On Tue, Sep 12, 2017 at 22:07 Tony Allevato <[email protected]> wrote:
> On Tue, Sep 12, 2017 at 7:10 PM Xiaodi Wu <[email protected]> wrote: > >> On Tue, Sep 12, 2017 at 9:58 AM, Thorsten Seitz via swift-evolution < >> [email protected]> wrote: >> >>> Good arguments, Tony, you have convinced me on all points. Transient is >>> the way to go. Thank you for your patience! >>> >> >> On many points, I agree with Tony, but I disagree that "transient" >> addresses the issue at hand. The challenge being made is that, as Gwendal >> puts it, it's _unwise_ to have a default implementation, because people >> might forget that there is a default implementation. "Transient" only works >> if you remember that there is a default implementation, and in that case, >> we already have a clear syntax for overriding the default. >> > > Right—I hope it hasn't sounded like I'm conflating the two concepts > completely. The reason I brought up "transient" is because nearly all of > the "risky" examples being cited so far have been of the variety "I have a > type where some properties happen to be Equatable but shouldn't be involved > in equality", so my intention has been to show that if we have a better > solution to that specific problem (which is, related to but not the same as > the question at hand), then there aren't enough risky cases left to warrant > adding this level of complexity to the protocol system. > > >> >> As others point out, there's a temptation here to write things like >> "transient(Equatable)" so as to control the synthesis of implementations on >> a per-protocol basis. By that point, you've invented a whole new syntax for >> implementing protocol requirements. (Ah, you might say, but it's hard to >> write a good hashValue implementation: sure, but that's adequately solved >> by a library-supplied combineHashes() function.) >> > > I totally agree with this. A design that would try to annotate "transient" > with a protocol or list of protocols is missing the point of the semantics > that "transient" is supposed to provide. It's not a series of switches to > that can be flipped on and off for arbitrary protocols—it's a semantic tag > that assigns additional meaning to properties and certain protocols (such > as Equatable, Hashable, and Codable, but possibly others that haven't been > designed yet) would have protocol-specific behavior for those properties. > > To better explain what I've been poking at, I'm kind of extrapolating this > out to a possible future where it may be possible to more generally (1) > define custom @attributes in Swift, like Java annotations, and then (2) use > some metaprogramming constructs to generate introspective default > implementations for a protocol at compile-time just as the compiler does > "magically" now, and the generator would be able to query attributes that > are defined by the same library author as the protocol and handle them > accordingly. > > In a world where that's possible, I think it's less helpful to think in > terms of "I need to distinguish between conforming to X and getting a > synthesized implementation and conforming to X and avoiding the synthesized > implementation because the default might be risky", but instead to think in > terms of "How can I provide enough semantic information about my types to > remove the risk?" > > In other words, the switches we offer developers to flip shouldn't be > about turning on/off entire features, but about giving the compiler enough > information to make it smart enough that we never need to turn it off in > the first place. As I alluded to before, if I have 10 properties in a type > and only 1 of those needs to be ignored in ==/hashValue/whatever, writing > "Equatable" instead of "derives Equatable" isn't all that helpful. Yes, it > spits out an error message where there wouldn't have been one, but it > doesn't reduce any of the burden of having to provide the appropriate > manual implementation. > > But all that stuff about custom attributes and metaprogramming > introspection is a big topic of it's own that isn't going to be solved in > Swift 5, so this is a bit of a digression. :) > That said, we could have enums EquatingKeys and HashingKeys, a la CodingKeys... That may not be a huge leap to propose and implement. > >> >> >>> -Thorsten >>> >>> Am 12.09.2017 um 16:38 schrieb Tony Allevato via swift-evolution < >>> [email protected]>: >>> >>> >>> >>> On Mon, Sep 11, 2017 at 10:05 PM Gwendal Roué <[email protected]> >>> wrote: >>> >>>> >>>>> This doesn't align with how Swift views the role of protocols, though. >>>>> One of the criteria that the core team has said they look for in a >>>>> protocol >>>>> is "what generic algorithms would be written using this protocol?" >>>>> AutoSynthesize doesn't satisfy that—there are no generic algorithms that >>>>> you would write with AutoEquatable that differ from what you would write >>>>> with Equatable. >>>>> >>>>> >>>>> And so