Hello Swift Evolution,
I’m bumping into an annoying problem with protocols. In a class or struct it is
common to have a `let` instance variable and assign it in `init`. Unfortunately
there is no way to translate this into a protocol with init in an extension. If
attempting to set the variable in init in an extension, it must be of type {
get set }, which means it cannot be a `let` constant in the conforming type.
AFAIK there is no way around this — if you want to set an instance variable in
an initializer in a protocol extension, it must be marked as { get set }. The
alternative is to write the initializer separately for each adopting type, but
this violates DRY.
Hence, I am proposing a third option to go along with `get` and `set` in a
protocol. This would indicate that the variable can be a constant, but is
settable in an initializer. In this case, the conforming type *must* use `let`
to declare the variable.
Option 1: the keyword `let`. If present, it would need to be the only thing in
the curly brackets because it simultaneously implies `get` and not `set`.
protocol P {
var x: Int { let }
init(_ x: Int)
func modifyX()
}
extension P {
init(_ x: Int) {
self.x = x // This is ok; would not be ok if x were marked { get }
}
func modifyX() {
self.x += 1 // Not allowed
}
}
struct S: P {
let x: Int // This is ok; would not be ok if x were marked { get set }
}
Option 2: `set(init)`. Can (and often will) coexist with `get`.
protocol P {
var x: Int { get set(init) }
init(_ x: Int)
func modifyX()
}
extension P {
init(_ x: Int) {
self.x = x // This is ok; would not be ok if x were marked { get }
}
func modifyX() {
self.x += 1 // Not allowed
}
}
struct S: P {
let x: Int // This is ok; would not be ok if x were marked { get set }
}
I’d like to hear all of your thoughts on this.
Best,
Robert
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