[swift-evolution] [Proposal] Property behaviors

[David Waite]

Joe,

There seem to be many new syntactic features to support this, both inside and outside of a behavior. I was wondering if you had considered/had comments an alternative approach where you declare an ordinary new value type which meets certain rules and ‘wraps’ the existing type, then define new syntax/features in Swift for having the wrapper (or possibly even multiple levels of wrappers) be more transparent for use.

For instance, the existing ‘lazy’ keyword functionality might have been implemented by a type  Lazy<T>

struct Lazy<T> {
    private var val:T?
    let supplier:()->T
    
    init(supplier:()->T) {
        self.supplier = supplier
    }

    var value:T {
        mutating get {
            if val == nil {
                val = supplier()
            }
            return val!
        }
        set {
            val = value
        }
    }
    mutating func clear() {
        val = nil
    }

}

With the following as an example of use (without any additional syntactic features)

var globally = "Test"
class Foo {
    var bar = Lazy<Int> {
        globally = "Side effect"
        return 1
    }
}

print(globally)
Foo().bar.value
print(globally)
Foo().bar.clear()

One could opt into a syntax to allow value to be hidden from view. In fact, I can hide the use of the Lazy struct today if I’m willing to write more code:

class Foo {
    private var internalbar = Lazy<Int> {
        return 1
    }
    var bar:Int {
        get {
            return internalbar.value
        }
        set {
            internalbar.value = newValue
        }
    }
}
print(Foo().bar)

Which actually has the benefit of being able to call the clear() method without new syntax, and being able to control access to clear separate from the getter/setter

-DW


> On Jan 13, 2016, at 3:07 PM, Joe Groff via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Thanks everyone for the first round of feedback on my behaviors proposal. I've revised it with the following changes:
> 
> - Instead of relying on mapping behaviors to function or type member lookup, I've introduced a new purpose-built 'var behavior' declaration, which declares the accessor and initializer requirements and provides the storage and behavior methods of the property. I think this gives a clearer design for authoring behaviors, and allows for a more efficient and flexible implementation model.
> - I've backed off from trying to include 'let' behaviors. As many of you noted, it's better to tackle immutable computed properties more holistically than to try to backdoor them in.
> - I suggest changing the declaration syntax to use a behavior to square brackets—'var [behavior] foo'—which avoids ambiguity with destructuring 'var' bindings, and also works with future candidates for behavior decoration, particularly `subscript`.
> 
> Here's the revised proposal:
> 
> https://gist.github.com/jckarter/50b838e7f036fe85eaa3
> 
> For reference, here's the previous iteration:
> 
> https://gist.github.com/jckarter/f3d392cf183c6b2b2ac3
> 
> Thanks for taking a look!
> 
> -Joe
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
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