[swift-corelibs-dev] #if __CORELIBS_FOUNDATION__

[Tony Parker]

Hi Drew,

> On Jan 5, 2016, at 12:08 AM, Drew Crawford <drew at sealedabstract.com> wrote:
> 
> 
>> On Jan 4, 2016, at 3:09 PM, Tony Parker <anthony.parker at apple.com <mailto:anthony.parker at apple.com>> wrote:
>> 
>> In this particular case, how would you use the #if? Any should be source compatible with AnyObject, since Any is a superset of AnyObject, right?
> 
> Well I don't know what you mean by "compatible" but the thing about strongly typed languages is we can't do
> 
> let a: Any
> 
> foo(a: AnyObject) { /* */ }
> 
> foo(a)
> 
> because that is "type error".  We can of course cast, but if in Darwin a.self is already AnyObject, casting again to AnyObject produces a warning, and I try to not have any warnings in my code.
> 
> This is kind of a side quest–there are a lot of ways to work around these problems–but the obviously straightforward one was to use #if to conditionally cast, and that wasn't available.
> 
> More broadly I find myself using the #if that I've created very often–my workflow seems to be to create an extension on an NSClass in-file that adds missing functionality and then PR it to corelibs-foundation afterwards–and conditional compilation really helps with that workflow.

My main objection is that I don’t want us to require clients to #if at all, in the long term. Putting it in now means that it will be easier to say “just #if” instead of fixing the underlying source incompatibility problem.

I think that if we get to a time nearer to the finish line of Swift 3 and we still have many source incompatible changes between the Core Libs Foundation and a corresponding Darwin Foundation then we should introduce something like the #if.

- Tony

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