[swift-evolution] [Pitch] Improving capturing semantics of local functions

[Mike Kluev]

On 14 November 2017 at 19:17, Mike Kluev <mike.kluev at gmail.com> wrote:

> On 14 November 2017 at 19:02, Mike Kluev <mike.kluev at gmail.com> wrote:
>
>> On Mon, 13 Nov 2017 22:30:25 +0100 David Hart <david at hartbit.com> wrote:
>>
>> > On 13 Nov 2017, at 05:52, Slava Pestov <spestov at apple.com> wrote:
>>> >
>>> >> On Nov 12, 2017, at 8:11 PM, Brent Royal-Gordon via swift-evolution <
>>> swift-evolution at swift.org> wrote:
>>> >>
>>> >> By analogy with the current closure syntax, the capture list ought to
>>> go somewhere before the parameter list, in one of these slots:
>>> >>
>>> >> 1.   func fn<T>[foo, bar](param: T) throws -> T where T: Equatable {
>>> … }
>>> >> 2.   func fn[foo, bar]<T>(param: T) throws -> T where T: Equatable {
>>> … }
>>> >> 3.   func [foo, bar] fn<T>(param: T) throws -> T where T: Equatable {
>>> … }
>>> >> 4.   [foo, bar] func fn<T>(param: T) throws -> T where T: Equatable {
>>> … }
>>> >>
>>> >> Of these options, I actually think #4 reads best; 1 and 2 are very
>>> cluttered, and 3 just seems weird. But it seems like the one that would be
>>> easiest to misparse.
>>> >
>>> > Another option that reads nicely IMHO is
>>> >
>>> > func fn<T>(param: T) throws -> T where T : Equatable [foo, bar] { … }
>>> >
>>> > I think #4 is ambiguous with array literals unfortunately.
>>>
>>
>> adding to the list of options:
>>
>> 6. func fn<T>(param: T) throws -> T where T: Equatable [foo, bar] { … }
>>
>> otherwise +1 to #1 and to the one in proposal. also see about #4 below.
>>
>> plus, if 90%+ of use cases in practice would be [weak self] -- (the only
>> examples shown in the proposals FTM) -- i would strongly consider this
>> syntax sugar in addition to a generic notation:
>>
>> weak func fn<T>(param: T) throws -> T where T: Equatable { … }
>>
>> works with "unowned" as a bonus.
>>
>> if implement this sugar than some variation of #4 looks appealing to have
>> these capture things close.
>>
>>
> the closer we have it to this English sentence the better IMHO:
>
> weak throwing function "fn", capturing "foo" and "bar" weakly, using
> generic type "T" which is "Equatable", having parameter "param" of type
> "T", returning type "T", with the following body { ... }
>
>
this is the closet one (hence +1 to #2):

weak throwing func fn [foo, bar] <T: Equatable> (param: T) -> T {...}

Mike
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