everybody has to swallow implicit and non-avoidable code >>>>> synthesis and shut up? >>>>> >>>> >>>> That's not what I said. I simply pointed out one of the barriers to >>>> getting a new protocol added to the language. >>>> >>>> Code synthesis is explicitly opt-in and quite avoidable—you either >>>> don't conform to the protocol, or you conform to the protocol and provide >>>> your own implementation. What folks are differing on is whether there >>>> should have to be *two* explicit switches that you flip instead of one. >>>> >>>> >>>> No. One does not add a protocol conformance by whim. One adds a >>>> protocol conformance by need. So the conformance to the protocol is a >>>> *given* in our analysis of the consequence of code synthesis. You can not >>>> say "just don't adopt it". >>>> >>>> As soon as I type the protocol name, I get synthesis. That's the reason >>>> why the synthesized code is implicit. The synthesis is explicitly written >>>> in the protocol documentation, if you want. But not in the programmer's >>>> code. >>>> >>>> I did use "non-avoidable" badly, you're right: one can avoid it, by >>>> providing its custom implementation. >>>> >>>> So the code synthesis out of a mere protocol adoption *is* implicit. >>>> >>>> Let's imagine a pie. The whole pie is the set of all Swift types. Some >>>> slice of that pie is the subset of those types that satisfy the conditions >>>> that allow one of our protocols to be synthesized. Now that slice of pie >>>> can be sliced again, into the subset of types where (1) the synthesized >>>> implementation is correct both in terms of strict value and of business >>>> logic, and (2) the subset where it is correct in terms of strict value but >>>> is not the right business logic because of something like transient data. >>>> >>>> >>>> Yes. >>>> >>>> What we have to consider is, how large is slice (2) relative to the >>>> whole pie, *and* what is the likelihood that developers are going to >>>> mistakenly conform to the protocol without providing their own >>>> implementation, *and* is the added complexity worth protecting against this >>>> case? >>>> >>>> >>>> That's quite a difficult job: do you think you can evaluate this >>>> likelihood? >>>> >>>> Explicit synthesis has big advantage: it avoids this question entirely. >>>> >>>> Remember that the main problem with slide (2) is that developers can >>>> not *learn* to avoid it. >>>> >>>> For each type is slide (2) there is a probability that it comes into >>>> existence with a forgotten explicit protocol adoption. And this probability >>>> will not go down as people learn Swift and discover the existence of slide >>>> (2). Why? because this probability is driven by unavoidable human >>>> behaviors: >>>> - developer doesn't see the problem (a programmer mistake) >>>> - the developper plans to add explicit conformance later and happens to >>>> forget (carelessness) >>>> - a developper extends an existing type with a transient property, and >>>> doesn't add the explicit protocol conformance that has become required. >>>> >>>> Case 2 and 3 bite even experienced developers. And they can't be >>>> improved by learning. >>>> >>>> Looks like the problem is better defined as an ergonomics issue, now. >>>> >>>> If someone can show me something that points to accidental synthesized >>>> implementations being a significant barrier to smooth development in Swift, >>>> I'm more than happy to consider that evidence. But right now, this all >>>> seems hypothetical ("I'm worried that...") and what's being proposed is >>>> adding complexity to the language (an entirely new axis of protocol >>>> conformance) that would (1) solve a problem that may not exist to any great >>>> degree, and (2) does not address the fact that if that problem does indeed >>>> exist, then the same problem just as likely exists with certain >>>> non-synthesized default implementations. >>>> >>>> >>>> There is this sample code by Thorsten Seitz with a cached property >>>> which is quite simple and clear : >>>> https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170911/039684.html >>>> >>>> This is the sample code that had me enter the "worried" camp.' >>>> >>> >>> I really like Thorsten's example, because it actually proves that >>> requiring explicit derivation is NOT the correct approach here. (Let's set >>> aside the fact that Optionals prevent synthesis because we don't have >>> conditional conformances yet, and assume that we've gotten that feature as >>> well for the sake of argument.) >>> >>> Let's look at two scenarios: >>> >>> 1) Imagine I have a value type with a number of simple Equatable >>> properties. In a world where synthesis is explicit, I tell that value type >>> to "derive Equatable". Everything is fine. Later, I decide to add some >>> cache property like in Thorsten's example, and that property just happens >>> to also be Equatable. After doing so, the correct thing to do would be to >>> remove