[swift-evolution] [Proposal] Explicit Synthetic Behaviour

Tony Allevato tony.allevato at gmail.com
Mon Sep 11 14:21:30 CDT 2017

On Mon, Sep 11, 2017 at 11:56 AM Thorsten Seitz via swift-evolution <
swift-evolution at swift.org> wrote:

> I think I do understand Haravikk's argument (actually it seems quite
> straightforward to me).
> An example should be:
> struct Foo : Equatable {
>     var x: Int
>     var cachedLabel: String? = nil
>     init(x: Int) {
>         self.x = x
>     }
>     mutating func label() {
>         if let label = cachedLabel {
>             return label
>         }
>         let label = calculateLabel()
>         cachedLabel = label
>         return cachedLabel
>     }
> }
> var foo1 = Foo(x: 1)
> var foo2 = Foo(x: 1)
> foo1 == foo2 // true
> var label = foo1.label()
> foo1 == foo2 // now false, due to cachedString being falsely included in
> the comparison
> The problem is that the developer was not required to implement the
> protocol and so might forget it.
> The difference to other default implementations is that those use the
> protocol itself as building blocks and so are correct with regards to the
> protocol's semantics, whereas the synthesized equality reaches deeply into
> the private innards of a struct and therefore is much more likely to be
> wrong as in the example above.

The example above wouldn't actually get a synthesized ==, because
Optional<T> doesn't conform to Equatable. We can't get that until we get
conditional conformances.

But if we ignore that, we see that the user has opted-in to the synthesized
== by explicitly claiming that their type conforms to a protocol that
contains a default implementation. Their decision—either by intention or
forgetful omission—to not provide a custom implementation means that the
default behavior will be used, as would any default method implementation.

I don't find the argument that "someone might say that their type conforms
to Equatable to shut the compiler up elsewhere and then forget to fill in
the implementation" very compelling—that's clearly programmer error. The
same could be true for any default method. I could say that my type
conforms to protocol X which comes with a default implementation of foo()
that is inappropriate for my type for some reason and then forget to fill
it in later. The argument that Equatable accesses properties that are not
among its requirements doesn't really make a difference—someone could just
as easily design a protocol X with some non-synthesized default method
implementations that are incorrect for some class of types (for example,
maybe there are preconditions on other requirements that can only be
verified at runtime).

If people want to argue that we need to distinguish between synthesized
default implementations and non-synthesized default implementations, it
would be helpful to provide evidence that there is a significantly large
class of bugs that will be difficult to diagnose and resolve because of
synthesized default implementations that warrants the creation of a
*completely new axis of protocol conformance* that will increase the
complexity of the language.

