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

[Hoon H.]

I like to point that closure capturing list syntax already allows very similar rebinding of “self” which changes only referencing semantics (strong -> weak), so I think this is pretty consistent.

   { [weak self = self] in 
       // ...
   }

For the prior proposal "Proposal: weakStrong self in completion handler closures” (by Jacob Bandes-Storch), I think my proposal is different thing.
My original intention was about rebinding of “self”, and then, I realised that eliding “self” qualification can make sense with guard statement and introduced it as an extra feature. I think it’s fine to develop my proposal separately, but please tell me if you have some opinions.

— Hoon H.





> On 2016/01/06, at 09:20 AM, Jordan Rose <jordan_rose at apple.com> 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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