[swift-evolution] [Proposal] Explicit Synthetic Behaviour

Thu Sep 7 16:02:21 CDT 2017

> On Sep 7, 2017, at 1:43 PM, Haravikk via swift-evolution <swift-evolution at swift.org> wrote:
> 
>> 
>> On 7 Sep 2017, at 19:36, Tony Allevato <tony.allevato at gmail.com <mailto:tony.allevato at gmail.com>> wrote:
>> 
>> 
>> 
>> On Thu, Sep 7, 2017 at 11:18 AM Haravikk via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>>> On 7 Sep 2017, at 18:53, Tony Allevato via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>> 
>>> 
>>> On Thu, Sep 7, 2017 at 10:39 AM Gwendal Roué <gwendal.roue at gmail.com <mailto:gwendal.roue at gmail.com>> wrote:
>>>> Le 7 sept. 2017 à 14:45, Tony Allevato <tony.allevato at gmail.com <mailto:tony.allevato at gmail.com>> a écrit :
>>>> 
>>>> Right, let's make sure we're talking about the right thing here. Gwendal, your issue isn't with synthesis in the form of Codable or the new additions to Equatable/Hashable which are opt-in-by-conformance, it's with the specific case of raw value enums or enums without associated values where the synthesis is implicit with no way to opt-out. That's a big difference.
>>> 
>>> Yes.
>>> 
>>>> I can definitely see the latter being an issue if it were more widespread, and I'd be supportive of those enums being required to declare their conformance for consistency (though it would be source breaking).
>>> 
>>> Yes, unfortunately.
>>> 
>>> 
>>>> However, I still haven't seen a real issue that has come up because of the distinction being drawn here between default implementations vs. implementations that can access other parts of the concrete type. It sounds like this discussion is trying to protect against a hypothetical problem that hasn't happened yet and may not happen; it would be helpful to show some motivating real-world cases where this is indeed a severe problem.
>>> 
>>> Yes. I'm not talking about implementation itself. I know this has been the main topic until I have tried to bring in the topic of the consequences of non-avoidable synthesis (extra methods that may conflict with userland methods).
>>> 
>>> If you ask me for a real-world case, then I think I gave one. Let me rephrase it:
>>> 
>>> it's impossible to define a value-backed enum without getting free Equatable conformance. This free conformance is sometimes unwanted, and I gave the example of DSLs. Now this problem is not *severe*. It's more a blind spot in the language, and finally just an unwanted side-effect of a compiler convenience,
>>> 
>>> Again, this is not the issue that Haravikk is describing in this thread.
>>> 
>>> I'll clarify—your issue is specifically with the fact that enums with raw values and enums without associated values receive Equatable even without explicitly conforming to it, and therefore users have no way of opting out of it. This predates SE-0185, and I didn't propose making any changes to the conformance of those enums for source compatibility reasons, though I wouldn't be opposed to it because it makes them consistent across the board.
>>> 
>>> Haravikk's argument is about synthesized conformances like Codable and Equatable/Hashable in SE-0185, where the user must explicitly conform the type to those protocols. His claim is that that act of opting in is not sufficient and that it is still dangerous if those synthesized conformances can access members that are not also declared in the protocol. That's a completely separate issue to yours, and one that I hope he'll present more evidence of. Right now, requiring that you not only explicitly conform to the protocol but also explicitly request the synthesis feels like a solution without an actual problem, and is a situation we already have today with default method implementations.
>> 
>> The simplest real-world case is easy:
>> 
>> 	struct Foo { var data:String }
>> 	extension Foo : Equatable {} // This currently produces an error, in future it will not
>> 
>> 
>> Why is this a problem? It's no different than if someone extended Foo to conform to a protocol with a default implementation that was written in code.
> 
> I'm sorry but I have now explained why it isn't multiple times; a non-reflective default conformance can ONLY act upon methods and properties that the protocol itself has defined, meaning that it knows everything it needs to know in order to do whatever it wants to do with those methods and properties because it defined them.
Just because it might have defined the properties does not necessarily mean that those properties are sufficient context for providing a default implementation:
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.

> Reflective/synthesised default implementations must by their very nature make assumptions about a concrete type that are not cannot be guaranteed to be correct. The properties and methods they may end up interacting with may have nothing at all to do with the protocol. Equatable remains by far the simplest example; just because a developer has used equatable properties does not guarantee that all of them should be compared during a check for equality.
In the same way that you might consider synthesized conformances to overreach into a type and touch things which are not related to a protocol, default implementations can be considered underreach in that they don’t know anything about properties which are necessary for providing a correct implementation.

> These things are two very different beasts.
I think they’re very much two sides of the same coin.

> While a developer may wish to override a default implementation, it should only be to provide optimisations that a protocol cannot; e.g- providing a logarithmic time search to replace to a linear time search. The end result however should be the same; i.e- there is no change in behaviour, only detail.
> 
> 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.
> 
>> That's not a fair characterization. Just because your concerns were disagreed with does not mean they were ignored; my understanding is that the core team views these synthesized conformances as a different kind of default method (and one which could be hoisted out of the compiler once sufficient metaprogramming facilities are available).
>> 
>> The way to handle synthesized conformances was discussed during the review period for Codable, during the earlier pitch a few months ago for what became SE-0185, and again during its formal review. It's not accurate to reduce the argument you disagree with to "but Codable does it" when what you're referring to is established precedent based on those prior discussions.
> 
> If that is the case then surely someone, including members of the core team, could have easily summarised what the rationale behind that was and why they think it applies to Equatable/Hashable. They did not, hence, ignored. I even framed my argument purely on the basis of why the Equatable/Hashable case differs from Codable and still nothing. It is absolutely fair for me to characterise that as being ignored.
> 
>> I feel like we keep going back to this, but this statement applies equally to non-synthesized default implementations. Are you suggesting that users should have to opt-in specifically to all default implementations provided by a protocol in some way beyond merely conforming to that protocol? If not, what specifically makes synthesized conformances a special case?
> 
> As mentioned, this is a special case because the behaviour being provided is not constrained to the protocol that provides it; it is invasive of a concrete type, and by its very nature can potentially produce incorrect results.
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