[swift-evolution] Required Callback

[Xiaodi Wu]

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 :)
>
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> 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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