> On Aug 3, 2016, at 03:23 , Dan Loewenherz <d...@lionheartsw.com> wrote:
>
> On Wed, Aug 3, 2016 at 3:51 AM, Rick Mann via swift-users
> <swift-users@swift.org> wrote:
>
> > On Aug 2, 2016, at 19:06 , Jordan Rose <jordan_r...@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
rm...@latencyzero.com
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