[swift-evolution] [Pitch] inout with capture variable

[Mark Lacey]

> On Oct 5, 2016, at 10:07 PM, Cao Jiannan <frogcjn at 163.com> wrote:
> 
> so if developer want to capture an variable inference, must declare a new variable.

I don’t follow what you mean here.

> 
> class A {
>     var value = 1
>     
>     func test() -> () -> Int {
>         var capturedValue = self.value
>         let editInfo = { () -> Int in 
>             capturedValue += 1
>             return capturedValue
>         }
>         return editInfo
>     }
> }
> 
> let a = A()
> 
> let editInfo = a.test()
> print(editInfo()) // 2
> print(a.value) // 1
> print(editInfo()) // 3
> print(a.value) // 1
> 
> what about:
> 
> class A {
>     var value = 1
>     
>     func test() -> () -> Int {
>         let editInfo = { [inout value] () -> Int in 
>             capturedValue += 1
>             return capturedValue
>         }
>         return editInfo
>     }
> }
> 
> let a = A()
> 
> let editInfo = a.test()
> print(editInfo()) // 2
> print(a.value) // 2
> print(editInfo()) // 3
> print(a.value) // 3

I’m not quite sure what you’re getting at, but this prints what you’re expecting in the output you have in the comments:

class A {
    var value = 1
    
    func test() -> () -> Int {
        let editInfo = { () -> Int in 
            self.value += 1
            return self.value
        }
        return editInfo
    }
}

let a = A()

let editInfo = a.test()
print(editInfo()) // 2
print(a.value) // 2
print(editInfo()) // 3
print(a.value) // 3


Mark

> 
> > > On Oct 5, 2016, at 9:06 PM, Cao Jiannan via swift-evolution<swift-evolution at swift.org <mailto:swift-evolution at swift.org>>wrote:
> > > 
> > > for example:
> > > 
> > > var a = 1
> > > 
> > > let block = { [inout a] in
> > > a += 1
> > > }
> > > 
> > > block()
> > > block()
> > > 
> > > print(a) // 3
> > This is already how captures work by default in closures and nested functions in Swift:
> > 
> > var a = 1
> > 
> > let block = { a += 1 }
> > 
> > block()
> > block()
> > 
> > print(a) // prints 3
> > 
> > 
> > If you want to capture something immutably you can put it in the capture list:
> > 
> > var a = 1
> > let block = { [a] in a += 1 } // error: left side of mutating operator isn't mutable: 'a' is a 'let' constant
> > 
> > Mark
> > 
> > > 
> > > 
> > > _______________________________________________
> > > swift-evolution mailing list
> > > swift-evolution at swift.org <mailto:swift-evolution at swift.org>
> > > https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>
> > 
> > 
> > 

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