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

[Christopher Rogers]

You can shadow self with a guard like you wrote it if use the keyword
escaping backquotes like so:

guard let `self` = self else { return }
On Wed, Jan 6, 2016 at 10:55 AM Jacob Bandes-Storch via swift-evolution <
swift-evolution at swift.org> wrote:

> FWIW, in a codebase of ~150 Swift files, I see 18 occurrences of "let
> strongSelf = self", and 26 occurrences of "self?." (which should arguably
> be changed to the former).
>
> Jacob
>
> On Tue, Jan 5, 2016 at 5:46 PM, 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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