> Why not just write
>      *struct* Foo : *deriving* Equatable {...}
> to request the synthesized implementation?
> -Thorsten
> Am 09.09.2017 um 19:42 schrieb Xiaodi Wu via swift-evolution <
> swift-evolution at swift.org>:
> On Sat, Sep 9, 2017 at 06:41 Haravikk via swift-evolution <
> swift-evolution at swift.org> wrote:
>> On 9 Sep 2017, at 09:33, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>> On Sat, Sep 9, 2017 at 02:47 Haravikk via swift-evolution <
>> swift-evolution at swift.org> wrote:
>>> On 9 Sep 2017, at 02:02, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>> On Fri, Sep 8, 2017 at 4:00 PM, Itai Ferber via swift-evolution <
>>> swift-evolution at swift.org> wrote:
>>>> On Sep 8, 2017, at 12:46 AM, Haravikk via swift-evolution <
>>>> swift-evolution at swift.org> wrote:
>>>> On 7 Sep 2017, at 22:02, Itai Ferber <iferber at apple.com> wrote:
>>>> protocol Fooable : Equatable { // Equatable is just a simple example
>>>>     var myFoo: Int { get }}
>>>> extension Fooable {
>>>>     static func ==(_ lhs: Self, _ rhs: Self) -> Bool {
>>>>         return lhs.myFoo == rhs.myFoo
>>>>     }}
>>>> struct X : Fooable {
>>>>     let myFoo: Int
>>>>     let myName: String
>>>>     // Whoops, forgot to give an implementation of ==}
>>>> print(X(myFoo: 42, myName: "Alice") == X(myFoo: 42, myName: "Bob")) // true
>>>> This property is *necessary*, but not *sufficient* to provide a
>>>> correct implementation. A default implementation might be able to
>>>> *assume* something about the types that it defines, but it does not
>>>> necessarily know enough.
>>>> Sorry but that's a bit of a contrived example; in this case the
>>>> protocol should *not* implement the equality operator if more
>>>> information may be required to define equality. It should only be
>>>> implemented if the protocol is absolutely clear that .myFoo is the only
>>>> part of a Fooable that can or should be compared as equatable, e.g- if a
>>>> Fooable is a database record and .myFoo is a primary key, the data could
>>>> differ but it would still be a reference to the same record.
>>>> To be clear, I'm not arguing that someone can't create a regular
>>>> default implementation that also makes flawed assumptions, but that
>>>> synthesised/reflective implementations *by their very nature have to*,
>>>> as they cannot under every circumstance guarantee correctness when using
>>>> parts of a concrete type that they know nothing about.
>>>> You can’t argue this both ways:
>>>>    - If you’re arguing this on principle, that in order for
>>>>    synthesized implementations to be correct, they *must* be able to — *under
>>>>    every circumstance* — guarantee correctness, then you have to apply
>>>>    the same reasoning to default protocol implementations. Given a default
>>>>    protocol implementation, it is possible to come up with a (no matter how
>>>>    contrived) case where the default implementation is wrong. Since you’re
>>>>    arguing this *on principle*, you cannot reject contrived examples.
>>>>    - If you are arguing this *in practice*, then you’re going to have
>>>>    to back up your argument with evidence that synthesized examples are more
>>>>    often wrong than default implementations. You can’t declare that
>>>>    synthesized implementations are *by nature* incorrect but allow
>>>>    default implementations to slide because *in practice*, many
>>>>    implementations are allowable. There’s a reason why synthesis passed code
>>>>    review and was accepted: in the majority of cases, synthesis was deemed to
>>>>    be beneficial, and would provide correct behavior. If you are willing to
>>>>    say that yes, sometimes default implementations are wrong but overall
>>>>    they’re correct, you’re going to have to provide hard evidence to back up
>>>>    the opposite case for synthesized implementations. You stated in a previous
>>>>    email that "A synthesised/reflective implementation however may
>>>>    return a result that is simply incorrect, because it is based on
>>>>    assumptions made by the protocol developer, with no input from the
>>>>    developer of the concrete type. In this case the developer must override it
>>>>    in to provide *correct* behaviour." — if you can back this up with
>>>>    evidence (say, taking a survey of a large number of model types and see if
>>>>    in the majority of cases synthesized implementation would be incorrect) to
>>>>    provide a compelling argument, then this is something that we should in
>>>>    that case reconsider.
>>> Well put, and I agree with this position 100%. However, to play devil's
>>> advocate here, let me summarize what I think Haravikk is saying:
>>> I think the "synthesized" part of this is a red herring, if I understand
>>> Haravikk's argument correctly. Instead, it is this:
>>> (1) In principle, it is possible to have a default implementation for a
>>> protocol requirement that produces the correct result--though not
>>> necessarily in the most performant way--for all possible conforming types,
>>> where by conforming we mean that the type respects both the syntactic
>>> requirements (enforced by the compiler) and the semantic requirements
>>> (which may not necessarily be enforceable by the compiler) of the protocol
>>> in question.
>>> (2) However, there exist *some* requirements that, by their very nature,
>>> cannot have default implementations which are guaranteed to produce the
>>> correct result for all conforming types. In Haravikk's view, no default
>>> implementations should be provided in these cases. (I don't necessarily
>>> subscribe to this view in absolute terms, but for the sake of argument
>>> let's grant this premise.)
>>> (3) Equatable, Hashable, and Codable requirements are, by their very
>>> nature, such requirements that cannot have default implementations
>>> guaranteed to be correct for all conforming types. Therefore, they should
>>> not have a default implementation. It just so happens that a default
>>> implementation cannot currently be written in Swift itself and must be
>>> synthesized, but Haravikk's point is that even if they could be written in
>>> native Swift through a hypothetical reflection facility, they should not
>>> be, just as many other protocol requirements currently could have default
>>> implementations written in Swift but should not have them because they
>>> cannot be guaranteed to produce the correct result.
>>> My response to this line of argumentation is as follows:
>>> For any open protocol (i.e., a protocol for which the universe of
>>> possible conforming types cannot be enumerated a priori by the protocol
>>> designer) worthy of being a protocol by the Swift standard ("what useful
>>> thing can you do with such a protocol that you could not without?"), any
