPair.count= count;self.count= count
>>> self.key= key;self.value= value
>>> }
>>> }
>>>
>>> Here the only important property in the key pair is the key, the value
>>> isn't important (only the keys are to be considered unique) and the
>>> count is just a throwaway value. The synthesised default
>>> implementation for this concrete type will therefore be completely
>>> wrong, likewise for Hashable, which will likely produce radically
>>> different results for instances that should be the same.
>>
>> I notice that despite asking endlessly for examples, the ones I've given
>> are being ignored. In future I shall remind people asking for examples
>> where they can shove them.
>
> And once again, totally ignored. You seem to love asking for "evidence" but why
> exactly should I bother giving anything if you ignore it when I try to?
>
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
> https://lists.swift.org/mailman/listinfo/swift-evolution
> <https://lists.swift.org/mailman/listinfo/swift-evolution>
>
>
>
>
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
>
More information about the swift-evolution
mailing list