[swift-evolution] Proposal: Add @requires_super attribute

[Marc Knaup]

"required" also doesn't mean that a subclass has to implement the required
initializer since it can be inherited.
Your example is an abstract function which should have it's own keyword (if
we ever get abstract functions).

On Wed, Dec 16, 2015 at 7:37 PM, Vester Gottfried <
vester.gottfried at gmail.com> wrote:

> I think reusing required would send the wrong message. Required would mean
> for me something like NSOperation subclasses maybe require to have a main()
> function, but that doesn't mean you have to call super. On the contrary,
> the documentation of NSOperation main() explicitly states not to call
> super.
>
> On Wed, Dec 16, 2015 at 7:08 PM, Marc Knaup <marc at knaup.koeln> wrote:
>
>> What about re-using the "required" keyword in the superclass which
>> already means something similar for initializers?
>> Subclass implementations are required to call super's implementation.
>> If a subclass doesn't implemented the required method it could mean that
>> it inherits the behavior from the superclass - just like initializers can
>> be inherited too.
>>
>> On Wed, Dec 16, 2015 at 7:02 PM, Jordan Rose <jordan_rose at apple.com>
>> wrote:
>>
>>> +1 from me. FWIW, the Objective-C one is syntactic.
>>>
>>> Information from Radar: the request for this is rdar://problem/17408107
>>> (plus a few duplicates). One of the dups suggests a variation where a
>>> subclass method can be declared as "refine" instead of "override" so that
>>> you can document that your *own* method is expected to call super. In
>>> this model, "@requires_super" could become something like "imposed". I
>>> personally think this doesn't add enough, especially since we wouldn't be
>>> publishing refine-vs-override in a library's public interface.
>>>
>>> Jordan
>>>
>>> On Dec 16, 2015, at 9:49 , Marc Knaup via swift-evolution <
>>> swift-evolution at swift.org> wrote:
>>>
>>> Sounds reasonable since even the best flow analysis cannot ensure that
>>> all codepaths call the super implementation.
>>>
>>> Some more edge cases:
>>>
>>>    - Calling super asynchronously by using it in a closure
>>>    - Referring to the super implementation by assign it to a variable
>>>    and call it later (is that really possible? never did that)
>>>
>>>
>>> On Wed, Dec 16, 2015 at 6:25 PM, Vester Gottfried <
>>> vester.gottfried at gmail.com> wrote:
>>>
>>>> I would suggest that @requires_super only checks if a call to super is
>>>> present at all. More detailed behaviour should be part of the functions
>>>> documentation, because I think all possibilities cannot be checked easily
>>>> by the compiler. For example a call to super my be required to happen early
>>>> or late inside the function. But when too early or too late is can probably
>>>> not been forseen by the compiler.
>>>>
>>>> On Wed, Dec 16, 2015 at 5:46 PM, Marc Knaup <marc at knaup.koeln> wrote:
>>>>
>>>>> +1 always had such issues with UIViewController's lifecycle methods.
>>>>>
>>>>> But edge cases need to be considered like "throws" for example.
>>>>> Do I need to call super before I throw something?
>>>>>
>>>>> On Wed, Dec 16, 2015 at 5:41 PM, Matthew Johnson via swift-evolution <
>>>>> swift-evolution at swift.org> wrote:
>>>>>
>>>>>> +1 to this.  Anything that helps ensure inheritance is thought
>>>>>> through carefully and used correctly is a win.
>>>>>>
>>>>>> On Dec 16, 2015, at 10:32 AM, Vester Gottfried via swift-evolution <
>>>>>> swift-evolution at swift.org> wrote:
>>>>>>
>>>>>> Some class based libraries/frameworks expect the consumer to subclass
>>>>>> certain classes and override specific method and require that the super
>>>>>> implementation of an overridden method is being called.
>>>>>>
>>>>>> Not calling the super implementation is a common source of bugs that
>>>>>> may be prevented if the compiler checks if super is called, like it does in
>>>>>> some cases of init().
>>>>>>
>>>>>> Example:
>>>>>>
>>>>>> class Box {
>>>>>>    @requires_super
>>>>>>     func addStuff() { ... }
>>>>>> }
>>>>>>
>>>>>> Overriding class Box's addStuff without calling super.addStuff()
>>>>>> should result in an error
>>>>>>
>>>>>> class Chest : Box {
>>>>>>     override addStuff() {
>>>>>>          // ERROR: addStuff() requires call to super.addStuff()
>>>>>>         ...
>>>>>>     }
>>>>>> }
>>>>>>
>>>>>> Objective-C developers know this as NS_REQUIRES_SUPER and I think its
>>>>>> worth thinking about adapting it.
>>>>>>
>>>>>> I hope my proposal was clear and thanks for reading,
>>>>>>
>>>>>> Gottfried
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>>>>>>
>>>>>>
>>>>>>
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>>>>>>
>>>>>>
>>>>>
>>>>
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>>>
>>
>
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