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

Adrian Zubarev adrian.zubarev at devandartist.com
Wed Jan 18 11:08:56 CST 2017

Yeah I’m looking for that day too. :)

class ImmutableThing : AnyValue { … }

Adrian Zubarev
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Am 18. Januar 2017 um 18:03:59, Matthew Johnson via swift-evolution (swift-evolution at swift.org) schrieb:

On Jan 18, 2017, at 11:00 AM, Joe Groff <jgroff at apple.com> wrote:

On Jan 18, 2017, at 8:57 AM, Matthew Johnson <matthew at anandabits.com> wrote:

On Jan 18, 2017, at 10:54 AM, Joe Groff via swift-evolution <swift-evolution at swift.org> wrote:

On Jan 18, 2017, at 8:50 AM, Tony Allevato <tony.allevato at gmail.com> wrote:

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?

It's conceivable that even computed `let` properties could be supported, if the getter implementation is a pure function of `self`.

How would that work when `self` is mutable?

The exact meaning of "pure" and "immutable" would have to be designed. To a first approximation, you could say a pure method would only be able to read immutable global or class data (which is itself `let` or `pure func`, not anything that's potentially mutable) in addition to its own arguments.

Got it.  That makes sense.  Looking forward to the expanded pure / immutable model someday!


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