[swift-evolution] Different types for getter and setter

[Tony Allevato]

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> 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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