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

Thu Feb 4 09:41:04 CST 2016

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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>>
>>
>>
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>
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-- 
kurt at CircleW.org
http://www.CircleW.org/kurt/
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