[swift-evolution] Allowing `guard let self = self else { … }` for weakly captured self in a closure.

Thu Feb 4 09:54:23 CST 2016

If you're having that problem, I might recommend swapping in
[weak self] ...
guard self != nil...

But seriously - how often do you have a closure where self has become nil
and you wanted to see if the closure was being called?

On Thu, Feb 4, 2016 at 7:48 AM, Jerome ALVES <j.alves at me.com> wrote:

> What about debugging ?
>
> How can we be sure the closure is called or not ? Where do you put the
> breakpoint ?
>
>
> Le 4 févr. 2016 à 16:41, Kurt Werle via swift-evolution <
> swift-evolution at swift.org> a écrit :
>
> I suggested exactly the same thing a few days ago as [firm self].  I like
> [guard self] even better.
> +1
>
> Kurt
>
> On Wed, Feb 3, 2016 at 9:57 PM, Evan Maloney via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>> I'm way-late to this discussion—I don't know how any of you get any
>> coding done trying to keep up with this mailing list!—but anyway...
>>
>> I propose:
>>
>> let doSomething: () -> Void = { [*guard* self] in
>>     ...
>> }
>>
>> [guard self] would effectively work like [weak self] in that it doesn't
>> cause the closure itself to hold a strong reference to whatever is pointed
>> to by self. But if self is still around when the closure is called, it
>> upgrades it to a strong reference for the duration of the closure's
>> execution.
>>
>> So, when doSomething() is just sitting around *not* doing something, it
>> doesn't prevent self from being deallocated.
>>
>> If, by the time doSomething() is called, self is no longer there,
>> doSomething() just returns without the closure executing. If self *is* there,
>> then the closure executes with self as a strong reference inside.
>>
>> That way, self within the closure is already strong, so you can use it
>> conveniently as a non-optional and without doing the strongSelf = self
>> dance. You get the memory management benefit of a weak reference without
>> the extra noise in your code needed to support it.
>>
>> Ok, so what closures with a return value, you ask? How about something
>> like:
>>
>> let maybeDoSomething: () -> Bool = { [guard self else false] in
>>     ...
>> }
>>
>> Here, maybeDoSomething() doesn't hold a strong reference to self. If it
>> executes when self is still alive, the code within the braces executes with
>> self as a strong reference. If self is gone, the value after the else is
>> returned (the "return" itself is implied).
>>
>> What do you think?
>>
>>
>> On Jan 28, 2016, at 7:32 PM, Hoon H. via swift-evolution <
>> swift-evolution at swift.org> wrote:
>>
>> Thanks for your opinions.
>> I am writing a formal proposal, and now I think it’d be fine to elide
>> explicit `self` qualification after `guard let … `.
>>
>> Also for your proposal, though I think mine is originated from different
>> intention, but final conclusion overlaps with your intention, and I am
>> still not sure what to do in this situation. Do you have some opinions?
>>
>> — Hoon H.
>>
>>
>>
>>
>>
>> On 2016/01/06, at 10:46 AM, Jacob Bandes-Storch <jtbandes at gmail.com>
>> wrote:
>>
>> +1.
>>
>> Merely using "self?.something" repeatedly might produce unexpected
>> behavior, if self becomes nil between calls. As I mentioned in another
>> thread, in Obj-C, there is a warning for this (-Warc-repeated-use-of-weak).
>>
>> In many cases, I use the pattern
>>
>>     somethingAsync { [weak self] in
>>         guard let strongSelf = self else { return }
>>
>>         // use strongSelf below
>>     }
>>
>> But of course, this leads to the unnatural/unwieldy "strongSelf.property"
>> all over the place.
>>
>> I agree with Jordan that "guard let self = self" isn't the most
>> satisfying syntax, but it has the advantage of being a *very* minimal
>> grammar/syntax change, and its behavior is completely clear as long as the
>> user is already familiar with guard.
>>
>> We should also consider whether "self." is required after "guard let self
>> = self". An explicit "guard let self = self" avoids the accidental-capture
>> problem, so I think it's reasonable to allow unqualified property access
>> for the remainder of the scope.
>>
>> Jacob
>>
>> On Tue, Jan 5, 2016 at 4:20 PM, Jordan Rose via swift-evolution <
>> swift-evolution at swift.org> wrote:
>>
>>> This has come up before, in a thread called "Proposal: weakStrong self
>>> in completion handler closures". I'm still not 100% happy with the
>>> syntax, but I like that "guard let" can handle non-Void non-Optional
>>> returns well, while 'weakStrong' cannot.
>>>
>>> Jordan
>>>
>>>
>>> On Jan 5, 2016, at 16:02, Hoon H. via swift-evolution <
>>> swift-evolution at swift.org> wrote:
>>>
>>> Currently, weakly captured `self` cannot be bound to `guard let …` with
>>> same name, and emits a compiler error.
>>>
>>> class Foo {
>>> func test2(f: ()->()) {
>>> // …
>>> }
>>> func test1() {
>>> test2 { [weak self] in
>>> guard let self = self else { return } // Error.
>>> print(self)
>>> }
>>> }
>>> }
>>>
>>> Do we have any reason to disallow making `self` back to strong
>>> reference? It’d be nice if I can do it. Please consider this case.
>>>
>>> class Foo {
>>> func getValue1() -> Int {
>>> return 1234
>>> }
>>> func test3(value: Int) {
>>> print(value)
>>> }
>>> func test2(f: ()->()) {
>>> // …
>>> }
>>> func test1() {
>>> test2 { [weak self] in
>>> self?.test3(self?.getValue1()) // Doesn't work because it's not
>>> unwrapped.
>>>
>>> self!.test3(self!.getValue1()) // Considered harmful due to `!`.
>>>
>>> guard self != nil else { return }
>>> self!.test3(self!.getValue1()) // OK, but still looks and feels harmful.
>>>
>>> guard let self1 = self else { return }
>>> self1.test3(self1.getValue1()) // OK, but feels ugly due to unnecessary
>>> new name `self1`.
>>>
>>> guard let self = self else { return }
>>> self.test3(self.getValue1()) // OK.
>>>
>>> }
>>> }
>>> }
>>>
>>> This also can be applied to `if let` or same sort of constructs.
>>>
>>> Even further, we can consider removing required reference to `self`
>>> after `guard let …` if appropriate.
>>>
>>> guard let self = self else { return }
>>> test3(getValue1()) // Referencing to `self` would not be required
>>> anymore. Seems arguable.
>>>
>>> I think this is almost fine because users have to express their
>>> intention explicitly with `guard` statement. If someone erases the `guard`
>>> later, compiler will require explicit self again, and that will prevent
>>> mistakes. But still, I am not sure this removal would be perfectly fine.
>>>
>>> I am not sure whether this is already supported or planned. But lacked
>>> at least in Swift 2.1.1.
>>>
>>> — Hoon H.
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
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>>> swift-evolution at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>
>>>
>>>
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>>>
>>>
>>
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>
>
> --
> kurt at CircleW.org <kurt at circlew.org>
> http://www.CircleW.org/kurt/ <http://www.circlew.org/kurt/>
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