[swift-evolution] [Pitch] Requiring proactive overrides for default protocol implementations.

[Douglas Gregor]

> On Apr 27, 2016, at 12:31 PM, Erica Sadun <erica at ericasadun.com> wrote:
> 
> On Apr 27, 2016, at 12:25 PM, Douglas Gregor <dgregor at apple.com <mailto:dgregor at apple.com>> wrote:
>> 
>> On Apr 27, 2016, at 10:10 AM, Erica Sadun <erica at ericasadun.com <mailto:erica at ericasadun.com>> wrote:
>>> 
>>> From the Swift Programming Language: Methods on a subclass that override the superclass’s implementation are marked with override—overriding a method by accident, without override, is detected by the compiler as an error. The compiler also detects methods with override that don’t actually override any method in the superclass.
>>> 
>>> I would like to extend this cautious approach to protocols, forcing the developer to deliberately override an implementation that’s inherited from a protocol extension. This would prevent accidental overrides and force the user to proactively choose to implement a version of a protocol member that already exists in the protocol extension.
>>> 
>>> I envision this as using the same `override` keyword that’s used in class based inheritance but extend it to protocol inheritance:
>>> 
>>> protocol A {
>>>     func foo()
>>> }
>>> 
>>> extension A {
>>>     func foo() { .. default implementation … }
>>> }
>>> 
>>> type B: A {
>>> 
>>>     override required func foo () { … overrides implementation … }
>>> }
>> 
>> A couple questions about your pitch:
>> 
>> 1) What is “required” doing there? 
> 
> I threw it in not because I’m tied to it but because I wanted it to be part of the conversation.
> This is a requirement from conforming to the protocol.

Ah. Note that this isn’t *quite* the meaning of “required” for initializers of classes (where it means “my subclasses must override this”), but in practice it’s basically the only reason why anyone uses “required” for initializers.

> 
>> 2) Is “override” only required when there is a default implementation of the protocol requirement, or is it required whenever you are implementing a protocol requirement?
> 
> Override is only because it is overriding the default implementation of the protocol requirement. Without that default implementation there would be no override, it would simply be satisfying the requirement.

For me, this doesn’t provide additional value of “required”: i.e., the value of having a keyword here is in telling me that I failed to implement a requirement when I’ve clearly said that I wanted to implement a requirement. Whether there was a default there or not isn’t really very interesting. Plus, a default could be added later to a requirement that I implement: that change has zero impact on how my code works (before or after), but now I’d be require to add an “override” keyword when I recompile.

Contrast that with classes: if you recompile against a new version of a library and the compiler tells you that you need to add “override”, it’s serious because the semantics of your program will change if you’re now overriding something that you weren’t before.

> 
>> 	* If the former, it might be the case that it’s too easy to forget to add the “override” keyword (because it’s needed for some implementations of protocol requirements but not others), which undercuts the value of having it.
> 
> Forcing the override keyword makes it clear at the definition point that the story extends beyond the method or whatever to point to a default implementation that is being replaced. I *really* like having that reference in terms of both code construction (“I am doing this as a deliberate act”) with the compiler complaining otherwise, and in terms of code self documentation (“I know this was added deliberately, what default did it override?”)

I see the former (“I am doing this as a deliberate act”)  as a very common complaint; the latter not nearly as much. What motivates that? And does it justify adding a *second* keyword to these declarations?

>> I’d also like to bring up two related topics, although they probably should at some point move to their own thread if they have any legs:
>>> 
>>> Related topic 1: How should a consumer handle a situation where two unrelated protocols both require the same member and offer different default implementations. Can they specify which implementation to accept or somehow run both? 
>>> 
>>> type B: A, C {
>>>     override required func foo() { A.foo(); C.foo() }
>>> }
>> 
>> I think the right answer here is for the compiler to produce an ambiguity if you don’t implement the requirement yourself, and then solving your “related topic 2” lets you choose which implementation you want.
> 
> How do you choose which one? What syntax? For example:
> 
> required func foo = A.foo
> 
> would be the simplest approach

type B: A, C {
  override required func foo() { A.foo(self)() }
}

> 
>> 
>>> Related topic 2: How can a consumer “inherit” the behavior of the default implementation (like calling super.foo() in classes) and then extend that behavior further. This is a bit similar to how the initialization chaining works. I’d like to be able to call A.foo() and then add custom follow-on behavior rather than entirely replacing the behavior.
>>> 
>>> type B: A {
>>>     override required func foo() { A.foo(); … my custom behavior … }
>>> }
>> 
>> Seems totally reasonable to me. One ugly syntax: A.foo(self)(), leveraging the currying of self?
> 
> Ugly but it would pretty much do it for me. It offers an expressive way to say “Please execute the A.foo behavior using the self instance”. Does 3 still support this?

Probably not? I actually don’t know ;)

	- Doug

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20160427/768e1944/attachment.html>