[swift-evolution] Allowing `guard let self = self else { … }` for weakly captured self in a closure.
Evan Maloney
emaloney at gilt.com
Thu Feb 4 10:03:25 CST 2016
Sorry, Kurt, I'm still slogging through old emails, or I would've given proper credit :)
As for Jerome's question of where to put the breakpoint, I'd put it in the line with the [guard self] in.
If 'self' went away before the closure executes and the guard fails, IMO the debugger should behave exactly the same as if it were entering and exiting a one-line closure, eg.:
{ return self.foo }
...except that it should do it on the line with the [guard self] regardless of how large the underlying closure is.
> On Feb 4, 2016, at 10:41 AM, Kurt Werle <kurt at circlew.org> wrote:
>
> 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 <mailto: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 <mailto: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 <mailto: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 <mailto: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 <mailto: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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>>>
>>>
>>>
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>>
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> --
> kurt at CircleW.org
> http://www.CircleW.org/kurt/ <http://www.circlew.org/kurt/>
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