[swift-evolution] Keyword for protocol conformance

[Xiaodi Wu]

Chris, as I wrote above, this idea has been discussed previously on several
occasions, and the principal reason why it has been abandoned each time is
that it cannot accommodate retroactive modeling. I'll refer you again to
three previous threads discussing this and closely related matters:

https://lists.swift.org/pipermail/swift-evolution/
Week-of-Mon-20160104/005380.html
https://lists.swift.org/pipermail/swift-evolution/
Week-of-Mon-20160229/011792.html
https://lists.swift.org/pipermail/swift-evolution/
Week-of-Mon-20160425/015920.html

If, after familiarizing yourself with the previous discussions, you feel
like there is an aspect of the topic that has not yet been considered, then
of course feel free to start another discussion.


On Thu, Aug 25, 2016 at 5:36 PM, Christopher Kornher via swift-evolution <
swift-evolution at swift.org> wrote:

>
> On Aug 25, 2016, at 4:32 PM, Christopher Kornher <ckornher at me.com> wrote:
>
> I hope that my brief email wasn’t the final word on this :(
>
> Does anyone think that a proposal to require “override” for overrides of
> protocol extension methods and variables (?) would be worth exploring?
>
> This would address a number of the issues raised in this thread, but not
> all.
>
> My intention is not to derail this thread, and I believe that the original
> topic of this thread, explicit protocol conformance, is worth exploring. I
> respond here because of these topics are strongly related. I will start a
> new thread, if there is interest.
>
> It is possible that tooling could address the remaining issues with
> overrides. JetBrains products, for example, provide navigable icons to
> overridden methods. There is no reason that Xcode and other tools could not
> automatically insert indicators like:
>
> ```
> class MyClass: MyProtocol {
>
> < conforms >
> func myMethod() -> String {
>          return “..."
>     }
>
> class MyClass2: MyProtocol1, MyProtocol2 {
>
> < conforms MyProtocol2 >
> func myMethod() -> String {
>         return “..."
>     }
> ```
> Of course, there is no reason that tooling can’t replace “override”
> completely:
>
> ```
> class MyClass: MyOtherClass, MyProtocol1, MyProtocol2 {
>
> < conforms MyProtocol2 >
> < overrides TheParentClassOfMyOtherClass>
> func myMethod() -> String {
>         return “..."
>     }
> ```
>
> Replace "<…>” with the navigable graphic of your choice and assume that
> user preferences would exist to show/hide these indicators.
>
>
>
> My cut and paste error left out this part:
>
> The roles of language and tooling in making code understandable is largely
> a matter of opinion. I personally like the current use of “override” since
> overriding is a common cause of serious programming errors and a mechanism
> for protecting against them should be built into the language and now,
> extended to overrides of protocol extensions.
>
> It is interesting that adding “override” to structs and enums would help
> to open the door to true inheritance (not polymorphism) for structs and
> possibly even enums, if anyone knows of a use for that. This is a feature
> that I could have used on a few projects, but there are far more important
> features in the queue, and that discussion should probably wait until after
> Swift 4, if it happens at all.
>
>
> - Chris K
>
>
>
> On Aug 24, 2016, at 11:13 AM, Christopher Kornher via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> Requiring "override" when anything overrides a method defined in a
> protocol extension should be added - structure and enumerated included, of
> course.
>
> Protocol extensions added inheritance to structs and enums and this should
> be made explicit.
>
> Sent from my iPad
>
> On Aug 24, 2016, at 12:55 AM, Charles Srstka via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> On Aug 24, 2016, at 1:20 AM, Robert Widmann <devteam.codafi at gmail.com>
> wrote:
>
> 2016/08/23 20:52、Charles Srstka <cocoadev at charlessoft.com> のメッセージ:
>
> On Aug 23, 2016, at 10:34 PM, Robert Widmann <devteam.codafi at gmail.com>
> wrote:
>
>
> 2016/08/23 15:29、Charles Srstka <cocoadev at charlessoft.com> のメッセージ:
>
> On Aug 23, 2016, at 2:33 PM, Robert Widmann via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>
> 2016/08/22 14:30、David Cordero via swift-evolution <
> swift-evolution at swift.org> のメッセージ:
>
>
> *The problem:*
> At the moment, looking at the code of a class or a struct implementing a
> protocol, it is hard to know what methods are actually implementing the
