[swift-evolution] Required Callback

[Xiaodi Wu]

I wonder if we can be a little more elegant:

- no need for an optional message: if the compiler is going to force you
the implementor to call something, it really doesn't matter why--you just
call it; any additional explanation can be given in doc comments

- at the implementation site, you the implementor would have to parrot
@required in the declaration; that in itself wins some clarity (the
implementor has to know it's required)

- instead of allowing any assignment or reference of the callback to
satisfy the requirement, either require actual invocation in the body, or
allow the callback to be called/assigned/whatever with an extra attribute
@invoked (or whatever the preferred bikeshed)

MyModule(@invoked end)
// or live dangerously:
// _ = @invoked end

On Tue, Aug 16, 2016 at 09:54 James Campbell <james at supmenow.com> wrote:

>
> I think as long as the end callback is referenced in some way that would
> still be better than what we have now, if you pass it into your own code
> but then continue on to not call it then I think it would be fair for the
> compiler to let you shoot yourself in the foot in that case.
>
> Calling end more than once is another case which isn't covered by this
> proposal.
>
> so this:
>
> run() { end in
>
> }
>
> would have an error:
>
> but
>
> run() end in {
>  MyModule(end)
> }
>
> would not since we have at least referenced it.
>
>
> *___________________________________*
>
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>
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> <http://supmenow.com>*
>
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>
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>
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> On 16 August 2016 at 15:49, Xiaodi Wu <xiaodi.wu at gmail.com> 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.
>>>
>>> *___________________________________*
>>>
>>> *James*
>>>
>>> *james at supmenow.com <james at supmenow.com>⎥supmenow.com
>>> <http://supmenow.com>*
>>>
>>> *Sup*
>>>
>>> *Runway East *
>>>
>>> *10 Finsbury Square*
>>>
>>> *London*
>>>
>>> * EC2A 1AF *
>>> _______________________________________________
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>>>
>>
>
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