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

[Hooman Mehr]

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> 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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