[swift-evolution] Removing var keyword in protocol property reqirements

Tony Allevato tony.allevato at gmail.com
Wed Jan 18 10:50:38 CST 2017

Good point—I hadn't considered the distinction.

Does that mean a future version of Swift might allow `let` in a protocol to
indicate a value that must be immutable after initialization, such that a
computed `var { get }` wouldn't satisfy it?

On Wed, Jan 18, 2017 at 8:48 AM Joe Groff <jgroff at apple.com> wrote:

> > On Jan 18, 2017, at 8:41 AM, Tony Allevato via swift-evolution <
> swift-evolution at swift.org> wrote:
> >
> > Personally, I don't feel that the problem is big enough to warrant a
> breaking change here.
> >
> > `var` is the general property declaration keyword in Swift for
> read-write stored properties and read-only/read-write computed properties.
> The fact that you can use `let` in place of `var` under one specific
> circumstance (a read-only property backed by a stored variable) doesn't
> necessarily make the entire keyword meaningless, IMO. `let` is just a
> shortcut for a read-only stored property, and I think dropping it or moving
> `{ get set }` elsewhere would be more confusing, since it would be
> syntactically inconsistent with how the property is implemented. (Protocol
> members look the same as class/struct members with the body removed.)
> >
> > In general, I'm skeptical of ideas that just aim to protect new or bad
> developers from themselves, especially at the expense of consistency.
> `let` more strongly guarantees **immutability**, not just "read-only". var
> { get } indicates that the protocol's interface only lets you read, but
> provides no guarantee that the implementation can't change the value you
> get. The language doesn't have the ability to reason about immutability for
> anything other than stored properties yet, but we want to leave syntactic
> space for that possibility without muddying the strong guarantee of `let`.
> For that reason, protocols only support `var { get }` requirements today.
> `let` properties naturally model the requirement since you can read them;
> their interface is a superset of the protocol requirement.
> -Joe
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