> On Dec 27, 2015, at 2:47 PM, John McCall <[email protected]> wrote:
>
>> On Dec 27, 2015, at 10:37 AM, Joe Groff via swift-evolution
>> <[email protected] <mailto:[email protected]>> wrote:
>>> Getters and setters can be written using dotted syntax within the
>>> back-ticks:
>>>
>>> let specificTitle = button.`currentTitle.get` // has type () -> String?
>>> let otherTitle = UIButton.`currentTitle.get` // has type (UIButton) -> ()
>>> -> String?
>>> let setTintColor = button.`tintColor.set` // has type (UIColor!) -> ()
>>> The same syntax works with subscript getters and setters as well, using the
>>> full name of the subscript:
>>>
>>> extension Matrix {
>>> subscript (row row: Int) -> [Double] {
>>> get { ... }
>>> set { ... }
>>> }
>>> }
>>>
>>> let getRow = someMatrix.`subscript(row:).get` // has type (Int) -> () ->
>>> [Double]
>>> let setRow = someMatrix.`subscript(row:).set` // has type (Int) ->
>>> ([Double]) -> ()
>> At least as far as pure Swift is concerned, for unapplied access, like
>> `UIButton.currentTitle`, I think it would be more consistent with the way
>> method references works for that to give you the getter (or lens) without
>> decoration. instance.instanceMethod has type Args -> Ret, and
>> Type.instanceMethod has type Self -> Args -> Ret; by analogy, since
>> instance.instanceProperty has type Ret or inout Ret, it's reasonable to
>> expect Type.instanceProperty to have type Self -> [inout] Ret. Forming a
>> getter or setter partially applied to an instance feels unmotivated to me—{
>> button.currentTitle } or { button.currentTitle = $0 } already work, and are
>> arguably clearer than this syntax.
>>
>> I acknowledge that this leaves forming selectors from setters out to dry,
>> but I feel like that's something that could be incorporated into a "lens"
>> design along with typed selectors. As a rough sketch, we could say that the
>> representation of @convention(selector) T -> inout U is a pair of
>> getter/setter selectors, and provide API on Selector to grab the individual
>> selectors from that, maybe Selector(getterFor:
>> UIView.currentTitle)/(setterFor: UIView.currentTitle). I don't think get/set
>> is a good interface for working with Swift properties, so I don't like the
>> idea of building in language support to codify it beyond what's needed for
>> ObjC interaction.
>
> I know this might be too early, but: what syntax are we thinking of for
> lenses? We might want to design this with future consistency in mind.
Vaguely, I think it could look something like this. You could define a lens
function by having it return `inout`. Calling the function produces an lvalue
whose access nests within the accesses of its input `inout` parameters, if any,
allowing for things like:
var localVar = 1
let localRef: () -> inout Int = { &localVar }
func second(inout array: [Int]) -> inout Int {
return &array[1]
}
// Maybe you can define an inout function with accessors too
func fahrenheit(inout celsius: Double) -> inout Double {
get {
return celsius * 9/5 + 32
}
set {
celsius = (newValue - 32) * 5/9
}
}
and you could access the unapplied lens for an instance property using
`Type.property` syntax, analogous to how `Type.method` works. I feel like if we
did that, then it would obviate the need for explicit `property.get` or
`property.set` for most native Swift uses, though maybe not ObjC interop uses.
-Joe_______________________________________________
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