[swift-evolution] Proposal - Allow properties in Extensions

[David Owens II]

> On Dec 18, 2015, at 3:34 PM, Joe Groff via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
>> On Dec 18, 2015, at 3:24 PM, Kevin Ballard via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> AFAIK there's no precedent today for extensions in the same module being able to do things that extensions in other modules can't do (beyond accessing `internal` members of course). I'm not completely opposed to the idea, but I would want to be very careful and make sure there's an extremely clear benefit to changing it so extensions in the same module can add extra storage.
>> 
>> One potentially large downside is you can no longer look at a type declaration and find out how large it is. For example, I could define
>> 
>> struct Foo {
>>    var x: Int
>> }
>> 
>> and it looks like a tiny struct, but I could then add an extension in a separate file that bloats that out to some ridiculously massive struct, and that wouldn't be obvious from looking at it.
>> 
>> That said, when looking at a definition imported from another module, you can't see which properties are computed and which are stored anyway, so it's maybe not a huge deal (although you can assume any property defined in an extension is computed). But it is still something to consider.
> 
> True, this is a tradeoff. We've discussed moving in the direction of allowing same-module extensions to do anything that can be done in the original type declaration, to allow freedom of organization without arbitrary rules about what must go in the type, but sizing instances was one of our concerns. (You could always peek at the LLVM IR if you want to know for sure what's getting generated.)


Have you considered using “partial” or “extendable" for these types of definitions (similar to C# partials) to reduce this confusion?

extendable struct Foo {
   var x: Int
}

When the keyword is there, all bets are off that the layout of the struct only contains a single member. Then extensions, defined within the same module, could add to it.

-David
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