the "derive" part and provide my custom implementation. But if I >>> forget to do that, the synthesized operator still exists and applies to >>> that type. If you're arguing that "derive Equatable" is better because its >>> explicitness prevents errors, you must also accept that there are possibly >>> just as many cases where that explicitness does *not* prevent errors. >>> >>> 2) Imagine I have a value type with 10 Equatable properties and one >>> caching property that also happens to be Equatable. The solution being >>> proposed here says that I'm better off with explicit synthesis because if I >>> conform that type to Equatable without "derive", I get an error, and then I >>> can provide my own custom implementation. But I have to provide that custom >>> implementation *anyway* to ignore the caching property even if we don't >>> make synthesis explicit. Making it explicit hasn't saved me any work—it's >>> only given me a compiler error for a problem that I already knew I needed >>> to resolve. If we tack on Hashable and Codable to that type, then I still >>> have to write a significant amount of boilerplate for those custom >>> operations. Furthermore, if synthesis is explicit, I have *more* work >>> because I have to declare it explicitly even for types where the problem >>> above does not occur. >>> >>> So, making derivation explicit is simply a non-useful dodge that doesn't >>> solve the underlying problem, which is this: Swift's type system currently >>> does not distinguish between Equatable properties that *do* contribute to >>> the "value" of their containing instance vs. Equatable properties that *do >>> not* contribute to the "value" of their containing instance. It's the >>> difference between behavior based on a type and additional business logic >>> implemented on top of those types. >>> >>> So, what I'm trying to encourage people to see is this: saying "there >>> are some cases where synthesis is risky because it's incompatible with >>> certain semantics, so let's make it explicit everywhere" is trying to fix >>> the wrong problem. What we should be looking at is *"how do we give >>> Swift the additional semantic information it needs to make the appropriate >>> decision about what to synthesize?"* >>> >>> That's where concepts like "transient" come in. If I have an >>> Equatable/Hashable/Codable type with 10 properties and one cache property, >>> I *still* want the synthesis for those first 10 properties. I don't want >>> the presence of *one* property to force me to write all of that boilerplate >>> myself. I just want to tell the compiler which properties to ignore. >>> >>> Imagine you're a stranger reading the code to such a type for the first >>> time. Which would be easier for you to quickly understand? The version with >>> custom implementations of ==, hashValue, init(from:), and encode(to:) all >>> covering 10 or more properties that you have to read through to figure out >>> what's being ignored (and make sure that the author has done so correctly), >>> or the version that conforms to those protocols, does not contain a custom >>> implementation, and has each transient property clearly marked? The latter >>> is more concise and "transient" carries semantic weight that gets buried in >>> a handwritten implementation. >>> >>> Here's a fun exercise—you can actually write something like "transient" >>> without any additional language support today: >>> https://gist.github.com/allevato/e1aab2b7b2ced72431c3cf4de71d306d. A >>> big drawback to this Transient type is that it's not as easy to use as an >>> Optional because of the additional sugar that Swift provides for the >>> latter, but one could expand it with some helper properties and methods to >>> sugar it up the best that the language will allow today. >>> >>> I would wager that this concept, either as a wrapper type or as a >>> built-in property attribute, would solve a significant majority of cases >>> where synthesis is viewed to be "risky". If we accept that premise, then we >>> can back to our slice of pie and all we're left with in terms of "risky" >>> types are "types that contain properties that conform to a certain protocol >>> but are not really transient but also shouldn't be included verbatim in >>> synthesized operations". I'm struggling to imagine a type that fits that >>> description, so if they do exist, it's doubtful that they're a common >>> enough problem to warrant introducing more complexity into the protocol >>> conformance system. >>> >>> >>> >>>> >>>> Gwendal >>>> >>>> _______________________________________________ >>> swift-evolution mailing list >>> [email protected] >>> https://lists.swift.org/mailman/listinfo/swift-evolution >>> >>> >>> _______________________________________________ >>> swift-evolution mailing list >>> [email protected] >>> https://lists.swift.org/mailman/listinfo/swift-evolution >>> >>>
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