> On Feb 15, 2017, at 8:35 AM, Rien via swift-evolution
> <[email protected]> wrote:
>
>>
>> On 15 Feb 2017, at 17:02, Matthew Johnson <[email protected]> wrote:
>>
>>>
>>> On Feb 15, 2017, at 9:59 AM, Rien <[email protected]> wrote:
>>>
>>>>
>>>> On 15 Feb 2017, at 16:45, Matthew Johnson <[email protected]> wrote:
>>>>
>>>>>
>>>>> On Feb 15, 2017, at 9:35 AM, Rien <[email protected]> wrote:
>>>>>
>>>>>
>>>>>> On 15 Feb 2017, at 16:11, Matthew Johnson via swift-evolution
>>>>>> <[email protected]> wrote:
>>>>>>
>>>>>>
>>>>>>> On Feb 15, 2017, at 5:59 AM, Jeremy Pereira via swift-evolution
>>>>>>> <[email protected]> wrote:
>>>>>>>
>>>>>>>
>>>>>>>> On 15 Feb 2017, at 11:11, Brent Royal-Gordon via swift-evolution
>>>>>>>> <[email protected]> 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.
>
> Regards,
> Rien.
The usual thing I've run into in practice follows this general pattern:
• A non-threadsafe class is introduced
• Years or decades pass
• Someone (me) tries to make the class threadsafe by adding locking
• In the intervening years, subclasses have been introduced that override
methods now called while holding a lock, and reentrantly call back into other
methods that take the lock
I've worked around this using recursive mutexes in some cases, but in more
extreme cases the subclass synchronously called out to another thread, and that
thread reentered the lock.
A defensive rule I've adopted in some new code is "public methods are not
allowed to call other public methods", which avoids reentrancy issues, but also
makes subclassing somewhat useless since the work is being done by private
non-overridable methods.
David
>
>>
>>>
>>> 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
>>>>>>> [email protected]
>>>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>>>
>>>>>> _______________________________________________
>>>>>> swift-evolution mailing list
>>>>>> [email protected]
>>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>
> _______________________________________________
> swift-evolution mailing list
> [email protected]
> https://lists.swift.org/mailman/listinfo/swift-evolution
_______________________________________________
swift-evolution mailing list
[email protected]
https://lists.swift.org/mailman/listinfo/swift-evolution