>>> sufficiently interesting requirement (i.e., one for which user ergonomics
>>> would measurably benefit from a default implementation) either cannot have
>>> a universally guaranteed correct implementation or has an implementation
>>> which is also going to be the most performant one (which can therefore be a
>>> non-overridable protocol extension method rather than an overridable
>>> protocol requirement with a default implementation).
>>> You're close, but still missing key points:
>>>    1. I am not arguing that features like these should *not* be
>>>    provided, but that they should *not* be provided implicitly, and
>>>    that the developer should actually be allowed to request them. That is
>>>    exactly what this proposal is about, yet no matter what I say everyone
>>>    seems to be treating me like I'm against these features entirely; *I
>>>    am not*.
>> You are entirely against Equatable having a default implementation for
>> ==. This is unequivocally stated. Others favor such a default
>> implementation and feel that in the absence of a way to spell this in Swift
>> itself, it should be magic for the time being. For the purposes of this
>> argument it really is not pertinent that you are not also against something
>> else; you're asking us to discuss why you are against a particular thing
>> that others are for.
>> FFS, how much clearer can I make this? *I AM NOT AGAINST THE FEATURE.*
>> What I am against is the way in which it is being provided implicitly
>> rather than explicitly, in particular as a retroactive change to existing
>> protocols in a way that introduces potential for bugs that are currently
>> impossible, but also in general.
> You are against a default implementation for ==, i.e. an implementation
> that is provided for you if you conform a type to the protocol and do
> nothing else ("implicitly rather than explicitly"), and you are against the
> default implementation being on the existing protocol Equatable
> ("retroactive change"). So, to summarize, what you are against is precisely
> a default implementation for the == requirement on Equatable.
> This is the topic of discussion here; I am attempting to convince you that
> you should be for rather than against these things.
>> As repeatedly answered by others, nothing here is specific to synthesized
>> default implementations, as more powerful reflection will gradually allow
>> them to be non-synthesised.
>> And as repeatedly stated by me; I am not treating synthesised vs.
>> run-time reflection any differently, I specifically included both in the
>> original proposal.
>> As pointed out very cogently by Itai, you assert but offer no evidence,
>> either in principle or empirically, that going too far by reflection is
>> worse than going not far enough without reflection in terms of likelihood
>> of a default implementation being inappropriate for conforming types.
>> As I have also repeatedly pointed out it is not an issue of "not going
>> far enough" vs. "going too far"; if a default implementation lacks
>> information then it should not be provided, doing so regardless is a flaw
>> in the protocol design and not something that this proposal attempts to
>> address (as such a thing is likely impossible).
> Right, one must consider the semantics of the specific protocol
> requirement and ask whether a reasonable default can be provided for it.
> Reflective implementations *necessarily* go too far, because they
>> literally know *nothing* about the concrete type with any certainty,
>> except for the properties that are defined in the protocol (which do not
>> require reflection or synthesis in the first place).
> I am confused why you are trying to argue in general terms about the
> universe of all possible default implementations that use reflection. This
> is necessarily a more difficult argument to make, and if it is to be
> convincing for all default implementations it must also be convincing for
> the two specific protocol requirements we are talking about here. Start
> small:
> We have agreed, as a community, that there is a reasonable default
> implementation for Equatable.== when certain conditions are met (for value
> types only at the moment, I believe). Namely, given two values of a type
> that has only Equatable stored properties, those values are equal if their
> stored properties are all equal. The author of a new value type who wishes
> to make her type Equatable but chooses not to implement a custom == then
> benefits from this default when all stored properties are Equatable.
> 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.
> 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 ==?
> And all of this continues to be a side-issue to the fact that in the
>>> specific case of Equatable/Hashable, which thus far has gone ignored, is
>>> that bolting this on retroactively to an existing protocol *hides bugs*.
>>> The issue of reflective default implementations is less of a concern on
>>> very clearly and well defined *new* protocols, though I still prefer
>>> more, rather than less, control, but in the specific case of *existing* protocols
>>> this fucking about with behaviours is reckless and foolish in the extreme,
>>> yet no-one on the core teams seems willing or able to justify it, which
>>> only opens much wider concerns (how am I to have any faith in Swift's
>>> development if the core team can't or won't justify the creation of new
>>> bugs?).
>> This has emphatically not gone ignored, as I have myself responded to
>> this point in an earlier thread in which you commented, as well as many
>> others. Crucially, no existing conforming type changes its behavior, as
>> they have all had to implement these requirements themselves. And as I said
>> to you already, the addition of a synthesized default implementation no
>> more "hides bugs" going forward than the addition of a non-synthesized
>> default implementation to an existing protocol, and we do that with some
>> frequency without even Swift Evolution review.
>> Feel free to a supply a non-synthesised default implementation for
>> Equatable without the use of reflection. Go-on, I'll wait.
>> You insist on suggesting these are the same thing, yet if you can't
>> provide one then clearly they are not.
> That is not the argument. The argument is that they are indistinguishable
> in the sense that the author of a type who intends to supply a custom
> implementation but neglects to do so will have a default implementation
> supplied for them. It is plainly true that this is no more or less likely
> to happen simply because the default implementation is synthesized.
> 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.
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