[swift-users] Protocol with instance var that's set on construction, otherwise read-only

[Rick Mann]

> On Aug 3, 2016, at 03:23 , Dan Loewenherz <dan at lionheartsw.com> wrote:
> 
> On Wed, Aug 3, 2016 at 3:51 AM, Rick Mann via swift-users <swift-users at swift.org> wrote:
> 
> > On Aug 2, 2016, at 19:06 , Jordan Rose <jordan_rose at apple.com> wrote:
> >
> > I don’t think it makes sense to do this. A protocol cannot control how a particular property is implemented (stored or computed), and any conforming type must initialize all of its stored properties before returning from its own initializer. (You can’t write an initializer in a protocol that doesn’t delegate to another initializer because you don’t know what other stored properties the conforming type might have.)
> >
> > Given that the protocol can’t control how the property gets initialized, it doesn’t make sense to allow the protocol to "set the variable, but only in the initializer”.
> 
> Really? It seems pretty natural for a conforming type to set a property once in the initializer, and it's immutable from then on out. I can do that quite cleanly with classes, but there's no way (that I know) to describe this using protocols. Ideally, I could just do:
> 
> protocol
> Element
> {
>     let uuid: UUID
> }
> 
> which implies that all conforming types must initialize that value on creation, or provide a getter with let semantics (the latter might be too easy to break, and could be disallowed, requiring conforming types to create storage for the property and set it in init()).
> 
> The compiler only knows as much as you tell it, and when you define a protocol and set it as { get } only, it's not going to let you set that property in a protocol extension for a protocol that doesn't expect a setter. Why or how would the compiler let you?
> 
> The solution is to create your init implementation in a concrete type, and define the let property in there as well. E.g.
> 
> protocol Element {
>     var uuid: UUID { get }
> }
> 
> class Item: Element {
>     let uuid: UUID
> 
>     init(uuid: UUID) {
>         self.uuid = uuid
>     }
> }
> 

Because then Element.uuid doesn't have let semantics. It can't be set, but it's not constant.

-- 
Rick Mann
rmann at latencyzero.com