[swift-evolution] [swift-evolution-announce] [Review #2] SE-0117: Default classes to be non-subclassable publicly

[John McCall]

> On Jul 18, 2016, at 9:31 AM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org> wrote:
> 
> On Mon, Jul 18, 2016 at 11:24 AM, Taras Zakharko via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> 
> > On 18 Jul 2016, at 14:07, Károly Lőrentey via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> >
> >
> > I see no drawback to this pattern; it is quite clear and simple. Therefore, in the interest of keeping the language free of needless complexity, I suggest we change the proposal to remove the implicit "sealed" level of public member overridability, and support only "open" or "final" class members.
> 
> 
> At the same time, your solution results in a lot of unnecessary boilerplate.
>  
> It's an exaggeration to say that it's *a lot* of boilerplate. It's one line or two in the base class.

The basic effect of Károly's counter-proposal is that every public member of an open class has to be marked either "open" or "final".  That's boilerplate.

I think you and Károly are evaluating the addition of non-open methods as if they were being added primarily to increase expressive capabilities.  They do marginally increase expressiveness, but I agree that it's not a common situation to explicitly want.  However, neither are non-open classes.  The goal here is not to create new expressive power, it's to establish a comprehensible intermediate position that's acceptable as a default so that publicizing an API doesn't require so much annotation and bookkeeping.  Otherwise, programmers are forced to immediately decide between over-promising (by making the method publicly overridable) or breaking their own code (if they have internal overrides).

Furthermore, I don't agree that non-open methods add significant new complexity.  For clients of a library, a non-open method is final; there are no semantically-detectable differences (ignoring covariant overrides).  Within a library, non-open methods remove the need for some unnecessary bookkeeping.  And just on a conceptual level, the analogy to class behavior is quite simple.

John.

>  
> Sure, it might be rare with methods, but don’t forget about properties! It makes perfect sense to have properties that should be only overridable internally while being accessible publicly.
> 
> My first reaction here was: of course, good point! But then, on reflection, what properties should behave this way? Can you give an example of a property that makes sense to override in internal subclasses but not in external subclasses, but that must be accessible publicly?
>  
> Imagine adding that boilerplate to every such property..
> 
> On balance, I think the number of `open` annotations would far exceed the amount of this boilerplate. I'm not convinced it is even a mildly common use case.
> 
> 
> Basically, if we do it your way, then it won’t be long that someone submits a proposal for a keyword for synthesising the boilerplate, which more or less brings us back to square one.
> 
> T.
> 
> 
> 
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