[swift-evolution] Making capturing semantics of local functions explicit

[Alex Lynch]

I agree; I think the 'in' should be there. This should be fairly
straight-forward to implement right? It's just alternative syntax over
existing behavior. (thinking about the proposal now...)

On Wed, Oct 25, 2017 at 5:45 PM, John McCall via swift-evolution <
swift-evolution at swift.org> wrote:

> On Oct 25, 2017, at 4:21 PM, David Hart <david at hartbit.com> wrote:
>
> On 25 Oct 2017, at 19:01, John McCall <rjmccall at apple.com> wrote:
>
> On Oct 25, 2017, at 7:41 AM, David Hart via swift-evolution <
> swift-evolution at swift.org> wrote:
> I got bit again by a sneaky memory leak concerning local functions and
> would like to discuss a small language change. I vaguely remember this
> being discussed in the past, but can’t find the thread (if anybody could
> point me to it, I’d appreciate it). Basically, here’s an example of the
> leak:
>
> class A {
>     func foo() {
>         func local() {
>             bar()
>         }
>
>         methodWithEscapingClosure { [unowned self] _ in
>             self.bar()
>             local() // this leaks because local captures self        }
>     }
>
>     func bar() {
>     }
> }
>
>
> Its sneaky because local’s capturing of self is not obvious if you’ve
> trained your brain to watch out for calls prefixed with self. I would
> suggest having the compiler force users to make self capturing explicit,
> the same way it does for closures:
>
>
> I think this is a good idea.  Ideally the proposal would also allow
> explicit capture lists in local functions.
>
>
> Ideally, yes. But the only sensible syntax I can come up for that seems
> odd in the context of functions:
>
> class A {
>     func foo() {
>         func local() -> Int { [weak self] in
>         }
>     }
> }
>
>
> Don’t you think?
>
>
> You could leave the "in" off, but it's only a little weird to have it, and
> the inconsistency would probably be worse.
>
> John.
>
>
> David.
>
> John.
>
>
> class A {
>     func foo() {
>         func local() {
>             bar() // error: Call to method ‘bar' in function ‘local' requires explicit 'self.' to make capture semantics explicit
>         }
>     	// ...
>     }
> }
>
>
> What do you think?
>
> David.
>
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