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

[David Hart]

> 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 <mailto: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?

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.
>> 
>> _______________________________________________
>> swift-evolution mailing list
>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>> https://lists.swift.org/mailman/listinfo/swift-evolution
> 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20171025/9ed84710/attachment.html>