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

[Nevin Brackett-Rozinsky]

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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