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

[Joe Groff]

> 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.

-Joe