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

[Jacob Bandes-Storch]

+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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