> On 15 Oct 2016, at 02:01, Paul Cantrell via swift-evolution 
> <swift-evolution@swift.org> wrote:
> 
> 
>> On Oct 9, 2016, at 3:43 PM, Charles Srstka via swift-evolution 
>> <swift-evolution@swift.org> wrote:
>> 
>> Let extensions introduce stored properties, but only in the same module as 
>> the type’s definition. Then, the compiler can just take any extensions into 
>> consideration when it’s determining the size of the type, just as if the 
>> properties had been declared in the type. Declaring stored properties on an 
>> extension outside of the type’s module results in a compiler error, exactly 
>> as today. This would, without any performance drawbacks, solve one of the 
>> big problems that people are hoping to solve via stored properties in 
>> extensions—the ability to organize members by protocol conformance.
> 
> Yes please! A big strong +1 to this from me. I can think of several specific 
> chunks of problem code that this would clean up immensely.

+1 from me too for internal extensions; I think this is the logical place to 
start for now so we can see how many use cases this actually covers. Personally 
I don't try to add functionality to types from other libraries, and when I have 
to I try as much as possible to do it via wrapping; obviously this doesn't 
cover the async type cases and others where it'd be hard to re-wrap the values, 
but I think I'd like to know more about how common such cases actually are.

However, allowing stored properties in local extensions is an absolute must for 
a first step, and would be very useful as a first step.


The problem I have with doing it for external types is that I just don't see 
how it can be done efficiently; associated objects means looking up the object 
to find what its associated values are, which isn't a negligible cost if you're 
doing it frequently, and it makes me very uncomfortable to think of hiding what 
is actually happening, as developers may think they're just using a regular 
property without really appreciating the actual costs involved. At the very 
least they need to handled through a .associated property or whatever to make 
it much clearer that these aren't native properties in the normal sense.
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