[swift-evolution] Different types for getter and setter

[Hooman Mehr]

These types of unusual property behaviors could be implemented if we had property behaviors proposal <https://github.com/apple/swift-evolution/blob/master/proposals/0030-property-behavior-decls.md> revised and implemented. How about resurrecting this for Swift 5.x? I think it will also be a useful feature that can help in the implementation of the concurrency model. To clarify the relation to concurrency: Will actors support public properties? What would a getter for such a property do? It is a similar asymmetric get/set issue, because an actor property may end up being set-only which is impossible in the current Swift property model.

> On Sep 19, 2017, at 1:36 PM, Philippe Hausler via swift-evolution <swift-evolution at swift.org> wrote:
> 
> There is another case to consider; similar to nil-resettable. Optional only by virtue of never being set, but setting nil values is invalid (e.g. Process.environment)
> 
>> On Sep 19, 2017, at 9:34 AM, Tony Allevato via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> There have been a couple times where I've wanted something like this:
>> 
>> 1) A nil-resettable property without having to resort to making it an IUO. It would be nice to have the setter able to take a T? but the getter return T, and the setter would provide a default value in the event that it receives nil.
>> 
>> 2) Once I was writing an API that would keep an array of things in a property, but I also wanted a shorthand version where the user could set it with a single value and have the setter transform that into the array internally. Looking back though, that's not really defensible; it's easy enough for the call site to just add two characters and write "foo.property = [x]", and I probably wouldn't stand by that example today.
>> 
>> 
>> On Tue, Sep 19, 2017 at 8:16 AM Nevin Brackett-Rozinsky via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> This may sound rather strange in the abstract, but recently I have encountered two situations where I would like to have a setter that accepts a different type than the getter returns.
>> 
>> In the first, the getter returns Foo and the setter should accept “@escaping @autoclosure () -> Foo”, so that the expression assigned to the property is not evaluated until it is needed. (The closure is stored in a private property, which the getter evaluates then caches the result.)
>> 
>> In the second, I want a subscript whose getter returns a concrete type (in my case, subscripting a matrix by row returns an ArraySlice<Element>) while the setter can accept something more generic (any kind of Collection with the correct Element type).
>> 
>> Thoughts?
>> 
>> Nevin
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