[swift-evolution] Proposal: Add @requires_super attribute

[Matthew Johnson]

Calling super asynchronously would not meet an @requires_super specification IMO.  I would interpret it to mean you must call super before returning.

The compiler already guarantees all code paths return and guarantees subclass initializers call super.  I don't think a sensible definition of @requires_super would be any more complex than that. 

Sent from my iPad

> On Dec 16, 2015, at 11:49 AM, Marc Knaup <marc at knaup.koeln> 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
>>>>>  _______________________________________________
>>>>> swift-evolution mailing list
>>>>> swift-evolution at swift.org
>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>> 
>>>> 
>>>> 
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> 
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