[swift-evolution] [Pitch] consistent public access modifiers

[Matthew Johnson]

> On Feb 15, 2017, at 10:35 AM, Rien <Rien at Balancingrock.nl> wrote:
> 
>> 
>> On 15 Feb 2017, at 17:02, Matthew Johnson <matthew at anandabits.com> wrote:
>> 
>>> 
>>> On Feb 15, 2017, at 9:59 AM, Rien <Rien at Balancingrock.nl> wrote:
>>> 
>>>> 
>>>> On 15 Feb 2017, at 16:45, Matthew Johnson <matthew at anandabits.com> wrote:
>>>> 
>>>>> 
>>>>> On Feb 15, 2017, at 9:35 AM, Rien <Rien at Balancingrock.nl> wrote:
>>>>> 
>>>>> 
>>>>>> On 15 Feb 2017, at 16:11, Matthew Johnson via swift-evolution <swift-evolution at swift.org> wrote:
>>>>>> 
>>>>>> 
>>>>>>> On Feb 15, 2017, at 5:59 AM, Jeremy Pereira via swift-evolution <swift-evolution at swift.org> wrote:
>>>>>>> 
>>>>>>> 
>>>>>>>> On 15 Feb 2017, at 11:11, Brent Royal-Gordon via swift-evolution <swift-evolution at swift.org> wrote:
>>>>>>>> 
>>>>>>>> 
>>>>>>>> Our philosophy in general, however, is to default to the behavior which preserves the most flexibility for the library designer.
>>>>>>> 
>>>>>>> Actually, I thought the philosophy was to preserver type safety. When did that change?
>>>>>>> 
>>>>>>> Also, when was the library designer prioritised ahead of the application developer?
>>>>>>> 
>>>>>>> 
>>>>>>>> Both open and non-open classes are common, but we chose to give non-open classes the `public` keyword because that's the flexibility-preserving option.
>>>>>>> 
>>>>>>> No it isn’t, it’s the flexibility restricting option. The consumer of an open class can subclass it. The consumer of a public class cannot subclass it. How is the second more flexible than the first?
>>>>>> 
>>>>>> It reduces complexity for the library author by allowing them to opt-out of the complexity involved in supporting unknown, user-defined subclasses.  It is important to allow libraries to have this flexibility. They are free to declare a class `open` if they want to allow subclassing. It’s even possibly for a library to declare all classes `open` if it wishes to do so.  But *requiring* that would reduce the design space libraries are allowed to explore and / or introduce fragility by moving the subclass restriction to a comment.
>>>>>> 
>>>>> 
>>>>> Why would a library author want to prohibit subclasses?
>>>>> A library user can always wrap the class and subclass the wrapper.
>>>> 
>>>> This is composition, not inheritance.  The most important difference is that a wrapper cannot override methods, it can only wrap and / or forward them.  This means that when the superclass calls a method on `self` that method *always* invokes its version of that method rather than a subclass override.  This is a very important difference.
>>>> 
>>> 
>>> Agreed, however that does not answer the question why would a library developer want to disallow subclassing?
>>> I do not see a use case for that. I.e. a feature that cannot be implemented without it. (without “open”)
>> 
>> The feature it enables is more robust libraries and the ability for library authors to better reason about their code.  You may not find this benefit enough to be worth a language feature, but many of us do.
> 
> You start of with a claim “more robust libraries”.
> I would really like to know the “how” of that. How does it make a library more robust?
> 
> I do write libraries myself, and if there is something I am missing, I very much would like to know.

This topic was well explored during the discussion and review of the proposal that introduced `open`.  If you would really like to know I suggest you take some time to read through that discussion.

> 
> Regards,
> Rien.
> 
>> 
>>> 
>>> Rien.
>>> 
>>>>> 
>>>>> There are cases where subclassing does not make sense. And thus preventing subclasses adds information for those users that don’t RTFM. But that imo is not worth the impact extra complexity places on all other users.
>>>>> 
>>>>> Rien.
>>>>> 
>>>>>>> 
>>>>>>> 
>>>>>>> _______________________________________________
>>>>>>> swift-evolution mailing list
>>>>>>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>>>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>
>>>>>> 
>>>>>> _______________________________________________
>>>>>> swift-evolution mailing list
>>>>>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20170215/19206abf/attachment.html>