[swift-users] Can you use @autoclosure in a setter?

[Slava Pestov]

Oh I see, the property itself does not have autoclosure type.

Slava

> On Sep 11, 2017, at 3:52 PM, Slava Pestov <spestov at apple.com> wrote:
> 
> No, newValue is a closure here, and valueSource will capture this closure. When valueSource is evaluated, the original closure will be evaluated.
> 
> Slava
> 
>> On Sep 11, 2017, at 3:11 PM, Hooman Mehr <hooman at mac.com <mailto:hooman at mac.com>> wrote:
>> 
>> But the expression that is assigned to the property will be eagerly evaluated to produce `newValue`. So this will not accomplish what Nevin is trying to do. 
>> 
>>> On Sep 11, 2017, at 3:08 PM, Slava Pestov via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
>>> 
>>> You can have valueSource store a closure that captures the autoclosure value. For example,
>>> 
>>> set {
>>>   valueSource = { newValue }
>>> }
>>> 
>>> Slava
>>> 
>>>> On Sep 11, 2017, at 11:04 AM, Nevin Brackett-Rozinsky via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
>>>> 
>>>> Hi, quick question here:
>>>> 
>>>> I have a class with a property that needs to be really *really* lazy. So lazy, in fact, that when you assign to that property, the class actually stores a closure of what you assigned, which is only evaluated if and when you actually attempt to read the property.
>>>> 
>>>> Simplified:
>>>> 
>>>> class Foo {
>>>>   private var valueSource: () -> Bar
>>>>   private var valueCache: Bar?
>>>>   
>>>>   init(_ v: @escaping @autoclosure () -> Bar) {
>>>>     valueSource = v
>>>>   }
>>>>   
>>>>   var value: Bar {
>>>>     get {
>>>>       if let v = valueCache { return v }
>>>>       let w = valueSource()
>>>>       valueCache = w
>>>>       return w
>>>>     }
>>>>     set {
>>>>       /* ??? */
>>>>     }
>>>>   }
>>>>   
>>>>   // I want this function's logic to go in the setter above
>>>>   func setValue(_ v: @escaping @autoclosure () -> Bar) {
>>>>     valueSource = v
>>>>     valueCache = nil
>>>>   }
>>>> }
>>>> 
>>>> The goal is to be able to write things like “someFoo.value = bar1 / bar2” (or even more complex expressions) and not evaluate them until/unless the result is actually needed.
>>>> 
>>>> Currently I am using “someFoo.setValue( bar1 / bar2 )”, which is not nearly as ergonomic as the assignment syntax. So, is there a way to make this work?
>>>> 
>>>> Nevin
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>>> 
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>> 
> 

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