[swift-evolution] [Proposal] Explicit Synthetic Behaviour
Vladimir.S
svabox at gmail.com
Wed Sep 13 10:40:46 CDT 2017
On 13.09.2017 7:14, Xiaodi Wu via swift-evolution wrote:
>
> On Tue, Sep 12, 2017 at 22:07 Tony Allevato <tony.allevato at gmail.com
> <mailto:tony.allevato at gmail.com>> wrote:
>
> On Tue, Sep 12, 2017 at 7:10 PM Xiaodi Wu <xiaodi.wu at gmail.com
> <mailto:xiaodi.wu at gmail.com>> wrote:
>
> On Tue, Sep 12, 2017 at 9:58 AM, Thorsten Seitz via swift-evolution
> <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> 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.
>
>
> RightБ─■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 protocolsБ─■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.
Actually, not taking into account a question of explicit marker for auto-generated
methods, this is IMO a great point.
Codable, which can auto-generate methods, *had* these CodingKeys from the moment of
birth. Currently, we have a proposal for auto-generating of methods for
Equatable/Hashable. Why we don't have a EquatingKeys/HashingKeys option for them in
symmetry with Codable? Why Codable already has a method to exclude fields, but for
Equatable/Hashable we are discussing some future esoteric '@transient' modifier(which
should describe the behaviour and destination of the property in details for compiler
and conformed protocols so all will "just work") ?
How this future '@transient' will live together with current CodingKeys ?
IMO the right solution will be:
1. introduce 'deriving'-like keyword to explicitly express that you request an
auto-synthesize of protocol requirements
2. introduce EquatingKeys/HashingKeys to be able to say which properties should be
included in generated requirements
3. Think what kind of '@transient' marker could be introduced in future to replace
the using of CodingKeys/EquatingKeys/HashingKeys.
Vladimir.
>
> -Thorsten
>
> Am 12.09.2017 um 16:38 schrieb Tony Allevato via swift-evolution
> <swift-evolution at swift.org <mailto:swift-evolution at swift.org>>:
>
>>
>>
>> On Mon, Sep 11, 2017 at 10:05 PM Gwendal Rouц╘ <gwendal.roue at gmail.com
>> <mailto:gwendal.roue at gmail.com>> 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 thatБ─■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 avoidableБ─■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 workБ─■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 exerciseБ─■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
>>
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