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

[Hoon H.]

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 <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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