[swift-evolution] Required Callback

[James Campbell]

That sounds fair, the closure that function returns could trigger a runtime
warning when it detects the closure has be deallocated without being
triggered (or even mutliple times but ofc not the focus for this proposal)



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On 16 August 2016 at 18:39, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:

> One alternative that comes to mind:
>
> The @escaping stuff came with a helper function, withoutActuallyEscaping,
> to deal with attribute mismatches. Perhaps we could have the same here, for
> consistency, something like withActuallyInvoking?
>
> On Tue, Aug 16, 2016 at 12:05 Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>
>> Well, the callsite @required would be what I call your escape hatch :)
>>
>> Hmm, lemme think on this...
>> On Tue, Aug 16, 2016 at 11:59 James Campbell <james at supmenow.com> wrote:
>>
>>> It would though, look :) :
>>>
>>> Backend.run() { requiredCallback in
>>>
>>>  NonAnotatedModuled({
>>>
>>> //We send a anon closure to the module and we call the callback with a
>>> required specifier to indicate to the compiler it will be called :)
>>> @required requiredCallback()
>>> })
>>> }
>>>
>>> This would be the same as this:
>>>
>>>
>>> Backend.run() { requiredCallback in
>>>
>>>  AnotatedModuled(requiredCallback)
>>> }
>>>
>>> And this:
>>>
>>>
>>> Backend.run() { requiredCallback in
>>>
>>> @required requiredCallback()
>>> }
>>>
>>> *___________________________________*
>>>
>>> *James⎥Lead Hustler*
>>>
>>> *james at supmenow.com <james at supmenow.com>⎥supmenow.com
>>> <http://supmenow.com>*
>>>
>>> *Sup*
>>>
>>> *Runway East *
>>>
>>> *10 Finsbury Square*
>>>
>>> *London*
>>>
>>> * EC2A 1AF *
>>>
>>> On 16 August 2016 at 17:55, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>>
>>>> Wait, doesn't work. Your anonymous closure would then be dinged for not
>>>> satisfying the requirement. Turtles all the way down. Still need an escape
>>>> hatch.
>>>>
>>>> On Tue, Aug 16, 2016 at 11:54 James Campbell <james at supmenow.com>
>>>> wrote:
>>>>
>>>>> I'll write up a draft proposal :) I think we have something nice :)
>>>>>
>>>>> *___________________________________*
>>>>>
>>>>> *James⎥Lead Hustler*
>>>>>
>>>>> *james at supmenow.com <james at supmenow.com>⎥supmenow.com
>>>>> <http://supmenow.com>*
>>>>>
>>>>> *Sup*
>>>>>
>>>>> *Runway East *
>>>>>
>>>>> *10 Finsbury Square*
>>>>>
>>>>> *London*
>>>>>
>>>>> * EC2A 1AF *
>>>>>
>>>>> On 16 August 2016 at 17:54, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>>>>
>>>>>> Nicer still!
>>>>>>
>>>>>> On Tue, Aug 16, 2016 at 11:53 James Campbell <james at supmenow.com>
>>>>>> wrote:
>>>>>>
>>>>>>> I guess that would make sense and you could wrap the callback up in
>>>>>>> a anon-closure if the module hadn't adpated the @required property so you
>>>>>>> get both compatibility, safety and clarity.
>>>>>>>
>>>>>>> *___________________________________*
>>>>>>>
>>>>>>> *James⎥Lead Hustler*
>>>>>>>
>>>>>>> *james at supmenow.com <james at supmenow.com>⎥supmenow.com
>>>>>>> <http://supmenow.com>*
>>>>>>>
>>>>>>> *Sup*
>>>>>>>
>>>>>>> *Runway East *
>>>>>>>
>>>>>>> *10 Finsbury Square*
>>>>>>>
>>>>>>> *London*
>>>>>>>
>>>>>>> * EC2A 1AF *
>>>>>>>
>>>>>>> On 16 August 2016 at 17:50, Haravikk <swift-evolution at haravikk.me>
>>>>>>> wrote:
>>>>>>>
>>>>>>>>
>>>>>>>> On 16 Aug 2016, at 15:49, Xiaodi Wu via swift-evolution <
>>>>>>>> swift-evolution at swift.org> wrote:
>>>>>>>>
>>>>>>>> I can see the use case, but it'd be annoying (or, impossible) to
>>>>>>>> work around if I intend to call `end` by passing it to a helper function in
>>>>>>>> another (let's say, precompiled) module. There's no way for the compiler to
>>>>>>>> inspect that `end` is always called by that other module, and if calling
>>>>>>>> `end` twice causes bad things to happen, I'm totally out of luck. You'd
>>>>>>>> need a companion annotation to pass along the requirement to the callee, or
>>>>>>>> some sort of force-unrequire, but the latter can't have teeth (i.e. can't
>>>>>>>> enforce at runtime) if the closure is escaping.
>>>>>>>> On Tue, Aug 16, 2016 at 08:39 James Campbell via swift-evolution <
>>>>>>>> swift-evolution at swift.org> wrote:
>>>>>>>>
>>>>>>>>> It would be handy if a callback could be marked as required with
>>>>>>>>> an optional descriptive message i.e
>>>>>>>>>
>>>>>>>>> class BackgroundTask {
>>>>>>>>>  func run(end: @required("You must call end otherwise iOS will
>>>>>>>>> penalise your app for being a bad citizen") () -> Void)
>>>>>>>>> }
>>>>>>>>>
>>>>>>>>> That was the developer can comunicate the bad things that can
>>>>>>>>> happen if this callback isn't called such as iOS peanlizing them for not
>>>>>>>>> ending a background task or perhaps memory leaks caused by clean up code
>>>>>>>>> unable to be triggered.
>>>>>>>>>
>>>>>>>>
>>>>>>>> Could this not just behave in the same way as @noescape, in which
>>>>>>>> case you can pass the closure on to other functions so long as they also
>>>>>>>> have the @noescape attribute? In this case passing it as a parameter to
>>>>>>>> another method with the @required attribute would be equivalent to calling
>>>>>>>> it directly (since you know the other method must eventually call it).
>>>>>>>>
>>>>>>>
>>>>>>>
>>>>>
>>>
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