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

[Slava Pestov]

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

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-users/attachments/20170911/a7bd6d2a/attachment.html>