> protocol and what other methods are just part of the code of the class or
> struct.
>
>
> That seems like a feature, not a bug.  Why should I as an author care
> whether a method contributes to a protocol conformance or not if the
> compiler can tell me that kind of information itself?
>
>
> Being able to reason about your code, what it does, and what it’s for is
> undesirable?
>
>
> That's not an answer to the question I asked.  Why is this significant
> enough to warrant an entire keyword?  The clutter of a whole keyword that
> does nothing but wait for a developer to make a source-compatible
> binary-breaking change to an interface does not seem worth it.  Maybe you
> can convince me otherwise.
>
>
> Same reason overriding a class method warrants a keyword. It expresses the
> purpose more clearly, and allows the compiler to catch mistakes for us.
>
>
> That's just it: The class of mistakes one can make by not being explicit
> about overrides is significantly more dangerous than the class of mistakes
> caused by dead code leftover from trimming protocols.
>
>
> I am in the middle of a large refactor of code that was originally
> Objective-C and then Swift written like Objective-C, to more idiomatic
> protocol-oriented Swift. I am finding that in Swift’s POP idiom, protocols
> with overrides are serving very nearly the same purpose that overrides were
> serving in the old design; hence, I don’t really think either is more or
> less dangerous than the other.
>
> ```
>
> protocol MyProtocol {
>     func myMethod() -> String
> }
>
> class MyClass: MyProtocol {
>
>     *conform* func myMethod() -> String {
>         return "Yuhuuu,I am conforming \\o//"
>     }
>
>     func whatever() {
>         print("I am a boring method and I don't conform anything")
>     }
> }
> ```
>
> It would be something similar to the current keyword `override` but for
> protocols.
>
> Apart from improving code readability, It would allow the detection, in
> compilation time, of errors due to code evolution. For example redundant
> methods that no longer conform anything once the requirement is removed
> from the protocol for whatever reason.
>
>
> If you make a breaking change to a protocol like this, you should have
> gone through a deprecation cycle to indicate to your clients the
> appropriate changes you're going to make to the protocol.  This aspect of
> the change seems to if not encourage, highlight, bad behavior.
>
>
> What if it’s your own code and all the callers are internal? What if
> you’re still developing the protocol and haven’t released the API interface
> yet?
>
>
> Then your concerns are local enough that you know where *all* implementations
> of the protocol lie and whether they require deletion or not.  The point
> about deprecation cycles still stands in all the cases you mention.  Just
> because the interface is private doesn't mean you can't take responsibility
> for keeping it as clean as you can.
>
>
> Charles
>
>
> tl;dr It seems like all of this can be subsumed by us warning about dead
> code.
>
>
> Did you look at my examples earlier in the thread? Neither of those would
> be caught by warning about dead code.
>
>
> The example involving the default implementation is most compelling, but
> it indicates that your proposed solution should focus on the protocol
> extension and not the implementing declaration.  Perhaps reusing one of our
> existing keywords can help here
>
> protocol P {
>   func foo() {}
> }
>
> extension P {
>   default func foo() {}
> }
>
> struct S: P {}
>
> Of course, this change would be potentially source-breaking either way - I
> don't like the sound of an "optional keyword”.
>
>
> I can come up with a similar example without the mistake being in the
> extension, though:
>
> protocol P {
> func foo() {}
> }
>
> extension P {
> func foo() { print(“Default Behavior”) }
> }
>
> struct S: P {
> func foo() { print(“Specific Behavior”) }
> }
>
> So far, so good. But now I realize that the original protocol needs an
> argument:
>
> protocol P {
> func foo(bar: String) {}
> }
>
> extension P {
> func foo(bar: String) { print(“Default Behavior; bar is \(bar)”) }
> }
>
> struct S: P {
> func foo() { print(“Specific Behavior”) } // Whoops, forgot to update this
> method, and now it won’t get called—and we of course won’t see the
> carnage until runtime.
> }
>
> Either way, we can all agree we need better diagnostics around these cases.
>
>
> No doubt.
>
> Charles
>
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