[swift-evolution] [Proposal] Sealed classes by default

Michael Peternell michael.peternell at gmx.at
Tue Jun 28 14:39:42 CDT 2016

Sealing classes by default is a terrible idea IMHO. Fortunately, my faith in the Swift core team is strong enough so that I cannot believe that this has a chance of ever being implemented at all :)

Why do I think it's terrible? I do subclass classes even when they say that you shouldn't do it. I even monkey-patched a few classes in the past. Why? Not because I prefer hacking over clean coding, but to get the job done. Every few months or so I encounter a situation where such hacking attempts are the only feasible way to make something work. Or I use them during research activities or unofficial "feasibility studies". (If that's not the case for you, then maybe you are working in a different job than I. Monkey patching is rarely necessary if you make iPhone apps.) These are situations where everyone else just says "that's not possible". Yes, you can make that USB driver class stop spamming the console; yes, you can stop that library from crashing randomly.

There is so much more to say on this topic. Sealing classes is also bad because it prevents forms of experimental coding. It doesn't solve a real problem. How often have I seen bugs where someone subclassed a class that shouldn't be subclassed? That's so rare. But if all classes are sealed by default this will lead to far worse problems. When a developer subclasses a class, he usually has a reason. There already is a `final` keyword in Swift, and a dev really thinks that a class shouldn't be subclass-able, he can use it already. And if I subclass it from Objective-C anyways, then maybe I know what I'm doing?

And I'm not buying this "performance" argument. If you want better performance, write the hot functions in C or maybe even in assembler. The dynamic dispatch overhead is not the feature that makes programs bad or slow. It's quadratic algorithms (e.g. unnecessary nested loops), calling functions multiple times (e.g. if(a.foo.bar(x[2]) == "joo" || a.foo.bar(x[2]) == "fooo" || a.foo.bar(x[2]) == nil) { ... }), or just overly complex code: all of them have more impact than dynamic dispatch. Even worse for performance is unnecessary IO or using graphics APIs inefficiently. A good profiler can help a lot with these issues.

Swift should be designed for the real world, not for an ideal world that does not exist and that will never exist.


> Am 28.06.2016 um 15:09 schrieb L. Mihalkovic via swift-evolution <swift-evolution at swift.org>:
> Regards
> LM
> (From mobile)
>> On Jun 28, 2016, at 1:57 PM, Alejandro Martinez via swift-evolution <swift-evolution at swift.org> wrote:
>> Anton Zhilin: That is one of the points if I’m not mistaken. Sealed
>> means that with whole-module-optimization the compiler can optimise
>> more things as it can treat it as final for other modules.
>> L. Mihalkovic: Could you give an example of what exactly do you mean?
>> I know one of the reasons behind the proposal is to actually improve
>> those situations by forcing us to think more on customisation when
>> designing APIs.
> In many situation it has been my experience that libraries can be extended DESPITE their authors, rather than only thanks to the skills the authors  have shown in planning for the future. It is what happened to me with AlamoFire, where i was able to extend it because some cracks existed which let me do something the designers did not think about (to me it was a lack of imagination to not have anticipated what i wanted to do).
> So if this can happen with a lib made by very experienced/talented developers, then imagine what happens with far less skilled developers.. it will mean having to copy code in order extend. It may sound pessimistic, but if u had seen as much bad code as i have seen, you might understand the view i am sharing.
> What's worse is that objc is not particularly conducive to good software architecture (it is a bit like perl and javascript where anything can be touched from anywhere, and has a limited set of constructs compared to c++, scala, java, c#, swift). So i do not believe that it has prepared objc devs to suddenly become great code designers with a language that has multiples levels of scoping/nesting (i would not use most of the swift libs i have seen on github to teach software design).
> Hence my conclusion that defaulting to sealed may be giving too much credit to the code that is currently available for reuse.
>> On Tue, Jun 28, 2016 at 12:44 PM, Anton Zhilin via swift-evolution
>> <swift-evolution at swift.org> wrote:
>>> Does `sealed` allow for any optimizations? Maybe somehow devirtualizing
>>> method calls?
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>> -- 
>> Alejandro Martinez
>> http://alejandromp.com
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